_The Egg._--Among the Hexapoda, as in Arthropods generally, the egg is
large, containing an accumulation of yolk for the nourishment of the
growing embryo. Most insect eggs are of an elongate oval shape; some
are globular, others flattened, while others again are flask-shaped,
and the outer envelope (_chorion_) is often beautifully sculptured
(figs. 20, d; 21, a, b). Various devices are adopted for the
protection of the eggs from mechanical injury or from the attacks of
enemies, and for fixing them in appropriate situations. For example,
the egg may be raised above the surface on which it is laid by an
elongate stalk; the eggs may be protected by a secretion, which in
some cases forms a hard protective capsule or "purse"; or they may be
covered with shed hairs of the mother, while among water-insects a
gelatinous envelope, often of rope-like form, is common. In various
groups of the Hexapoda--aphids and some flesh-flies (_Sarcophaga_),
for example--the egg undergoes development within the body of the
mother, and the young insect is born in an active state; such insects
are said to be "viviparous."
_Parthenogenesis._--A number of cases are known among the Hexapoda of
the development of young from the eggs of virgin females. In insects
so widely separated as bristle-tails and moths this occurs
occasionally. In certain gall-flies (_Cynipidae_) no males are known
to exist at all, and the species seems to be preserved entirely by
successive parthenogenetic generations. In other gall-flies and in
aphids we find that a sexual generation alternates with one or with
many virgin generations. The offspring of the virgin females are in
most of these instances females; but among the bees and wasps
parthenogenesis occurs normally and always results in the development
of males, the "queen" insect laying either a fertilized or
unfertilized egg at will.
_Maturation, Fertilization and Segmentation._--Polar bodies were first
observed in the eggs of Hexapoda by F. Blochmann in 1887. The two
nuclei are successively divided from the egg nucleus in the usual way,
but they frequently become absorbed in the peripheral protoplasm
instead of being extruded from the egg-cell altogether. It appears
that in parthenogenetic eggs two polar nuclei are formed. According to
A. Petrunkevich (1901-1903), the second polar nucleus uniting with one
daughter-nucleus of the first polar body gives rise to the germ-cells
of the parthenogenetically-produced male. There is no reunion of the
second polar nucleus with the female pronucleus, but, according to the
recent work of L. Doncaster (1906-1907) on the eggs of sawflies, the
number of chromosomes is not reduced in parthenogenetic egg-nuclei,
while, in eggs capable of fertilization, the usual reduction-divisions
occur. Fertilization takes place as the egg is laid, the spermatozoa
being ejected from the spermatheca of the female and making their way
to the protoplasm of the egg through openings (micropyles) in its firm
envelope. The segmentation of the fertilized nucleus results in the
formation of a number of nuclei which arrange themselves around the
periphery of the egg and, the protoplasm surrounding them becoming
constricted, a blastoderm or layer of cells, enclosing the central
yolk, is formed. Within the yolk the nuclei of some "yolk cells" can
be distinguished.
[Illustration: From Nussbaum in Miall and Denny's, _Cockroach_,
Lovell, Reeve & Co.
FIG. 15.--Diagram showing Formation of Germinal Layers. E, ectoderm;
M, inner layer. Magnified.]
_Germinal Layers and Food-Canal._--The embryo begins to develop as an
elongate, thickened, ventral region of the blastoderm which is known
as the ventral plate or germ band. Along this band a median furrow
appears, and a mass of cells sinks within, the one-layered germ band
thus becoming transformed into a band of two cell-layers (fig. 15). In
some cases the inner layer is formed not by invagination but by
proliferation or by delamination. The outer of these two layers (fig.
