_a._ The egg
[Illustration:
FIG. 482.—Female Dyticus, laying eggs: _A_, ovipositor extended. _B_,
egg of Notonecta, attached to stem of rush. _C_, egg of Dyticus,
laid in excavation in rush.—After Régimbart, from Miall.
]
Insects as a rule arise from eggs which are laid in a great variety of
situations, those species which are viviparous being exceedingly few in
number compared with the class as a whole. It is noteworthy that Leydig
has found in the same Aphis, and even in the same ovary, an egg-tube
producing eggs, while a neighboring tube was producing viviparous
individuals.[77] The viviparous species are confined to certain
May-flies, the Aphidæ, Diptera (Sarcophaga, Tachinidæ, Œstridæ, and
Pupipara), and to certain Coleoptera (Stylopidæ and some Staphylinidæ).
The number of eggs laid varies from a very few, as in the Collembola and
in the Psocidæ, or 15 or even less in certain fossorial wasps, and from
20 to 35 in some locusts to many thousands in the social insects, the
honey-bee laying by estimate over 1,000,000 eggs in the course of her
life. Dr. Sharp thinks that from 50 to 100 may perhaps be taken as an
average number for one female to produce. The eggs of insects with a
complete metamorphosis are said by Brauer to be smaller in proportion to
the parent than those laid by ametabolous or heterometabolous insects.
In this respect the insects are paralleled by the birds, the highest
forms laying smaller eggs than the water birds, ostrich, Apteryx, etc.
[Illustration:
FIG. 483.—Eggs (_e_) of Hydrobius (?) and their capsules, from which
the larva, Fig. 452, hatched.—Emerton _del._
]
The egg, or ovum, when laid is not always ripe or perfect, but, as in
those of ants, continues to grow after oviposition. Others are laid some
time after the embryo has begun to form; and in the flesh-flies the
larva hatches before the egg is deposited.
[Illustration:
FIG. 484.—Egg-masses of Chironomus: _A_, string of eggs of _C.
dorsalis_, divided into sections to show both sides. _B_, twisted
fibres which traverse the string of eggs. _C_, egg-mass of
Chironomus (_sp_). _D_, egg-mass of a third species. _E_, part of
_D_, more highly magnified. _F_, developing eggs, two stages.—After
Miall.
]
Insects as a rule instinctively lay their eggs near or upon objects
destined to be the food of the larva; those of caterpillars on leaves,
those of many flies on meat or carrion, those of Copris and other
dung-beetles in dung, those of aquatic insects in water, while many
oviposit in the earth or in plants (Fig. 482), or in the bodies of
animals destined to be the hosts of the parasitic larvæ. As the eggs are
preyed upon by mites and other animals, the contrivances and
modifications of the mode of egg-laying, and the situations in which
they are placed, are almost endless. Many insects lay their eggs in a
mass, covered with a gummy substance; or those laid in the water, as the
eggs of dragon-flies, caddis-flies, Chironomus (Fig. 484), etc., are
enveloped by a jelly-like mass.
[Illustration:
FIG. 485.—Egg-capsule of _Periplaneta americana_: _a_, side; _b_, end
view; _c_, natural size.—After Howard and Marlatt, Bull. 4, Div.
Ent. U. S. Dept. Agr.
]
The _oötheca_ of the cockroach (Fig. 485) is a solid, dense case, which,
after being carried about by the mother, can be left without harm in the
crevices of the floors of houses. The oötheca of Mantis (Fig. 486) is
formed by a large mass of frothy matter, which hardens and is attached
to stems of plants.
[Illustration:
FIG. 486.—Egg-capsules of _Mantis carolina_.—After Riley.
]
On the other hand, the female “walking-stick” (_Diapheromera
femoratum_) drops her eggs, says Riley, loosely upon the ground,
from whatever height she may happen to be, and “one hears a constant
pattering, not unlike drops of rain, that results from the abundant
dropping of these eggs, which, in places, lay so thick among and
under the dead leaves that they may be scraped up in great
quantities.” (Report for 1879.)
The eggs of the lace-winged flies are supported on pedicels, above the
reach of ovivorous mites.
