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M E M O I R. S
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PEABODY ACADEMY OFSCIENCE.
VOLUME I. NUM BER II.
SALEM, MAss.
PU B I, IS H E D BY T H E A C A D E M Y .
- MARCHI, 1871.
*RE88; F. W. l’UTN AM, I'ROPRIRTOIt






P E A B O D Y A. C. A D E M Y O F SCIE N C E .
SE COND MEMOIR.
EMBRYOLOGICAL STUDIES
ON
Diplay, Perilhemis, and the Thysanurous Genus soloma.
B Y
A. S. P.A.C.K.A.R.D, Jr.
SALEM, MASS.
PUBLISHED BY THE PEABODY ACADEMY OF SCIENCE.
1871.
ACCEPTED FOR PUBLICATION
J U L Y, 1870.
EMBRYOLOGICAL STUDIES
ON
DIPLAX, PERITHEMIS, AND ISOTOMA.
THE DEVELOPMENT OF DIPLAX.”
THE eggs of a species of this genus were found July 2d., by Mr.
A. Hyatt, attached to the leaves of a submerged sedge. The nidus,
evidently (as suggested to me by my friend Dr. Hagen) too large to
have been laid by a single dragon-fly, consisted of a long ropy mass
of a gelatinous consistency, about a quarter of an inch thick, which
twined about the leaves of the grass. Through this mass the eggs
were thickly dispersed. They must have been laid for a week or more,
as on the 16th of July large numbers had already hatched out. They
continued to hatch while in glass jars, till the first week of September,
those eggs situated in the middle of the gelatinous mass seeming to
hatch last; in this way a succession of young dragon-flies was disclosed
through the summer. I was unable to carry them farther on in their
development, and did not learn what species they belonged to.
The eggs are oval, cylindrical, .03 of an inch in length, and about half
as thick. Some were observed to be contained in an ovate, transparent,
gelatinous envelope, about twice the size of the egg itself, and at the
anterior pole contracted into a short neck (Pl. 1, fig. 3). On some
eggs was observed a tubercle attached to the anterior pole of the shell,
and retained quite late in embryonic life, as in Pl. 2, fig. 13, t.
In all the eggs observed the blastoderm had apparently been formed,
the blastodermic cells having disappeared. Such eggs (Pl. 1, fig. 1, 1a)
presented a clear space around the periphery of the egg, the yolk-mass
being formed of granules of very unequal size. At this stage (fig. 1),
the granules are massed more towards what is probably the posterior
pole of the egg. In the next stage observed (Pl.. 1, fig. 2, 2d), the
-
*Nearly all the drawings referring to Diplax, and which give this chapter most of its value, were
made under my direction by Mr. J. H. Emerton, the eggs illustrating the points I wished to describe,
having been, in most cases, selected for him to draw. A part of the article, especially that relating to the
revolution of the embryo, is described from his drawings, as I did not observe it.
i.
i



4 EMBRYOLOGICAL STUDIES ON
position of the embryo is internal to the yolk, being surrounded by
a layer of embryonal cells; the rudimentary body and limbs floating
in the middle of the egg. The cephalic lobes are partially devel-
oped, forming an obtusely rounded mass projecting from the yolk,
and situated nearer the middle of the egg than in the next stage.
The cephalic lobes overhang the base of the antennal rudiments, indi- .
cating that these lobes belong to the antennal ring, being probably the
tergal portion. The appendages of the head and abdomen have budded
out, and are grouped into two masses, i.e. the mouth-parts (fig. 2, 1–
Iv), and the thoracic appendages (fig. 2, V-VII). Of the remainder
of the body only the ventral wall (sternites) of the head, the thorax,
and two basal abdominal segments appear. The sternites of the first and
second maxillary segments are now distinct, and exactly the same in
size and development as those of the thorax or abdomen, and as they
disappear in after life, their appearance at this stage is worthy of note,
as indicating that the segments composing the head are in the embryo
exactly like those of the thorax or abdomen, and, so to speak, of the
same rank, not being, as some authors have believed, subsegments.
When, at a latter stage, the tergal walls of the cephalic segments are
completed, we have in each arthromere of the head, a ring nearly
as perfect as in the thorax. In after growth, the sternal portion
of the arthromere is the first to be absorbed, by the approximation
of the base of the cephalic appendages on the median line of the head.
Seen from beneath (Pl. 1, fig. 2a), the antennae, mandibles, and first
maxillae form a group by themselves, while the second maxillae (la-
bium) are turned backwards and grouped with the legs.
The development of the abdomen is, then, later than that of the head
and thorax, the development of the germ progressing from the head
backwards. The extremity of the abdomen was not detected, though I
have no doubt but that farther and more skilled observation would have
enabled me to see it.
The mandibles are from this point of view short, thick, and appear to
arise just beneath the base of the antennae. The maxillae are twice as
large as the mandibles and one-half the size of the labium. They
are broad at base, and situated much farther apart than the mandi-
bles. The second maxillae are twice the size of the first pair of max-
illae, and one half as large as the first pair of legs. The bases
of the two appendages are far apart, in this respect also corresponding
with the thoracic appendages. They are bent inwards in the middle,
and their extremities reach near to the middle of the first pair of
legs. The three pairs of legs are almost exactly similar in size, and are
inserted at nearly equal distances apart. They are folded upon the ven-
tral side of the body, and are long enough to be bent inwards at nearly
right angles, so that the extremities nearly meet over the median line
of the body.


rº- . * * rº-. . . . . ºr -a < …. ------------------------ ~~~~~~~~~~------------ ---------- 3 gº ºr -- . ~~
DIPLAx, PERITHEMIS, AND ISOTOMA. 5
In the next stage (Pl. 1, fig. 4) the rudiments of the eyes appear,
the head having enlarged and approached nearer the anterior end of the
egg. The appendages of the head are much the same, the antennae,
however, being a little longer, and the second maxillae (labium) are con-
siderably longer, but still in their position associated with the legs.
These latter have grown longer, and are laid along the side of the body,
underlapping each other; the hind pair are the longest and are bent
upwards at their extremities. The abdomen is now clearly sketched out,
forming a broad obtuse lobe incurved at the tip, and bent beneath the
body, with traces of four segments.
I will now describe more fully the changes undergone during this
period. August 25th, the embryo had assumed the form indicated by
Plates 1, 2, figures 5, 6, and my remarks regarding this period are
mostly suggested by the faithful studies of Mr. Emerton, as I did not
myself observe the revolution of the embryo. Figure 5 (though a little
rupturned, giving a partial view from beneath, and showing very finely
the relative positions and attachments of the limbs and mouth-parts) |
represents the embryo in its usual position, seen laterally and previous $
to revolving in the egg. The head is not distinctly separated from the
thorax, the tergites not being visible, while those (t") of the post-
abdomen (pat), as we may call the temporary subdivision of the ab-
domen, are clearly marked out. The clypeus (c) is prominent, and, in
the position here given, nearly reaches to the ends of the antennae.