15, E) is the ectoderm. With regard to the inner layer (_endoblast_ of
some authors, fig. 15, M) much difference of opinion has prevailed. It
has usually been regarded as representing both endoderm and mesoderm,
and the groove which usually leads to its formation has been compared
to the abnormally elongated blastopore of a typical gastrula. No doubt
can be entertained that the greater part of the inner layer
corresponds to the mesoderm of more ordinary embryos, for the coelomic
pouches, the germ-cells, the musculature and the vascular system all
arise from it. Further, there is general agreement that the
chitin-lined fore-gut and hind-gut, which form the greater part of
the digestive tract, arise from ectodermal invaginations (stomodaeum
and proctodaeum respectively) at the positions of the future mouth and
anus. The origin of the mid-gut (mesenteron), that has no chitinous
lining in the developed insect, is the disputed point. According to
the classical researches of A. Kowalevsky (1871 and 1887) on the
embryology of the water-beetle _Hydrophilus_ and of the muscid flies,
an anterior and a posterior endoderm-rudiment both derived from the
"endoblast" become apparent at an early stage, in close association
with the stomodaeum and the proctodaeum respectively. These two
endoderm-rudiments ultimately grow together and give rise to the
epithelium of the mid-gut. These results were confirmed by the
observations of K. Heider and W. M. Wheeler (1889) on the embryos of
two beetles--_Hydrophilus_ and _Doryphora_ respectively. V. Graber,
however (1889), stated that in the _Muscidae_, while the anterior
endoderm-rudiment arises as Kowalevsky had observed, the posterior
part of the "mid-gut" has its origin as a direct outgrowth from the
proctodaeum. The recent researches of R. Heymons (1895) on the
Orthoptera, and of A. Lecaillon (1898) on various leaf beetles, tend
to show that the whole of the "mid-gut" arises from the proliferation
of cells at the extremity of the stomodaeum and of the proctodaeum. On
this view the entire food-canal in most Hexapoda must be regarded as
of ectodermal origin, the "endoblast" represents mesoderm only, and
the median furrow whence it arises can be no longer compared with the
blastopore. According to Heymons, the yolk-cells must be regarded as
the true endoderm in the hexapod embryo, for he states (1897) that in
the bristle-tail _Lepisma_ and in dragon-flies they give rise to the
mid-gut. These views are not, however, supported by other recent
observers. J. Carriere's researches (1897) on the embryology of the
mason bee (_Chalicodoma_) agree entirely with the interpretations of
Kowalevsky and Heider, and so on the whole do those of F. Schwangart,
who has studied (1904) the embryonic development of Lepidoptera. He
finds that the endoderm arises from an anterior and a posterior
rudiment derived from the "endoblast," that many of the cells of these
rudiments wander into the yolk, and that the mesenteric epithelium
becomes reinforced by cells that migrate from the yolk. K. Escherich
(1901), after a new research on the embryology of the muscid Diptera,
claims that the fore and hind endodermal rudiments arise from the
blastoderm by invagination, and are from their origin distinct from
the mesoderm. On the whole it seems likely that the endoderm is
represented in part by the yolk, and in part by those anterior and
posterior rudiments which usually form the mesenteron, but that in
some Hexapoda the whole digestive tract may be ectodermal. It must be
admitted that some or the later work on insect embryology has
justified the growing scepticism in the universal applicability of the
"germ-layer theory." Heider has suggested, however, that the apparent
origin of the mid-gut from the stomodaeum and proctodaeum may be
explained by the presence of a "latent endoderm-group" in those
invaginations.
[Illustration: From Nussbaum in Miall and Denny, _The Cockroach_,
Lovell Reeve & Co.
FIG. 16.--Cross section of Embryo of German Cockroach
(_Phyllodromia_). S, serosa; A, amnion; E, ectoderm; N, rudiment of
nerve-cord; M, mesodermal pouches.]
_Embryonic Membranes._--A remarkable feature in the embryonic
development of most Hexapoda is the formation of a protective membrane
analogous to the amnion of higher Vertebrates and known by the same
term. Usually there arises around the edge of the germ band a double
fold in the undifferentiated blastoderm, which grows over the surface
of the embryo, so that its inner and outer layers become continuous,
forming respectively the _amnion_ and the _serosa_ (fig. 16, A, S).
The embryo of a moth, a dragon-fly or a bug is invaginated into the
yolk at the head end, the portion of the blastoderm necessarily pushed
in with it forming the amnion. The embryo thus becomes transferred to
the dorsal face of the egg, but at a later stage it undergoes
reversion to its original ventral position. In some parasitic
Hymenoptera there is only a single embryonic membrane formed by
delamination from the blastoderm, while in a few insects, including
the wingless spring-tails, the embryonic membranes are vestigial or
entirely wanting. In the bristle-tails _Lepisma_ and _Machilis_, an
interesting transitional condition of the embryonic membranes has
lately been shown by Heymons. The embryo is invaginated into the yolk,
but the surface edges of the blastoderm do not close over, so that a
groove or pore puts the insunken space that represents the amniotic
cavity into communication with the outside. Heymons believes that the
"dorsal organ" in the embryos of the lower Arthropoda corresponds with
the region invaginated to form the serosa of the hexapod embryo.