The female Chrysopa usually lays between 40 and 50 eggs. In one
case, we observed that 18 egg-stalks were deposited, but there were
only nine well-formed eggs in the batch, and nine eggless stalks,
some only half the usual height, others with the knob of cement at
the end to which the egg is ordinarily fastened. The eggs are
evidently stuck on to the end of the pedicel after the latter has
been formed, as, in one instance, an egg was glued to the stalk very
much out of centre, the insect’s abdomen not having been aimed
straight, so to speak, at the mass of cement.
[Illustration:
FIG. 487.—Eggs of Chrysopa, with larva and fly.
]
The eggs of Rhodites are fixed to a long stalk thickened at the end;
those of Inquilines and certain Chalcids (_Leucospis gigas_, Fig.
489, _A_) are also stalked; and the use of this stalk in the eggs of
Cynips (_E_) is thought by Adler to be respiratory, while, also, he
states that the egg-cavity communicates with the egg-stalk, so that
a part of the egg-contents can pass into the latter, and this
happens at the laying of each egg. The egg of certain ichneumons
(Paniscus, Fig. 488) ends in a short stalk, which is inserted in the
skin of the caterpillar destined to serve as the host of the
parasite, the eggs, as stated by De Geer, being retained more firmly
in the integument by the stalk so swelling as to form two knobs
(Fig. 498, _c_).
[Illustration:
FIG. 488.—Young larva of Paniscus in position of feeding on the skin
of a caterpillar: _a_, the egg-shell.—After Newport, from Sharp.
]
Certain Homoptera also have stalked eggs, as those of _Psylla
pyricola_ (Fig. 489, _B_), those of _Aleyrodes citri_ (_C_, _a_,
_b_), and of an allied form, _Aleurodicus cocois_ (_D_), and those
of Corixa (Fig. 493).
[Illustration:
FIG. 489.—Stalked eggs: _A_, of a Chalcid (after Fabre); _B_, of
Psylla (after Slingerland); _C_, of Aleyrodes; _D_, of Aleurodicus
(after Riley and Howard); _E_, of _Dryophanta scutellaris_ (after
Adler).
]
[Illustration:
FIG. 490.—Eggs of ox bot-fly, enlarged.—After Riley.
]
Reference should also be made to the eggs of lice, which are oval
and attached to the hairs of their host. Those of the ox bot-fly
(_Hypoderma lineata_) are usually placed four to six together, and
fastened to a hair. The lower portion of the egg is admirably
adapted for clasping a hair. “It consists of two lobes, forming a
bulbous enlargement, which is attached to the egg by a broad, but
rather thin, neck, so that, when the latter is viewed sidewise, it
appears as a slender pedicel” (Fig. 490, _a-d_). (Riley in Insect
Life, iv, p. 307.) The egg of another fly (_Drosophila ampelophila_,
Fig. 491) bears a pair of long, slender appendages near the anterior
end. “The egg is inserted into the soft pulp of the decaying fruit;
these appendages leave the ovipositor last, and are spread out upon
the surface of the mass. They, in this way, serve to keep the egg in
place, and thus insure the emergence of the larva into the open air
instead of into the more or less fluid mass in which the egg is
situated. The larva issues from the egg just above the base of these
appendages.” (Comstock.)
[Illustration:
FIG. 491.—Egg of Drosophila.—After Comstock.
]
=Mode of deposition.=—The exact process of oviposition has been rarely
observed, or at least not observed in detail, and further observations
are much needed. In the cockroach (Phyllodromia), Wheeler has seen the
eggs pass out of the oviduct and become arranged in the oötheca, in a
way similar to that in the account published by Kadyi on Periplaneta.
[Illustration:
FIG. 492.—Rocky Mountain locust (_aa_) depositing its eggs (_c_); _d_,
the earth partially removed, showing (_e_) an egg-mass already in
place, and (_d_) one being placed; _f_ shows where such a mass has
been covered over. _A_, oviposition; _j_, position of oviduct; _g_,
egg-guide; _e_, egg. _B_, egg-mass of the same; _a_, from side, _b_,
from beneath, _c_, from above.—After Riley.