The antennae are about a third longer than the mandibles and maxillae,
which are both very equal in size. The position of the labium is inter- Y.
esting as showing that even in the embryo of insects the second pair of ->
maxillae are intermediate in position and form between the first maxillae
and the legs. The temporary postabdomen, so well marked in this
stage, may be said to represent the permanent postabdomen of the
macrurous Crustacea. The body is cylindrical, oval, the head is very &P
small in proportion to the rest of the body, the thorax forming about :
one half the bulk of the body. The two terminal lobes of the eleventh | 4
abdominal segments are large and conical in form. The mode of de- dº
velopment and relation of the rudiments of the eleventh ring to these
lobes will be interesting to trace hereafter. The sternites (st) are now
indicated, and the rudiments of the intestine (i) first appear in the
postabdomen. & - -
Two days. later, August 27th (fig. 6b), an embryo was observed rolled
on itself and placed across the shorter diameter of the egg. . This and
figures 6, 6a, 7, evidently represent the embryo in the process of turn- :
ing; up to this time the head has laid in the posterior pole. Figures - ;
8 and 8a represent the embryo after it has assumed its final position, ' ' ' ',
with the head in the anterior pole of the egg. The bulk of the head as
compared with that of the rest of the body is well shown. A slight
indentation marks the base of the clypeus, the front edge lying between
º, ſº , .* * * . * ... : . . - - - ----. * 3. . - - ... • - *
iſiſ | iſºaſ sº.--- - *-- * - - - -- - ---------- —- –––––– - - - - - - - * . . .--> -- a - - ſt


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6 EMBRYOLOGICAL STUDIES ON
the antennae. The ventral walls are well developed between the rudi-
mentary appendages. A day after, August 28th, the embryo still mostly
rolled upon itself, presents more of a dorsal aspect, and the post-
abdomen (pat) is brought out more in relief, showing the dark central
spot, or rudiments of the intestine (i). The legs are a little farther
along in their development, and the thickening of the ventral walls, or
sternites (or perhaps the rudiments of both sternites and pleurites, the
two not having yet been differentiated), is more marked, the region
seeming to widen and extend higher up the sides of the body. The
sutures between the sternites are also well indicated.
In an embryo observed August 28th, the body is unrolled and lies
nearly straight in the egg. The broad and short vertex of the head
shows a broad plano-convex, transversely narrow, oblong area, at each
side of which the eyes form a dark rounded oval spot. The yolk mass
fills up the bulk of the body. The arthromeres are plainly indicated,
but no sutures are apparent. Those defining the segments of the post-
abdomen are however clearly seen, the four basal joints being well
shown ventrally, while the free postabdomen is upcurved, the tergites
being distinct. The rudiments of the intestine have extended along the
whole length from the anal extremity, which begins between the ter-
minal lobes, and extends to near the base. The central darker mass is
perhaps the rudiments of the anal plexus of tracheae, where the blood is
to be ačrated.
A ventral view shows the broad sternal area, on the edges of which
the appendages overlap. The head is nearly as broad as the rest of the
body. The antennae are inserted near to, and, opposite, the eyes, their
tips nearly meeting, and extending opposite the first maxillae. The sec-
ond maxillae do not appear, from this view of the body, to greatly ex-
ceed the size of the first pair, and are grouped with them. The form
of the postabdomen, with the rudimentary intestine, is much as when
seen from above.
The same specimen seen two days later, August 30th (fig. 6a), has
changed but slightly though it shows distinctly the base of the post-
abdomen, and the number of segments composing the abdomen. If we
are right in supposing that the ring marked 1 is the basal segment of
the abdomen, then the body of the abdomen consists of six well marked
segments, and the small slender postabdomen consists of five segments,
making eleven in all. The anal stylets hence appear to belong to the
eleventh segment. Later in this (8a) stage the bases of the stylets are
retracted, as it were, within the tenth ring so as to appear appended to
it, and the eleventh segment is only represented by a small tergite.
The anal stylets therefore in this genus, and undoubtedly in the Libel-
lulidae generally, must be considered as appendages to the eleventh ab-
dominal segment.
In this drawing only the base of the antennae is visible, the remainder

indu
DIPLAx, PERITHEMIS, AND ISOTOMA. 7
being laid along the middle of the body. The second maxillae are still
bent backward and resemble the legs, the tips being still simply
rounded. & - *
In figure 7 the embryo is observed lying across the egg, showing the
opposite side of the embryo to what is shown in figure 6. The post-
abdomen is still free from the body, which is rolled closely on itself, so
as to show very distinctly the insertion of the appendages.
The bulk of the head is about the same. The antennae (1) have be-
come more differentiated, being slenderer and inserted much higher up
and above the middle of the head so far as shown by the drawing.
The eyes are nearer the base of the head, and nearer the vertex than
before. The mandibles and maxillae have apparently remained the same.
In the second maxillae the changes are most interesting. The outer
edge of the tip of each limb is produced into a point, while the tip
itself is somewhat square. The two limbs are however separate, and
still grouped with the legs, though directed more anteriorly than in the
previous stages. The insertions of the legs seem obscured by the yolk
mass overlapping and extending nearly to the middle of them. .
Comparing the stage indicated by figures 6 and 7 with the previous
one (fig. 5), the principal changes are these : the body from being
oblong oval has greatly thickened so as to become almost spherical,
though it afterwards, at a period preceding that represented by figures
8, 8a, becomes oval again; the head has grown larger in proportion to
the bulk of its appendages; this is due to the more rapid growth of the
parts between the base of the appendages and the eyes. Two more
abdominal segments have been added, both apparently at the base of the
postabdomen. As in the Myriapods (Julus) and worms, increase in
the number of segments seems to take place near the terminal segment
of the abdomen. Another important change is the concentration of the
yolk towards the back. In a little more advanced stage (fig. 7a) than
represented by figure 7, the embryo has assumed a longitudinal position
in relation to the egg, the head pressing against the anterior end of the
egg. The second maxillae are longer and intermediate in form between
what is indicated by figures 7 and 8. The end of the abdomen is
more advanced, the tip being acute and ending in rudiments of bristles
arising from the anal stylets. The rudimentary eyes consist of seven
large epithelial cells of irregular shape surrounding a central larger one,
and situated on a distinct piece of the head, indicated by the dotted line
in the figure, dividing the clypeus from the epicranium. The labrum is
distinct and separated from the clypeus by a suture. The legs are also
considerably longer than in figure 7. º
At the next stage (figs. 8, 8a) the body is more elongated, the head
is larger, and the anal stylets are fully formed, the rudiments of the
setae being more apparent than before, while the yolk mass is still more
circumscribed. *
------ ~~~~~~~~~~sºrse- - - -
8 EMBRYOLOGICAL STUDIES ON
The head now occupies the anterior fourth of the egg. The eye-
bearing piece of the epicranium now stands out; its lower edge is dis-
tinct from and overlaps the base of the mandibles (this not shown in
the figure) but merges insensibly into the clypeus, there being no
suture between, though there is apparently a slight depression where the
clypeus seems to join with the eye-bearing piece. The clypeus is quite
large and well developed, being clearly defined and separated by a well
marked suture from the labrum. The labrum (fig. 8, lb) seen sidewise
appears somewhat scoop-shaped and resembles in form that of the cricket,
the outer edge, when seen laterally, being slightly curved, the inner very
convex. The antennae show no signs of articulations and are filled with
fine granules. They are slenderer and (fig. 8, 1) twice as long as the
mandibles, and curved around so as to just touch the ends of the max-
illae (fig. 8b, III), impinging on the second maxillae. The mandibles
are directed a little obliquely backwards, but are not curved, and no
appearance of teeth can yet be detected. The first maxillae are a little
longer than the mandibles, their insertion, or base, being higher up, and
the ends extending a little beyond. The origin of the second maxillae
(fig. 8, Iv) could not be made out; it appeared to be below that of the
first maxillae. They are now evidently grouped with the other cephalic
appendages, and are no longer so plainly associated with the legs as in
the previous stages. They are nearly four times the bulk of the first
maxillae. There is a constriction beyond the middle, beyond which the
appendage again bulges out, and the tip is somewhat truncated.