Wheeler, however, compares with the "dorsal organ" the peculiar extra
embryonic membrane or indusium which he has observed between serosa
and amnion in the embryo of the grasshopper _Xiphidium_.
_Metameric Segmentation._--The segments are perceptible at a very
early stage of the development as a number of transverse bands
arranged in a linear sequence. The first segmentation of the ventral
plate is not, however, very definite, and the segmentation does not
make its appearance simultaneously throughout the whole length of the
plate; the anterior parts are segmented before the posterior. In
Orthoptera and Thysanura, as well as some others of the lower insects,
twenty-one of these divisions--not, however, all similar--may be
readily distinguished, six of which subsequently enter into the
formation of the head, three going to the thorax and twelve to the
abdomen. In Hemiptera only eleven and in Collembola only six abdominal
segments have been detected. The first and last of these twenty-one
divisions are so different from the others that they can scarcely be
considered true segments.
_Head Segments._--In the adult insect the head is insignificant in
size compared with the thorax or abdomen, but in the embryo it forms a
much larger portion of the body than it does in the adult. Its
composition has been the subject of prolonged difference of opinion.
Formerly it was said that the head consisted of four divisions, viz.
three segments and the procephalic or prae-oral lobes. It is now
ascertained that the procephalic lobes consist of three divisions, so
that the head must certainly be formed from at least six segments. The
first of these, according to the nomenclature of Heymons (see fig.
17), is the mouth or oral piece; the second, the antennal segment; the
third, the intercalary or prae-mandibular segment; while the fourth,
fifth, and sixth are respectively the segments of the mandibles and of
the first and second maxillae. These six divisions of the head are
diverse in kind, and subsequently undergo so much change that the part
each of them takes in the formation of the head-capsule is not finally
determined. The labrum and clypeus are developed as a single
prolongation of the oral piece, not as a pair of appendages. The
antennal segment apparently entirely disappears, with the exception of
a pair of appendages it bears; these become the antennae; it is
possible that the original segment, or some part of it, may even
become a portion of the actual antennae. The intercalary segment has
no appendages, nor rudiments thereof, except, according to H. Uzel
(1897), in the thysanuran _Campodea_, and probably entirely
disappears, though J. H. Comstock and C. Kochi believe that the labrum
belongs to it. The appendages of the posterior three or trophal
segments become the parts of the mouth. The appendages of the two
maxillary segments arise as treble instead of single projections, thus
differing from other appendages. From these facts it appears that the
anterior three divisions of the head differ strongly from the
posterior three, which greatly resemble thoracic segments; hence it
has been thought possible that the anterior divisions may represent a
primitive head, to which three segments and their leg-like appendages
were subsequently added to form the head as it now exists. This is,
however, very doubtful, and an entirely different inference is
possible. Besides the five limb-bearing somites just enumerated, two
others must now be recognized in the head. One of these is the ocular
segment, in front of the antennal, and behind the primitive pre-oral
segment. The other is the segment of the maxillulae (see above, under
_Jaws_), behind the mandibular somite; the presence of this in the
embryo of the collembolan _Anurida_ has been lately shown (1900) by J.
W. Folsom (fig. 18, v. 5), who terms the maxillulae "superlinguae" on
account of their close association with the hypopharynx or lingua. In
reference to the structure of the head-capsule in the imago, it
appears that the clypeus and labrum represent, as already said, an
unpaired median outgrowth of the oral piece. According to W. A. Riley
(1904) the epicranium or "vertex," the compound eyes and the front
divisions of the genae are formed by the cephalic lobes of the embryo
(belonging to the ocular segment), while the mandibular and maxillary
segments form the hinder parts of the genae and the hypopharynx.
[Illustration: After Heymons.