]
“When about to form the capsule, the female Blatta closes the
genital armature, and the two folds of the white membrane which
lines the oöthecal cavity close vertically in the middle line. Then
some of the contents of the colleterial glands are poured into the
chamber, and bathe the inner surface of the posterior wall. The
first egg glides down the vagina from the left ovary, describes an
arc, still keeping its germarium-pole uppermost, after having
pressed the micropylar area against the mouth of the spermatheca,
passes to the right side of the back of the chamber, and is placed
perpendicularly two-thirds to the right of the longitudinal axis of
the insect’s body. The next egg comes from the right ovary,
describes an arc to the opposite side of the body, decussating with
the path of the first egg, and is placed completely on the left side
of the median line. The third egg comes from the left ovary, and is
made to lie completely on the right side of the median line; and so
the process continues, the ovaries discharging the eggs alternately,
and each egg describing an arc to the opposite side of the capsule.
The oöthecal chamber soon becomes too small to contain all the
constantly accumulating eggs, so the anal armature opens and allows
the end of the capsule to project. A raised line, the impression of
the edges of the white membrane, runs down the end of the capsule.
The last egg deposited comes from the right ovary, and lies
two-thirds on the left, and one-third to the right, of the median
line. As soon as the egg is laid, a further discharge from the
colleterial glands spreads over the vaginal or anterior wall of the
cavity, and becomes evenly continuous with the secretion which has
before been spread over the back and the sides of the capsule by the
white membrane.
“The crista, a cord-like ridge running the full length of the dorsal
surface of the capsule, is a thick-walled tube, either half of which
is formed by the edge of the side walls of the capsule split into
two laminæ. The rhythmical clasping of the three pairs of palpi
which guard the vaginal opening is registered in an exquisite
pattern on the inner face of either half of the crista.”[78]
The mode of oviposition in the locust has been fully described by Riley,
who states that the eggs pass down and out of the oviduct, and “guided
by a little finger-like style” (Fig. 298), they pass in between the
horny valves of the ovipositor, and issue at their tips amid the mucous
fluid which forms the egg-capsule (Fig. 492).
=Vitality of eggs.=—It is well known that the eggs of phyllopod and
other fresh-water Crustacea have wonderful vitality, withstanding
extreme dryness for several years, at least from two to ten. Such cases
are unknown among insects. It has been observed, however, by T. W.
Brigham, and also by L. Trouvelot, that the eggs of the walking-stick
(_Diapheromera femorata_) for the most part hatch only after the
interval of two years.[79]
The eggs of Bittacus are said by Brauer to lie over unhatched for two
years; indeed, the first condition of their hatching is a complete
drying of the earth in which the eggs lie, the second is a succeeding
thorough wetting of the ground in spring.
_Appearance and structure of the ripe egg._—The eggs of insects are on
the whole rather large in proportion to the size of the parent,
especially so in many minute forms, as the fleas, lice, etc.
Their general shape is spherical or oval, often cylindrical; where the
eggs are long and cylindrical a dorsal and ventral side can be
distinguished (Fig. 502). They are in the Tortricidæ and Limacodid moths
flattened, thin, and scale-like. In the eggs of locusts and
grasshoppers, as well as certain Diptera, the ventral side of the embryo
corresponds to the convex side, and the concave side of the egg to the
dorsal region of the embryo (Figs. 502 and 493).
There is an anterior and posterior end or pole, the anterior end being
that which in the body of the parent lies towards her head, or towards
the upper or distal end of the ovarian tube. Towards this end lies in
the later stages of embryonic life the head-end of the embryo, while the
posterior end of the embryo is turned towards the hinder pole of the egg
(Figs. 493 and 520).
=The egg-shell and yolk-membrane.=—The ripe egg is protected by two
membranes: 1, an inner or _vitelline membrane_ or _oölemma_ (_dh_) (Fig.
500, _d_), produced in the egg by a hardening of the outer layer, and 2,
the outer or _chorion_ (_c_), which is secreted by the cells of the
ovarian follicle. The latter is divided into two layers: an inner, the
_endochorion_, and an outer, the _exochorion_.
[Illustration:
FIG. 503.—Fertilization of the egg of a round-worm (_Ascaris
megalocephala_): _A_, the ends (centrosomes) of the spindle formed.
_B_, the spindle completed; _sp_, sperm-nucleus, with its
chromosomes; _ei_, egg-nucleus; _p_, polar bodies.—After Boveri,
from Field’s Hertwig.