The postabdomen is nearly as bulky as the head, the tip of the anal
stylets reaching half way to the middle of the egg. The stylets are
nearly equal in size. They are not articulated, and end very acutely,
the outermost being the longer and slenderer. The figure shows their
relation to the abdomen. The rudiment of the terminal bristle is seen
on the outer stylet. The large nervous ganglia (fig. 8, n) are seen
laterally to be nearly square, very approximate, the commissures being
very short. -
Two days later, the specimen being observed July 27th, the second
maxillae approached each other and the sides were closely appressed,
taking on somewhat the form indicated in figure 8a, thus forming the
labium, with its tip touching the ends of the anal stylets. The tip of
each second maxilla is square, where before it was subacute and cylin-
drical. The mandibles and first maxillae remain unchanged, no signs of
the terminal teeth of the mandibles being visible. The antennae are,
however, much longer and slenderer, the tips reaching beyond the middle
of the labium. The eyes are more distinct, are larger, and formed of
dermal scale-like cells. The abdominal segments have become more
distinct. The legs have grown longer and are better defined, the artic-
ulations beginning to appear.
Another embryo, observed July 27th, had reached about the same stage
.." . . . . , , , , , , , , "... . ; , ;
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jw.rºw" wº wr------------~~~~wº----------->isr- -ºr-y- z-as -x **.* -º-;
DIPLAX, PERITHEMIS, AND ISOTOMA. 9
of growth. The cephalic lobes, including the antennary segment, have
assumed much more of the form as seen in the mature larva. The entire
head is divided into two very distinct regions, i.e., one before the mouth-
opening (preoral, including the clypeus and labrum, and antennary, or
first segment of the head), and the other behind the mouth (post-
oral, including the mandibular or second segment, the first maxillary
or third segment, and the labial, second maxillary or fourth cephalic
segment).
At this period the embryo is quite fully formed and is about ready to
leave the egg. The three regions of the body are well defined. The
articulations of the tergum have become apparent. The body is so bent
on itself that the extremities of the second maxillae just overlap the tip
of the abdomen. The front of the head is now still farther differenti-
ated. The supraclypeal piece seems to be merged into the antennary
ring, the sutures between them having disappeared. The antennae are
inserted higher up, just in front of the eyes, or rather the eyes have,
as it were, dropped down. The clypeus is broad and large, and the
slightly bilobate labrum is separated from it by a suture. The mandi-
bles and first maxillae are still tubercular in shape, the teeth of the
former not yet appearing. The two halves of the labium are now placed
side by side, with the prominent spinous appendage on the outer edge
of each side of the tip. These spines are probably the rudiments of
the labial, or second maxillary palpi, not afterwards developed, but as
seen in figure 8b, forming inarticulate spines, or, as in the adult larva of
this family, stout, curved, raptorial hooks. The legs. are long and par-
tially bent back on themselves, and at the angles the articulations begin
to appear. The coxo-femoral joints are very distinct; the tarsi are di-
rected upwards, and the two claws in which they terminate are simple,
straight, and equal in size. The tip of the abdomen ends in two un-
equal pairs of stylets, terminating each in a long bristle.
On removing the embryo from the egg when a little farther advanced
(fig. 9, observed August 16th), and on straightening the body out and
viewing it laterally, we are strikingly reminded of the general form of
the Lepismae, and the inference is strongly suggested that they are
embryonic, degraded Neuroptera, and should therefore probably be con-
sidered as a division standing at the foot of that suborder. The resem-
blance is still carried out when we examine the shape of the head, the
shape of the mandibles (better seen however at a subsequent stage),
the homonymous rings of the body, and the form of the end of the
abdomen, with the small terminal tergite resting between the large ter-
minal spines tipped with a long bristle, thus resembling the abdominal
tip of Lepisma. Cut 1, a is a dorsal, and b is a ventral, view of the end
of the abdomen at this stage. The rudimentary eleventh ring is repre-
sented by a triangular supra-anal piece, or plate (corresponding to the
supra-anal plate, as we are accustomed to call it, of other Neuropterous
Q
*
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*:: ~~~~~~~-------...-----------. . - - - - - - ....... . . .----------------~ * ~ : " ' ' ' ' ' "
- '. * - - - -- - - - - - - -- - -
|| | |
10 - EMBRYOLOGICAL STUDIES ON
larvae, and of Lepidopterous larvae). Its base overhangs the base of the
two pairs of anal stylets. Of these latter the upper pair are lamellar,
and broad, and rounded at the end; the lower pair are cylindrical and
* long and slender, bearing each a long straight
bristle, twice as long as the stylet itself. Com-
paring the end of the abdomen of this embryo
with the same parts in the recently hatched larva
(see cut 3, y) the four stylets are seen in the
latter to have been shortened to serve as anal
valves, and the bristles are still shorter. These
stylets in the larva appear to be appendages of
the tenth segment. - - p
The head is large, and the body gradually
tapers from it to the end of the abdomen, and is
flattened, especially the abdomen. The head is
divided into two divisions, indicated by a suture
leading from the vertex to the base of the first
maxillae, thus separating the occiput (fig. 9, 4) from the epicranium, and
showing that the occiput, or that part of the head behind the eyes
2. and insertion of the antennae in adult
insects, is the most posterior part of
the head, and is the tergite of the
labial, or second maxillary segment.