FIG. 17.--Morphology of an Insect: the embryo of _Gryllotalpa_,
somewhat diagrammatic. The longitudinal segmented band along the
middle line represents the early segmentation of the nervous system
and the subsequent median field of each sternite; the lateral
transverse unshaded bands are the lateral fields of each segment; the
shaded areas indicate the more internally placed mesoderm layer. The
segments are numbered 1-21; 1-6 will form the head, 7-9 the thorax,
10-21 the abdomen. A, anus; Abx1 Abx11, appendage of 1st and of 11th
abdominal segments; Ans, anal piece = telson or 12th abdominal
segment; Ant, antenna; De, deuterencephalon; Md, mandible; Mx1, first
maxilla; Mx2, second maxilla or labium; O, mouth; Obcl, rudimentary
labrum and clypeus; Pre, protencephalon; St1 St10, stigmata 1 and 10;
Terg, tergite; Thx1, appendage of first thoracic segment; Tre,
tritencephalon; Ul, a thickening at hinder margin of the mouth.]
Great difference of opinion exists as to the hypopharynx, which has
even been thought to represent a distinct segment, or the pair of
appendages of a distinct segment. Heymons considers that it represents
the sternites of the three trophal segments, and that the gula is
merely a secondary development. Folsom looks on the hypopharynx as a
secondary development. Riley holds that the hypopharynx belongs to the
mandibular and maxillary segments, while the cervical sclerites or
gula represent the sternum of the labial segment. The ganglia of the
nervous system offer some important evidence as to the morphology of
the head, and are alluded to below.
_Thoracic Segments._--These are always three in number. The three
pairs of legs appear very early as rudiments. Though the thoracic
segments bear the wings, no trace of these appendages exists till the
close of the embryonic life, nor even, in many cases, till much later.
The thoracic segments, as seen in an early stage of the ventral plate,
display in a well-marked manner the essential elements of the insect
segment. These elements are a central piece or sternite, and a lateral
field on each side bearing the leg-rudiment. The external part of the
lateral field subsequently grows up, and by coalescence with its
fellow forms the tergite or dorsal part of the segment.
_Abdominal Segments and Appendages._--We have already seen that in
numerous lower insects the abdomen is formed from twelve divisions
placed in linear fashion. Eleven of these may perhaps be considered as
true segments, but the twelfth or terminal one is different, and is
called by Heymons a telson; in it is placed the anal orifice, and the
mass subsequently becomes the upper and lower laminae anales. In
Hemiptera this telson is absent, and the anal orifice is placed quite
at the termination of the eleventh segment. Moreover, in this order
the abdomen shows at first a division into only nine segments and a
terminal mass, which last subsequently becomes divided into two. The
appendages of the abdomen are called cerci, stylets and gonapophyses.
They differ much according to the kind of insect, and in the adult
according to sex. Difference of opinion as to the nature of the
abdominal appendages prevails. The cerci, when present, appear in the
mature insect to be attached to the tenth segment, but according to
Heymons they are really appendages of the eleventh segment, their
connexion with the tenth being secondary and the result of
considerable changes that take place in the terminal segments. It has
been disputed whether any true cerci exist in the higher insects, but
they are probably represented in the Diptera and in the scorpion-flies
(Mecaptera). In those insects in which a median terminal appendage
exists between the two cerci this is considered to be a prolongation
of the eleventh tergite. The stylets, when present, are placed on the
ninth segment, and in some Thysanura exist also on the eighth segment;
their development takes place later in life than that of the cerci.
The gonapophyses are the projections near the extremity of the body
that surround the sexual orifices, and vary extremely according to the
kind of insect. They have chiefly been studied in the female, and form
the sting and ovipositor, organs peculiar to this sex. They are
developed on the ventral surface of the body and are six in number,
one pair arising from the eighth ventral plate and two pairs from the
ninth. This has been found to be the case in insects so widely
different as Orthoptera and Aculeate Hymenoptera. The genital armature
of the male is formed to a considerable extent by modifications of the
segments themselves. The development of the armature has been little
studied, and the question whether there may be present gonapophyses
homologous with those of the female is open.
[Illustration: A. After Wheeler, _Journ. Morph._ vol. viii., and
Folsom, _Bull. Mus. Harvard_, xxxvi.
B. After Folsom.
FIG. 18.--Embryos of Springtail (_Anuridamaritima_). Magnified. A,
Head-region of germ band. B, Section through head and thorax. The
neuromeres are shown in Arabic, the appendages in Roman numerals.