]
[Illustration:
FIG. 493.—Eggs of Corixa: _A_, early stage before formation of the
embryo, from one side. _B_, the same viewed in the plane of
symmetry. _C_, the embryo in its final position; _a_, anterior, _p_,
posterior, end; _l_, left, _r_, right, _v_, ventral, _d_, dorsal,
aspect. (The letters refer to the _final_ position of the embryo,
which is nearly diametrically opposite to that in which it first
develops); _m_, micropyle; _p_, pedicle.—After Metschnikoff, from
Wilson.
]
[Illustration:
FIG. 494.—Eggs of Phasmidæ: _A_, _Lonchodes duivenbodi_. _B_,
_Platycrania edulis_. _C_, _Haplopus grayi_. _D_, _Phyllium
siccifolium_.—After Kaup, from Sharp.
]
While the yolk-membrane is usually a completely homogeneous, thin,
structureless membrane, the chorion or shell of the egg is usually
covered with a network of ridges enclosing polygonal areas, varying in
shape according to the species or genus. These external markings are due
to the impress of the cellular structure of the epithelium of the
ovarian follicle.
In the chorion of the cockroach the surface appears to be finely
granular, the finest granules being arranged in large, more or less
regularly hexagonal areas, which are bounded by narrow, dark spaces,
containing somewhat larger though less dense granules. The surface of
the eggs of certain Phasmids are variously sculptured (Fig. 494).
The true structure of the chorion can only be, as Wheeler observes,
seen in cross-sections, as shown by Blochmann, and also by Wheeler.
The chorion consists of two chitinous laminæ kept in close
apposition by means of numerous minute trabeculæ or pillars. It is
the ends of these pillars that look like granules. In the spaces
between the hexagonal areas, the trabeculæ are more scattered and
individually thicker than those of the hexagons.
[Illustration:
FIG. 495.—Egg of cotton-worm moth, Aletia: _a_, top view, showing the
micropyle.—After Comstock.
]
[Illustration:
FIG. 496.—Egg of _Danais archippus_.—After Riley.
]
These markings are of singular beauty and complexity in the eggs of many
Lepidoptera, whose ova are variously ribbed, forming a beautiful
fretwork of raised lines (Figs. 495 and 496), while in the Diptera and
Hymenoptera the chorion is less solid, and usually smooth under low
powers. The exochorion of the egg of the house and meat fly (_C.
vomitoria_) is pitted with elongated hexagonal depressions, which cross
the egg transversely. That of the honey-bee is also divided into long
hexagonal areas (Fig. 497).
[Illustration:
FIG. 497.—Egg with embryo of honey-bee, × 40: _ch_, chorion; _ga_,
ganglia; _s. ga_, brain; _jm_, jaw-muscles forming; _c_, œsophageal
collar; _fb_, fore intestine; _mb_, mid-intestine; _ab_,
hind-intestine.—After Cheshire.
]
[Illustration:
FIG. 498.—Micropyle (_Mk_) of eggs; _a_, of a fly, Antomyia; _b_,
_Drosophila cellaris_; _c_, stalked egg of _Paniscus
testaceus_.—After Leuckart, from Perrier.
]
When the eggs are deposited in exposed places, and remain in such
situations for several days, or weeks, or even through the winter, the
shell is either solid and strengthened by the ribs and ridges; or the
shell, if of winter eggs, is unornamented, and is dense and solid, to
resist extremes in temperature or the attacks of egg-eating birds,
mites, etc.
=The micropyle.=—This is an opening or canal, or, as in most insects, a
group of canals situated at the anterior end of the egg for the entrance
of the spermatozoa during the process of fertilization of the ovum (Fig.
498). In Acrydians, however, the micropyle is situated at the posterior
end of the egg. The micropyle (Fig. 499) is a complicated apparatus
within whose circumference the vitelline membrane appears to be firmly
attached to the chorion, so that the perforation passes through the
chorion as well as the yolk-membrane.
The micropyles of the cockroach are probably as simple and generalized
as in any insect. Wheeler states that they are in Phyllodromia scattered
over the end of the egg, “over a quadrant of the upper hemisphere, where
the beautiful hexagonal pattern of the chorion gives away to an even
trabeculation.” The micropyles are wide-mouthed, very oblique,
funnel-shaped canals, perforating the chorion, the apertures of the
funnels appearing under a low power as clear, oval spots, the long axis
of which is parallel to the long axis of the egg.