The eyes now consist of an irregular
group of rudely formed facets, and
remind us of the few large facets
forming the eye of the Lepismidae.
× The antennae are inserted in front
W. of and a little below the eyes, and
2 .. consist of three joints. The man-
sº dibles and maxillae are of the same
#. size and form as before, while the
- labium (second maxillae) is appressed
to the under side of the body, and
reaches as far back as the insertion
of the hindermost pair of legs. The
long slender legs once bent at the
coxo-femoral joint, reach, when laid
along the body, to the middle of
the tenth abdominal ring. The ter-
N. gite of the eleventh ring (fig. 9) is
now distinct, forming a short trian-
gular piece resting upon the base of the large stout terminal spines
destined to form the anal valves of the free swimming larva (cuts 2, 3,
gy) where, however, they are very much shorter.

DIPLAX, PERITHEMIS, AND ISOTOMA. 11
The nervous system (fig. 9, m) is now proportionally smaller than be-
fore, the ganglia being oval in outline, seen laterally. The intestinal
canal is now formed, and at a, is surrounded by a plexus of tra-
cheae, in which the main tracheae (t) terminate. The main branches of
the tracheae form a terminal loop in the head. The yolk mass is now
much restricted, being confined to the labial segment of the head and
the thoracic segments. … Figure 12 shows the earliest form of the
tracheae to consist of two main trunks giving off lateral twigs at nearly
right angles, forming a loop opposite the second pair of legs, and bifur-
cating (fig. 9) in the head. The twigs at the posterior termination
form the respiratory plexus. ſº .
At a subsequent period in the life of the embryo (fig. 10, observed
July 25th), upon removing an embryo from the egg, the principal change
is in the labium, which is now comparatively shorter than before, and is
drawn up nearer the front of the head, thus standing out freer, from the
body than before ; the articulations in the antennae and legs are more
distinct, and the edges of the eyes are better defined. - 2
On the same day (July 25th), another embryo (fig. 11) was taken
from the egg, apparently being just ready to hatch. The principal
change that has occurred is in the
position of the second maxillae, which
now stand out at nearly right angles
from the head, while the mandibles
are dentate at the end, and remind
us of the shape of the mandibles in
Lepisma. The head is also freer
from the body. The labium is dis-
tinctly bilobate, and the occiput is
separated by a well defined suture
from the épicranium. The labium
(second maxillae) is now broad, flat * >
seen from above, oblong in shape, square at the end, with the rudiments
of the palpi (fig. 8b). The body is covered with minute tubercles,
which in the next, or free swimming stage, give rise to hairs. -
The Larva (cuts 2 and 3). When hatched it is .05 of an inch in
length. The head is now free and the antennae stand out free from the
3.*
* Cut 3. Side view of the head of the larva of Diplax before the first moult; c, deciduous tubercles
terminating in a slender stylet; their use is unknown; they have not been observed in the full-grown
larva; e, the compound eyes. 1, the three jointed antennae, the terminal joint nearly three times as long
as the two basal ones. 2, the mandibles, and also enlarged, showing the cutting edge divided into four
teeth. 3, maxilla dividing into two lobes; d, the outer and anterior lobe, 2-jointed, the basal joint termi-
nating in two setae; and a, the inner lobe concealed from view, in its natural position, by the outer lobe,
d. 4, the base, or pedicel, of the second maxillae, or labium, the expanded terminal portion being drawn
separately; d. and a, two movable stout stylets representing, perhaps, the labial palpi; the lobe to which
they are attached is multidentate, and adapted for seizing its prey; on the right side the two stylets are
appressed to the lobe; ac, represents, perhaps, the ligula; but we have not yet studied its homologies
carefully; this part is attached to a transversely linear piece soldered to the main part of the labium; y,
the 11th abdominal tergite, with its pair of conical anal stylets; 2, the last tarsal joint and pair of long
slender claws. -
º,
. . . . . . . sºvº.

12 EMBRYOLOGICAL STUDIES ON
front. The thorax has greatly diminished in size, while the abdomen
has become wider, and the limbs very long; and the numerous minute
tubercles seen in the preceding stage have given origin to hairs. The
dorsal vessel can now, for the first time, be seen. When in motion,
the resemblance to a spider is most striking. The nervous ganglia are
from one fourth to one fifth as wide as the abdomen itself, and are con-
nected by two slender commissures. The flow of blood to the head,
and the return currents through the lacunar, or venous circulation along
the sides of the body are easily observed. The vessels were not
crowded with blood discs, the latter being few in number, only one or
two passing along at a time. Two currents passing in opposite direc-
tions were observed in the legs. -
The young larva differs remarkably from the mature larva (which for
the most part differs from the pupa (cut 4) only in the smaller eyes and
4. absence of the rudiments of wings) in the much longer legs
and shorter abdomen, and in the presence of the deciduous tu-
bercles (cut 3, c) situated on the vertex of the head.
Individuals were seen which were supposed to have undergone
their first moulting, being nearly as large again as, the freshly
hatched larva, and with the abdomen a little longer.
The larvae, after hatching, were not observed through their
transformations. It is probable that, like the young Chloéon, which,
according to Lubbock, moults twenty times before assuming the imago,
or winged state, the young dragon-fly moults more than four or five
times, the usual number in the insects with complete metamorphoses.
THE DEVELOPMENT OF PERITHEMIS.
During the summer of 1870, I had an opportunity of witnessing
some steps in the development of Perithemis domitia, which show
that in all respects the embryology of this genus is identical with that
of the species of Diplax, and confirms the general accuracy of the draw-
ings by Mr. Emerton, and my own observations. On a sunny day
(July 28th), I observed a dragon-fly hovering over the surface of a
pond, dipping its abdomen (the body being in a perpendicular attitude,
with the wings in rapid motion) lightly into the water which just covered
a piece of floating cow dung, and then fly off to return again and repeat
the operation. Several dragon-flies were coursing over the small pond,
and I am inclined to think that two or more dragon-flies pushed their
eggs into this same mass of ordure. The dipping motion was the work
of an instant; whether one, or a packet of eggs were deposited at a
single dip of the abdominal tip I could not say, but from the arrange-
ment of the eggs I should suppose they were deposited one by one at
each dip of the abdomen.
&


DIPLAx, PERITHEMIS, AND ISOTOMA. 13
Upon examining the piece of dung, which was soft, the upper side
for a couple of square inches was seen to be covered with a jelly-like
mass of thick consistency (thicker than the ropy mass enclosing the eggs
of Diplax) which filled up the cavity in the piece of dung. The eggs
were scattered through the mass several deep, and were not arranged in
any order.
On examination the embryos were mostly in an advanced stage of
growth, some having hatched, and it is accordingly probable that the
dragon-flies had visited this favorite spot for oviposition for a period of .
two or three weeks.