1, Ocular segment.
2, Antennal.
3, Trito-cerebral.
4, Mandibular.
5, Maxillular.
6, Maxillary.
7, Labial.
8, Prothoracic.
9, Mesothoracic.
10, Metathoracic.]
In the adult state no insect possesses more than six legs, and they
are always attached to the thorax; in many Thysanura there are,
however, processes on the abdomen that, as to their position, are
similar to legs. In the embryos of many insects there are projections
from the segments of the abdomen similar, to a considerable extent, to
the rudimentary thoracic legs. The question whether these projections
can be considered an indication of former polypody in insects has been
raised. They do not long persist in the embryo, but disappear, and the
area each one occupied becomes part of the sternite. In some embryos
there is but a single pair of these rudiments (or vestiges) situate on
the first abdominal segment, and in some cases they become
invaginations of a glandular nature. Whether cerci, stylets and
gonapophyses are developed from these rudiments has been much debated.
It appears that it is possible to accept cerci and stylets as
modifications of the temporary pseudopods, but it is more difficult to
believe that this is the case with the gonapophyses, for they
apparently commence their development considerably later than cerci
and stylets and only after the apparently complete disappearance of
the embryonic pseudopods. The fact that there are two pairs of
gonapophyses on the ninth abdominal segment would be fatal to the view
that they are in any way homologous with legs, were it not that there
is some evidence that the division into two pairs is secondary and
incomplete. But another and apparently insuperable objection may be
raised--that the appendages of the ninth segment are the stylets, and
that the gonapophyses cannot therefore be appendicular. The pseudopods
that exist on the abdomen of numerous caterpillars may possibly arise
from the embryonic pseudopods, but this also is far from being
established.
_Nervous System._--The nervous system is ectodermal in origin, and is
developed and segmented to a large extent in connexion with the outer
part of the body, so that it affords important evidence as to the
segmentation thereof. The continuous layer of cells from which the
nervous system is developed undergoes a segmentation analogous with
that we have described as occurring in the ventral plate; there is
thus formed a pair of contiguous ganglia for each segment of the body,
but there is no ganglion for the telson. The ganglia become greatly
changed in position during the later life, and it is usually said that
there are only ten pairs of abdominal ganglia even in the embryo. In
Orthoptera, Heymons has demonstrated the existence of eleven pairs,
the terminal pair becoming, however, soon united with the tenth. The
nervous system of the embryonic head exhibits three ganglionic masses,
anterior to the thoracic ganglionic masses; these three masses
subsequently amalgamate and form the sub-oesophageal ganglion, which
supplies the trophal segments. In front of the three masses that will
form the sub-oesophageal ganglion the mass of cells that is to form
the nervous system is very large, and projects on each side; this
anterior or "brain" mass consists of three lobes (the prot-, deut-,
and tritencephalon of Viallanes and others), each of which might be
thought to represent a segmental ganglion. But the protocerebrum
contains the ganglia of the ocular segment in addition to those of the
procephalic lobes. These three divisions subsequently form the
supra-oesophageal ganglion or brain proper. There are other ganglia in
addition to those of the ventral chain, and Janet supposes that the
ganglia of the sympathetic system indicate the existence of three
anterior head-segments; the remains of the segments themselves are, in
accordance with this view, to be sought in the stomodaeum. Folsom has
detected in the embryo of _Anurida_ a pair of ganglia (fig. 18, 5)
belonging to the maxillular (or superlingual) segment, thus
establishing seven sets of cephalic ganglia, and supporting his view
as to the composition of the head.
_Air-tubes._--The air-tubes, like the food-canal, are formed by
invaginations of the ectoderm, which arise close to the developing
appendages, the rudimentary spiracles appearing soon after the budding
limbs. The pits leading from these lengthen into tubes, and undergo
repeated branching as development proceeds.