[Illustration:
FIG. 499.—_a_, fragment of a micropylar papilla, showing its lumen;
_b_, optical section of another papilla, in this one the lumen
extends to the vitelline membrane, but does not pass beyond it; _c_,
_d_, _e_, and _f_, papillæ of different forms. _A_, anterior end of
an ovarian egg, showing mode of growth of the micropylar papillæ:
_a_, _b_, two successive stages; _c_, surface view of modified
papillæ from the lower edges of the cap; _d_, tunica propria of the
ovariole; _e_, remnant of the cell-mass that secreted (?) the
micropylar cap.—After Ayers.
]
[Illustration:
FIG. 500.—Egg of _Perla maxima_: _c_, chorion; _d_, oölemma; _gs_,
glass-like covering of micropyle; _l_, cavity under same; _g_,
canals penetrating chorion.—After Imhof, from Sharp.
]
“With a higher power the tube of each funnel is clearly visible as a
thin canal which dilates rapidly into the large oval aperture on the
outer face of the chorion. The narrow tube is sometimes fully as
long as the large orifice. The micropylar perforations are all
directed from the germarium to the vaginal pole of the egg. Hence a
line, the hypothetical path of the spermatozoön, drawn through one
of these oblique micropyles, and continued into the egg, would
strike the equatorial plane. The female pronucleus, as we shall see
further on, moves in this plane.” (Wheeler, p. 289.)
[Illustration:
FIG. 501.—Micropyles: _a_, of _Nepa cinerea_; _b_, of _Locusta
viridissima_; _c_, of a bug (_Pyrrhocoris apterus_).—From
Gerstäcker.
]
The micropylar region is generally, at least in Orthoptera and Odonata,
covered by a gelatinous cap (Figs. 499 and 500, _gs_), which may form a
covering membrane which extends over a large part of the egg, or may
envelop the entire outer surface. In some cases micropyles are scattered
over the entire surface of the egg, but usually the perforation is
situated at the end, and is often guarded by raised processes, either
one or several, like bristles, or toadstools, etc., these being
especially characteristic of the eggs of certain Hemiptera (Nepa, Fig.
501, _a_, and Ranatra), or the region is variously sculptured, as in the
eggs of butterflies. In the micropylar apparatus of Œcanthus the papillæ
have a distinct lumen (Fig. 499), or a channel for the ingress of the
male filament.
[Illustration:
FIG. 502.—Diagrammatic median section through egg of Musca in stage of
fertilization (incorporating the figures of Henking and Blochmann):
_ch_, chorion; _d_, dorsal; _v_, ventral side of the egg; _dh_,
yolk-membrane; _do_, nutritive yolk; _g_, gelatinous cap over the
micropyle (_m_); _K_, outer layer of plasma (Keimhautblastem); _p_,
male and female pronucleus before copulation; _r_, directive body
(Richtungskörper).—After Korschelt and Heider.
]
Another use of the micropylar apparatus noticed by Ayers in the egg of
the tree-cricket is that it “serves as a thick, roughened plate, against
which the insect may push when ovipositing, without injury to the egg,
and without danger that the ovipositor slips from its place.” In
Chrysopa eggs the micropyle forms a conspicuous button-like knob,
resembling the finely milled head of a certain kind of screw.
=Internal structure of the egg.=—The egg-contents are surrounded by an
outer layer of protoplasm or formative yolk, which is separate from the
inner parts of the egg (Fig. 502, _do_), the latter being mostly
composed of the nutritive yolk-element. The superficial protoplasmic
layer, called by Weismann _Keimhautblastem_ (_K_) is, in a few cases,
afterwards entirely lost, but in most instances forms a very thin layer
of clear protoplasm, slight in extent compared with the yolk-mass
within.
The eggs of insects are rich in yolk, only certain eggs, such as those
of the Aphides and the egg parasites (Proctotrypidæ) being poor in yolk.
The eggs of heterometabolous insects have been said by Brauer to contain
relatively more yolk than those of the Metabola, particularly the
Diptera; though, as Wheeler observes, this rule has some exceptions, the
eggs of the 17–year Cicada being very numerous and small.