The eggs are regularly oval and about one half as large as in Diplax,
though of exactly the same form. The contents (observed with a 4-10
Zentmayer and A eye-piece) consisted of larger cells with more nu-
merous minute ones, much as in the eggs of a species of Lecanium from
the grape-vine which I was examining the same day.
Most of the eggs were provided with an acute tubercle like that fig-
ured on Plate 2, figure 13, c, and of the same form and relative size,
being a little more than twice as long as the base is thick, and situated
at the anterior end of the egg. It is transparent and evidently arises
from the shell, beneath the layer of gum. I did not examine it under
a higher power, but am inclined to think that it is the micropyle.
The egg is surrounded by a network of rudely hexagonal areoles,
some being almost round, and others oblong; about twenty perfect are-
oles could be seen on the surface of the egg next to the eye. Similar
areoles are described and figured by Dr. A. Brandt as surrounding the
egg of Agrion and Calopteryx. He also states that Prof. R. Leuckart
has described them in the eggs of Æschna grandis (Müller's Archiv,
1855, p. 204). This solid coating of gum is annoying to the observer,
but in many eggs falls off. The chorion is slightly horn-colored, and a
little more transparent than in Diplax.
While most of the eggs were well advanced I was fortunate enough
to observe the primitive band in one egg. Like that of Agrion and
Calopteryx, and of the Lecanium eggs then under observation, it was
internal to the yolk, but situated nearer to one side of the egg than the
other. The next stage observed corresponds exactly to that indicated
by Plates 1, 2, figures 8, 8a. The appendages both of the head and
thorax were as indicated in the figures referred to, the joints of the
limbs not being formed. The yolk-sac was confined to the back and
subdivided into several large compartments filled with the ordinary yolk-
cells. -
On squeezing out an embryo just ready to hatch, it exactly repeated
the form represented by Plate 2, figure 9, the division between the
front-head, or cephalic lobes, and the second maxillary segment, being
well marked. The tracheae and the nervous cord were the same in ap-
pearance.
*:::=-3 --~~~#xºr---ºr-ser----->
14 EMBERYOLOGICAL STUDIES ON
The only difference between the recently hatched larva and that of
Diplax was, that the head of the former is squarer, the eyes being more
prominent. The nervous ganglia are large and rounded, and connected
by two short, broad commissures. The heart is tubular, with the valves
corresponding in position to each suture of the body. The valves are
long, oblique at a slight angle to the walls of the vessel, and, when
closed, form an acute v. The two valves approach each other at each
pulsation, nearly but not quite closing together, like the wires in a cer-
tain kind of rat-trap. The pulsations were quite irregular, sometimes
but a single, rather long, oval blood globule passing slowly along; at
other times a number of globules pass along together. At times four or
five beats intervened without the passage of a single globule. Indeed
much fewer globules were apparently sent forward through the heart
5.” than flowed back by the lateral pair of veins,
along which they passed obliquely. -
Fº The larva is active, swimming up and down in
in the water, and feeds on large infusoria, catching
them by suddenly dropping its labial mask, and
retaining its victim in its grasp by its mandi-
bles. Some it rejected, while others it swallowed
with a relish. -
Comparison with the Embryo of Calopterya, and
Agrion. —Since the observations on Diplax were
made, and abstracts read at the meeting at Bur-
ation for the Advancement of Science, and pub-
* lished in the “American Naturalist” for February,
1868, and in the “Proceedings of the Boston So-
ciety of Natural History” (vol. xi) for January 22d,
- 1868, I have received, through the kindness of
Dr. Alexander Brandt,f of St. Petersburg, his admirable paper on the
embryology of Agrion, Calopteryx, and certain Hemiptera. -
Brandt’s studies were directed chiefly to the development of the em-
bryonal membranes. His conclusions are: “1st. Calopteryx and Agrion
are developed according to the type of the development as shown by
Metschnikow to exist in the Hemiptera, namely, the germ or primitive
band is internal to the yolk.
“2d. In those insects with an internal germ we need to distinguish an
embryonal membrane, which is divided into a visceral and a parietal layer.
*The accompanying figure (cut 5), from Professor Nicolaus Melnikow’s “Beiträge zur Embryonalent-
wicklung der Insekten.” (Wiegmann's Archiv für Naturgeschichte, 1869, p. 136), of the embryo of Pedi-
culus capitis, will show plainly the relations of the parietal and visceral layers to the germ; vk, indicates
the cephalic lobes (scheitelplatten), the rudimentary appendages of the head and thorax being situated
in the middle and posterior pole of the egg, the embryo being about to revolve in its egg; am, is the
parietal (amnion), and db, the visceral layer; as, the antennae.
fibeiträge zur Entwicklungsgeschichte der Libelluliden und Hemipteren mit besonderer Berücksich-
tigung der Embryonalhülle derselben. Von Alexander Brandt, Jun. Read October 29th, 1868, before the
Académie Impériale des Sciences de St. Petersburg. 4to, pp. 33. With 3 plates. St. Petersburg, 1869.

DIPLAx, PERITHEMIS, AND ISOTOMA. 15
“3d. The visceral layer (veiled or plaited layer of Metschnikow) does
not become united with the extremities, but enters, together with the pa-
rietal layer (amnion of Metschnikow), into the formation of the yolk sac.
“4th. The formation of the yolk sac, together with the revolution or
turning of the embryo on its transverse axis, consists in an independent
contraction of the parietal layer of the embryonal membrane.” "
As my attention was directed to morphological points, we can only
infer from the few data given above that Diplax and Perithemis have
the same arrangement of the embryonal membranes, and that these
membranes later in the life of the embryo form the yolk sac, through
the contraction of the parietal layer of the embryonal membrane, as in
Agrion, Calopteryx, and certain Hemiptera. -
As regards the changes of the embryo after the rudiments of the ap-
pendages have appeared, they seem in Diplax and Perithemis to be the
same as in Calopteryx and Agrion. My figures 2, 2a may be compared
with Brandt's figure 11; figure 4 with Brandt's figure 12. The embryo
of Diplax is much thicker and shorter, corresponding to the shorter,
more ovate egg. The attitude of the germ during its turning in the egg
is identical with that of Agrion and Calopteryx. Finally, Brandt’s figure
19 may be compared with my figures 8, .8a, and 9, the yolk now being
confined to a small area on the back of the embryo, which is now
segmented and nearly ready to hatch, the claws being indicated, the
eyes formed, the appendages partly jointed, and otherwise much as in
the larva.
THE DEVELOPMENT OF ISOTOMA.
The following fragmentary notes and rather rude drawings are pub-
lished simply because any knowledge, however slight, of the mode of
development of the Thysanura is very desirable at this time, as bearing
both on the principles of embryology of insects and the morphology of
this interesting and difficult group. &
All that we know of the embryology of the Thysanura are some “un-
satisfactory observations” on Podura recorded by Nicolet (Dohrn, Ent.