_Dorsal Closure._--The germ band evidently marks the ventral aspect of
the developing insect, whose body must be completed by the extension
of the embryo so as to enclose the yolk dorsally. The method of this
dorsal closure varies in different insects. In the Colorado beetle
(_Doryphora_), whose development has been studied by W. M. Wheeler,
the amnion is ruptured and turned back from covering the germ band,
enclosing the yolk dorsally and becoming finally absorbed, as the
ectoderm of the germ band itself spreads to form the dorsal wall. In
some midges and in caddis-flies the serosa becomes ruptured and
absorbed, while the germ band, still clothed with the amnion, grows
around the yolk. In moths and certain saw-flies there is no rupture of
the membranes; the Russian zoologists Tichomirov and Kovalevsky have
described the growth of both amnion and embryonic ectoderm around the
yolk, the embryo being thus completely enclosed until hatching time by
both amnion and serosa. V. Graber has described a similar method of
dorsal closure in the saw-fly _Hylotoma_.
[Illustration: After Heymons, _Zeit. Wiss. Zoolog._ vol. 53.
FIG. 19.--Cross sections through Abdomen of German Cockroach Embryo. A
(later than fig. 16) magnified. B (still more advanced, dorsal closure
complete) magnified.
ec, Ectoderm.
en, Endoderm.
sp, Splanchnic layer of mesoderm.
y, Yolk.
h, Heart.
p, Pericardial septum.
c, Coelom.
g, Germ-cells surrounded by rudiment-cells of ovarian tubes.
m, Muscle-rudiment.
n, Nerve-chain.
f, Fat body.
s, Inpushing of ectoderm to form air-tubes.
x, Secondary body-cavity.]
_Mesoderm, Coelom and Blood-System._--From the mesoderm most of the
organs of the body--muscular, circulatory, reproductive--take their
origin. The mass of cells undergoes segmentation corresponding with
the outer segmentation of the embryo, and a pair of cavities--the
coelomic pouches (fig. 16, M)--are formed in each segment. Each
coelomic pouch--as traced by Heymons in his study on the development
of the cockroach (_Phyllodromia_)--divides into three parts, of which
the most dorsal contains the primitive germ-cells, the median
disappears, and the ventral loses its boundaries as it becomes filled
up with the growing fat body (fig. 19). This latter, as well as the
heart and the walls of the blood spaces, arises by the modification of
mesodermal cells, and the body cavity is formed by the enlargement and
coalescence of the blood channels and by the splitting of the fat
body. It is therefore a haemocoel, the coelom of the developed insect
being represented only by the cavities of the genital glands and their
ducts.
_Reproductive Organs._--In the cockroach embryo, before the
segmentation of the germ-band has begun, the primitive germ-cells can
be recognized at the hinder end of the mesoderm, from whose ordinary
cells they can be distinguished by their larger size. At a later stage
further germ-cells arise from the epithelium of the coelomic pouches
from the second to the seventh abdominal segments, and become
surrounded by other mesoderm cells which form the ovarian or
testicular tubes and ducts (fig. 19, g). In the male of _Phyllodromia_
the rudiment of a vestigial ovary becomes separated from the
developing testis, indicating perhaps an originally hermaphrodite
condition. An exceedingly early differentiation of the primitive
germ-cells occurs in certain Diptera. E. Metchnikoff observed (1866)
in the development of the parthenogenetic eggs produced by the
precocious larva of the gall-midge _Cecidomyia_ that a large
"polar-cell" appeared at one extremity during the primitive
cell-segmentation. This by successive divisions forms a group of four
to eight cells, which subsequently pass through the blastoderm, and
dividing into two groups become symmetrically arranged and surrounded
by the rudiments of the ovarian tubes. E. G. Balbiani and R. Ritter
(1890) have since observed a similar early origin for the germ-cells
in the midge _Chironomus_ and in the _Aphidae_.
The paired oviducts and vasa deferentia are, as we have seen,
mesodermal in origin. The median vagina, spermatheca and ejaculatory
duct are, on the other hand, formed by ectodermal inpushings. The
classical researches of J. A. Palmen (1884) on these ducts have shown
that in may-flies and in female earwigs the paired mesodermal ducts
open directly to the exterior, while in male earwigs there is a single
mesodermal duct, due either to the coalescence of the two or to the
suppression of one. In the absence of the external ectodermal ducts
usual in winged insects, these two groups resemble therefore the
primitive Aptera. The presence of rudiments of the genital ducts of
both sexes in the embryo of either sex is interesting and suggestive.
The ejaculatory duct which opens on the ninth abdominal sternum in the
adult male arises in the tenth abdominal embryonic segment and
subsequently moves forward.