-Zeitung, Stettin 1869, p. 244), and a figure of the embryo of “Podu-
rella” in Agassiz and Gould's “Principles of Zoology,” figure 110, p. 144
(Edition of 1854). This represents the embryo just ready to hatch.
Dr. Dohrn has evidently gathered enough from Nicolet's observations to
determine him to place Podura among those insects in which the germ,
or primitive band, is external to the yolk (Insecta ectoblasta).
Sir John Lubbock, in his memoir “On the Generative Organs, and on
the Formation of the Egg in the Annulosa” (Phil. Trans. London, 1861,
p. 595), has described the ovarian egg of Petrobius maritimus, and its
mode of formation. The egg is compound, as in the insects generally;
the “original epithelial, or at least ovarian cell, which becomes the
16 EMBRYOLOGICAL STUDIES ON
&
whole egg in Argulus, Limulus, Argas, Mermis, etc., forms in other
cases only the Purkingean vesicle,” the eggs of Limulus, etc., being,
according to Lubbock's view, simple. He states that “the mature egg is
elongated fusiform, about 335th of an inch in length, and enclosed in a
tough, somewhat transparent chorion. The yolk consisted as usual, of
a viscid substance, containing fine granules and oil globules, varying up
to rºoth of an inch in diameter.”
The eggs of Isotoma Walkerii were found by Mr. C. A. Walker, at
Chelsea, under the bark of an apple tree, on the 25th of April, 1870.
They were either laid singly or in small scattered groups on the damp
under surface of the bark. The eggs are spherical, glistening white,
with the chorion very transparent. They measure .0625 of an inch in
diameter. At the date above mentioned many of the embryos had
hatched, as the young were found in various stages of growth. They
continued to hatch until May 6th. Numerous eggs were observed in
which the blastoderm had not yet been formed. In these, amid a mass
of minute granules, floated from two to four large oil globules (Pl. 3,
fig. 2, c), the largest of which was one fourth the diameter of the egg
itself, while there were numerous smaller globules measuring about one
fourth the diameter of the largest cells.
The earliest stage I was able to observe (Pl. 3, figs. 1, 2) was the
period of formation of the primitive band. I could not detect any eggs
with blastodermic cells, or a blastoderm fully formed. Figure 2 repre-
sents the primitive band lying upon the outside of the yolk, next .
to the chorion, and divided into the external tegumental layer (t) and
the inner, or muscular, layer (m). The two ends of the primitive band
meet at a, the band thinning out so as to be of an imperceptible thick-
ness just before a (fig. 1), where there is a clear space and an infold-
ing (a) of the primitive band, which seems to be the place of meeting
of the anterior and posterior end of the germ. A little later the fold
grows deeper (as at b, fig. 2), and a seems to indicate the rudiments
of the cephalic lobes. The exact mode of origin of these lobes (Scheitel-
platte of Zaddach) I did not observe.
At a more advanced period of development, the primitive band, now
more properly speaking, the primitive body-walls, is clearly defined, and
is more homogeneous, the small cells previously scattered through its
substance (as in fig. 2) having disappeared. This stage is signalized
by the appearance of the primitive arthromeres, or segments of the body
(protozoonites of Claparede). They originate just as they do in the Phry-
ganidae, according to Zaddach. They arise six in number (fig. 3), repre-
senting probably three cephalic and three thoracic segments (unless the
fourth is the 2d maxillary arthromere, which more extended observations
may prove to be the case). It will be noticed that they arise on the
opposite side of the egg from the fold a, so that the cephalic lobes,
(pl) extend from a around to I. The central portion, or muscular layer
". "...' ' &” . - … . . . * { " . . .e. ** * * . . . .”.” ... t . . . . . .-- . ---
*** - - - -º-º-º-º-º-º-º-º-º-º-º-º-, …"--": ...is ºxº-Se......N.' ..…..:



DIPLAx, PERITHEMIS, AND ISOTOMA. 17
of the egg, is now more homogeneous than before, the fat globules
having disappeared in the centre of the egg. Plate 3, figures 4 and
5, represent the germ farther advanced and the budding out of the
appendages. º
The cephalic lobes are, as in figure 5, quite clearly indicated, and the
rudimentary appendages, in most cases, well shown. At about this
period the yolk sac is more circumscribed than before in the germ. .
Whether there is an internal division of the embryonal membrane (vis-
ceral membrane) which forms the yolk sac, as in Agrion and the Hemip-
tera, I was not able to determine. The outer layer (“amnion,” or parietal
layer), which surrounds the embryo, I did not at this stage observe, but
as soon as the appendages are formed, as in figure 8, the chorion bursts
on slight pressure, and the embryonal membrane is readily detected,
enveloping the embryo, like the “larval skin” of many crustacea.
At the period indicated by figure 6 the dorsal walls (tergites) of the
arthromeres are closed in, and the rudiments of the spring (figs. 6 and
7, Sp) appears. It is liable to be mistaken for the antennae (1) so
large and well developed is it. It is evidently in all respects, both in
its origin from the under side of the penultimate segment of the abdo-
men, and in its form, homologous with the cephalic and thoracic append-
ages. .
In figure 7 the mandibles are just behind, and but little smaller than
the antennae (1), and in a vertical view (fig. 9) of the embryo when
somewhat older, a pair of tubercles (II) are seen next to the rudimen-
tary antennae (1), which are probably the rudiments of the mandibles.
In figure 10, i indicates the position of the rudiments of the alimentary
canal; the yolk-cells composing it are much smaller than those scattered
over the other portions of the body, and in the appendages. Figure 11
shows the antennae of much greater length than before, with the rudi-
ments of the articulations scarcely indicated.
At a later period the antennae, especially, seem to show traces of ar-
ticulations, and have grown much longer, while the end of the abdomen
is divided deeply by the median furrow into two lobes. The mandibles
(II) and first maxillae (III) are distinct. I was unable at this or any
other period to discover any traces of the second maxillae. Though ex-
isting in a very rudimentary state in the adult I could not detect them
after repeated attempts, but do not doubt but that a more skilled ob-
server would have made them out. Indeed it is a most difficult thing
to discover their rudiments in the adult; I failed, at the time these
observations were made, to detect them, though since then I have suc-
ceeded in making out their structure and relation to the surrounding
parts of the mouth. -
The cephalic lobes are very distinct, and posteriorly defined by a
slightly marked suture from the postoral portion of the head. They
are deeply cleft by the median line of the body. There are no indica-

| . . ."
18 EMBRYOLOGICAL STUDIES ON
tions of the basal tergites of the head, the segments to which they
belong being not yet differentiated from the thorax.
A later period (figs. 13, 13a) is characterized by the differentiation of
the head as a distinct region of the body, the posterior portion, or post-
oral division of the head (including the mandibular and first maxillary
arthromeres) uniting with the cephalic lobes to form the head, which
is distinctly seen to move freely on the thorax. The ocelli are eight in
number, and arranged obliquely in pairs, being situated on an elongated
oval area just above the base of the antennae. The two pairs of rudi-
mentary postoral appendages, comprising the mandibles and maxillae, are
now greatly increased in size, both of much the same size and form, ex-
cept that the hinder pair are divided by a slightly marked articulation,
which is not observable in the mandibles. The basal division of the
maxilla probably represents the cardo and stipes together, the distal ar-
ticulation representing the future palpus, galea and lacinia. When seen
in a front view we can better observe the relations of these organs to
the labrum. This latter partially overlaps the mandibles on their inner
edge, while the maxillae are more external, though partly covered in
front by the mandibles.
The front of the head is so entirely different from what it is in the
adult, that certain points demand our attention. It is evident that at
this period the development of the insect has gone on in all important
particulars much as in other insects, especially the Neuropterous Mysta-
cides as described by Zaddach. The head is longer vertically than hori-
zontally, the frontal, or clypeal, region is broad, and greater in extent
than the epicranio-occipital region. The antennae are inserted high up
on the head, next the ocelli,” falling down over the clypeal region. The
clypeus, however, is merged with the epicranium, and the usual suture
between them does not appear distinctly in after life, though its place is
seen in figure 13 to be indicated by a slight indentation. The labrum
is distinctly defined by a well marked suture, and forms a squarish knob-
like protuberance, and in size is quite large compared to the clypeus.
From this time begins the process of degradation, when the insect
assumes its Thysanurous characters, which consist in an approach to the
form of the Myriapodous head, the front, or clypeal region being re-
duced to a minimum, and the antennae and eyes brought in closer prox-
imity to the mouth than in any other insects.
That other most essential Thysanurous characteristic, the spring, is now
fully formed. It arises as a thick tubercle from the sternite of the pe-
multimate segment of the abdomen, and subdivides into a pair of two-
jointed finger-shaped prolongations.
The tip of the abdomen is deeply bilobate, the median line of the
body being deeply impressed.
The final stage in the life of the embryo is just previous to hatching,
* The basal joint is not shown in ſig. 13 a, but is well indicated in the side view of the embryo, fig. 13.

DIPLAx, PERITHEMIS, AND ISOTOMA. 19
(figs. 14, 14a). At this time the animal lies with the body so curved
that the tip of the abdomen just touches the mouth. The ocelli are sit-
uated on an irregular lunate spot. The mandibles and maxillae are long,
slender, blade-like, concealed within the head, so that the mouth is
somewhat tubular, as it appears in a front view of the head. They
move back and forth upon one another, and, in their relation to the
head, may be compared with the base of the mandibles and maxillae in
the head of Cimea, lectularius and Coreus tristis. At this period I could
not detect any traces of a labium. The feet end in two claws, one
being minute and very slender. Neither at this, nor in the larval state,
could any traces of the tracheae be observed, and I doubt whether they
exist.
The embryo when about to hatch, throws off the eggshell and “am-
nion” (or “larva skin”) in a few seconds. The larva is perfectly white,
and is very active in its movements, running over the damp, inner
surface of the bark. The larva (figs. 15, 15a, spring; 16, 16a under
side of the head, showing the mandibles and maxillae, II, III; 17, the same
seen ventrally) is a little over one hundredth of an inch in length, and
differs from the adult in being shorter and thicker, with the spring
very short and stout; while the head is much rounded, and the antennae
are stout and thick. In fact the larva assumes the form of the lower
genera of the family, such as Achorutes and Lipura, the adult more
closely resembling Degeeria.
The larva after moulting retains its early form, and is still white. It
is then two and a half hundredths of an inch in length. After a second
moult the body becomes purplish, translucent, and in form much more
slender, resembling the adult. -
The eggs are laid and the young are hatched apparently within a
period of from six to ten days.
The adult seems to be undescribed, and I take pleasure in naming it
Isotoma Walkerii, after Mr. C. A. Walker through whose instrumentality
I have been able to study its embryology.
It belongs to Nicolet's first section of the genus, of which the Eu-
ropean I. glacialis is a type. It is .04 of an inch in length, and is dull
yellowish snuff-green in color. The four-jointed antennae have the third
joint smaller and shorter than the second, which is larger than the first ;
the terminal joint is long oval, and equals in length the second and
third together. The abdomen is broadest just before the end; the
spring arises from a short stalk, while the fork is long and slender,
the branches being bowed outward near the base. It was found in
abundance in the spring, and during the past autumn in November and
the following spring again, under the bark of the same tree.
Comparing the development of Isotoma with that of Mystacides, a
Phryganidan, as worked out so thoroughly by Zaddach, the correspond-
ence throughout the different stages is very striking, so that we may
20 EMBERYOLOGICAL STUDIES ON
feel warranted in saying that the formation of the germ, and the growth
of the embryo of Isotoma is, in the most important points, almost
identical with that of the Phryganidae.
As in the Phryganidae (assuming that Mystacides fairly represents the
mode of development in the family generally), the Isotoma germ is an
“ectoblast,” i.e., the primitive band is developed on the outside of the
yolk, and my figures 1, 2 and 3 may be compared with Zaddach's figures
13–19. Melnikow in his “Beitrage zur Embryonalentwickelung der In-
sekten” in “Wiegmann and Troschel's Archiv,” 1869, p. 148, describes
the outer (“amnion” or parietal) and inner (faltenblatt, or visceral)
membrane of Mystacides, which Zaddach had overlooked. The parietal
layer of Isotoma was readily perceived, but the visceral layer was
not detected. The embryo, at the time represented by figure 12, may
be compared with Zaddach's figure 40. At this period the dermal walls
of the posterior portion of the head (including the mandibular and
maxillary segments and thorax) have not been completed. The yolk
also has been more completely absorbed, and the abdomen is doubled
upon itself; this is due to the great length of
the germ of Mystacides as compared with that
of Isotoma, rather than to any important struc-
tural difference. A succeeding step, represented
by figures 13 and 13a, may be compared with
Zaddach's figure 52 (as shown in the adjoining
cut 6). The position of the two embryos is
the same, though in Isotoma, the development
of the antennae has been accelerated over that of
the mandibles and maxillae, where in Mysta-
cides they are coördinated in their development with the last named ap-
pendages, otherwise the two embryos are much alike. The growth of
the young Isotoma henceforth remains almost at a standstill ; and there
is only an elaboration of the articulations, the formation of claws, of hairs,
and a slight change in the form of the head. Much greater changes are
effected after this stage in Mystacides, i.e., the mouth-parts and antennae
undergo a greater modification, the legs are greatly elongated and the
different pairs vary greatly in form ; the tracheal system is perfected,
and the tubercles on the basal ring of the abdomen, by which it is re-
tained in place in its case, are developed, so that there is a vast differ-
ence between the freshly-born young of the two insects; the one form
having gone on in the natural course of development, the other being,
in many very essential points, retarded.
An interesting point in the embryology of Isotoma is the homology of
the spring. Though its earliest development was not observed, it is
evidently homologous with the third pair of blades comprising the un-
jointed ovipositor of the higher insects, and seems to be homologous with
the legs and cephalic appendages.

DIPLAx, PERITHEMIS, AND ISOTOMA. 21
The question naturally arises whether after all, contrary to my own
conclusions,” the three pairs of non-articulated tubercles, which form the
ovipositor of the winged hexapodous insects, may not be modified ab-
dominal limbs, and homologous with the three pairs of spinnerets of the
spiders, and the abdominal feet of Myriapods. Another point, that I have
been led by these and other embryological studies to reconsider, is the
important question whether the eyes of insects (including spiders and
Myriapods) are homologues of the limbs, and are developed on separate
cephalic segments. My attention was called forcibly to reconsider this
point, by the embryo of Limulus, in which a pair of ocelli are situated
indubitably on the first segment, and the compound eyes on the third
segment of the cephalothorax, each of these segments bearing a pair of
limbs l Evidently they are, in Limulus, modified dermal sense-cells, and
developed without reference to whether the segment bears limbs or not.
In certain mites, as Hydrachna, Pontarachna, Thalassarachna, and prob-
ably all the Hydrachnidae, the ocelli are situated over the second pair of
legs at a considerable distance behind the head. In a genus of Annelids
(Polyopthalmus) organs of sight are developed on each ring of the body,
or, as in certain Planarians, scattered irregularly over the body. In the
embryo of Isotoma and Diplax the ocelli are evidently developed on
the antennary segment, as they are epithelial cells primarily situated
on the cephalic lobes, which seem to form the tergite of the antennary
segments. With this view, the supposition I have expressed (led thereto
by the generally received opinion of Milne-Edwards, Dana, and others,
that the eyes of insects and Crustacea represent limbs and therefore de-
mand separate segments) seems incorrect. Accordingly, we seem forced
to the belief that the head of the hexapodous insects consists of but
jour segments, i.e., the second maxillary, first maxillary, and mandibular
segments, situated behind the mouth-opening, and the antennary, or first
and preoral segment, situated in front of the mouth. The cephalic
plates, which fold back upon the head, forming the main expansion of
the insectean head, is apparently the tergum of the antennary segment.
The clypeus and labrum are apparently differentiated from the cephalic
lobes, and thus seem to form a portion, or fold, of the antennary seg-
ment. These cephalic lobes in the Arachnids are described and figured
by Claparède as folding back on top of the mandibular and maxillary
segments, bearing ocelli and forming the anterior wall of the mouth.
The upper part of the head of an insect is, then, largely tergal, rather
than pleural as previously stated by me. -
* Proceedings of the Boston Society of Natural History. Vol. 11. p. 393. 1868.
' , AA.
*
IEEDXCIE IT...AL INT.A.TIO INT O IET IEP II, _A_TIERS -
PLATEs 1, 2. EMBRYology of DIPLAx.
Figures 1, 1a. Side and front view of the egg after the blastoderm has probably formed;
2b, another egg before primitive band has been formed.
Figure 2. Lateral, and 2d, ventral, view of the embryo; pl, cephalic lobes; I, antennae; II,
mandibles; III, first maxillae; Iv, second maxillae.
Figure 3. Egg enclosed in a gelatinous capsule.
Figure 4. Embryo farther advanced; e, eyes; ab, abdomen.
Figure 5. A later stage; c, clypeus; st, sternum; pat, temporary postabdomen; i, rudiments
of intestine; th’, tergites of abdomen.
Figure 6. Revolution of the embryo, view from behind; 6a, side view; 1–11, the eleven seg-
ments, or arthromeres, of the abdomen; 6b, the embryo lying across the egg, side view.
Figure 7. The embryo a little older, and lying across the egg; 7a, the eye in its relation to
the appendages of the head. &
Figure 8. Embryo nearly ready to hatch, ventral view; 8a, the same seen laterally; 8b,
front view of the head, showing the labium with the lateral terminal spines. +
Figure 9. An embryo removed from the egg and straightened out; 1–3, the three anterior
cephalic segments; 4, the fourth, or second maxillary, segment of the head; t, main
trachea; 2, respiratory plexus of tracheae surrounding the intestine; m, nervous system.
Figure 10. An embryo a little farther advanced, the labium being a little shorter.
Figure 11. An embryo still farther advanced, the labium (iv) being still shorter.
Figure 12. An embryo showing the mode of origin of the tracheae.
Figure 13. An egg showing the acute tubercle (c), supposed to be the micropyle.
Figures 14, 14a. An embryo just previous to hatching.
PLATE 3. EMBRYOLOGY OF ISOTOMA.
Figure 1. Portion of egg showing the fold a, in the primitive band. The lettering is the
same in all the figures.
Figure 2. An egg a little farther advanced; a, fold; b, much deeper than in fig. 1; t, tegu-
mental; m, muscular layer of the primitive band; c, large yolk cells.
Figure 3. The primitive band fully formed, with the protozoonites, I, II, III, being the rudi-
ments of the antennary, mandibular, and first maxillary segments; v - VIII, the three
thoracic segments; a, the fold, indicating where the two ends of the body meet; pl., the
cephalic lobes. e
Figure 4. The same farther advanced.
Figure 5. The same still farther advanced. º
Figure 6. The segments of the body completed; pl, the cephalic plates; 1, antennae; sp, the
rudiments of the spring.
Figure 7. Another egg, with the antennae and spring in view, and probably the mandibles.
The yolk mass is more circumscribed than before. -
Figure 8. Tergal view of the germ, with the chorion (ch) broken, showing the embryo sur-
rounded by the parietal membrane or larval skin (am).
Figure 9. The embryo seen tergally, farther advanced.
Figure 10. Showing the antennae arising from beneath the cephalic lobes; and the three pairs
of legs.
Figure 11. An embryo a little farther advanced, the antennae being divided into joints.
Figure 12. An embryo much farther advanced, showing the cephalic plates distinctly sepa-
rated from the posterior portion bearing the mandibles (II) and first maxillae (III), which
have not yet become differentiated from the posterior portion of the body. e
Figures 18, 13a. Side view of an older embryo showing the eyes, and labrum (l) differen-
tiated from the clypeus; and the three-jointed spring and bilobate extremity of the
abdomen.
Figures 14, 14a. The embryo just ready to hatch, the tip of the abdomen just meeting the
mouth; 14b, front view of the head showing the mandibles and maxillae.
Figure 15. Freshly hatched larva; 15a, spring.
Figures 16, 16a. Under side of head showing relative position of the mandibles and maxillae.
Figure 17. The under side of head showing the gular area.
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