This he thinks is a greater advantage to the insect than the
production of a few large eggs, “when we consider the extremely long
period of larval life and the vicissitudes to which the larvæ may be
subjected during all this time.” “Similarly, _Meloë angusticollis_
produces a large number of very small eggs, while the eggs of the
smaller beetles (Doryphora, _e.g._) are much larger. But Meloë is a
parasitic form, and probably only a few of its many offspring ever
succeed in gaining access to the egg of the bee.”
In the eggs of Chrysopa the yolk-granules are remarkably small, so that
the primitive band is in strong contrast to the yolk in color and
density. When crushed, the yolk does not flow out as a liquid, but in a
pasty mass, and we have questioned whether, as in the eggs of Limulus,
whose yolk is solid with fine granules, the denseness of the yolk is not
connected in the way of cause and effect with their exposed situation.
The central or yolk-mass (Fig. 502, _do_) consists chiefly of rounded
masses of yolk, with fat-globules, between which extends a fine network
of protoplasm.
The elements of the yolk are spherical and strongly refractive, by
pressure becoming polygonal structureless homogeneous bodies.
The germinal vesicle of the ripe insect-egg lies in the centre of the
yolk, where it appears as a large vesicle-like cell-nucleus containing a
few chromatin elements.
_b._ Maturation or ripening of the egg
Before the eggs of animals can be fertilized, they require in some
observed cases, and probably in animals in general, to undergo a series
of changes, which, as observed in the starfish, etc., consists in the
replacement of the germinal vesicle by a very much smaller egg-nucleus,
and also at the same time the construction at one pole of the egg of the
directive or polar bodies (Fig. 502, _r_). Towards the end of the
ripening process of the insect egg this vesicle, according to Blochmann,
passes to the dorsal surface of the egg, and is transformed into the
directive spindles (_Richtungspindel_).
_c._ Fertilization of the egg
The egg next requires the penetration and admission into the
yolk-interior of a spermatozoön.
This process is essentially in insects, as in other animals, the fusion
of the sperm-nucleus with the nucleus of the egg. Under normal
conditions but a single spermatozoön is required for fertilization. As
shown by Hertwig, in the sea-urchin, after the spermatozoön has
penetrated into the egg, the head, and the small rounded body, called a
_centrosome_, can still be recognized, but the tail becomes fused with
the yolk of the egg. In the protoplasm of the egg (called _cytoplasm_)
the achromatic end of the sperm-nucleus gives rise to conspicuous rays,
like those observed in ordinary cell-division. Preceded by these rays,
the sperm-nucleus or male pronucleus (Fig. 502, _p_) moves towards the
nucleus of the egg, and finally fuses with it, thus forming a new single
nucleus. This latter, which is called “the cleavage nucleus,” rapidly
forms a nuclear or “cleavage spindle” (Fig. 503). This act gives an
impulse to the cleavage of the egg, which is the first step in the
formation of the embryo. All these changes have yet to be worked out in
detail in insects by microscopic sections of the egg, whose generally
hard and opaque egg-shells present great obstacles to such work.
_d._ Division and formation of the blastoderm[80]
In insects as in most other Arthropoda the segmentation of the yolk is
superficial and not total. The ovum is _centrolicithal_, _i.e._ the yolk
is concentrated at the centre of the egg, and surrounded by a peripheral
layer of transparent protoplasm (the _Keimhautblastem_).
[Illustration:
FIG. 504.—Formation of the blastoderm of _Pieris cratægi_: _A_,
longitudinal section through the egg, with two masses of protoplasm
in the yolk. _B_, a blastoderm-cell at the upper end. _C_, a later
stage, with more blastoderm-cells.—After Bobretsky.
]
The first step in segmentation is the movement of the first
division-nucleus (_i.e._ that in the fertilized egg arising from the
union of the sperm-nucleus with the female pronucleus) towards the
interior of the egg in order to multiply itself by the mode of indirect
nuclear division (Figs. 504, _A_, and 507).
[Illustration:
FIG. 505.—Embryology of the mole-cricket: 1, egg in which the amœboid
nuclei (_abc_) are moving toward the surface; 2, egg in which the
nuclei (_abc_) have reached the surface, and show an active
nucleus-formation; 3, the blastoderm-cells have no nucleus, and are
placed at equal distances apart; 4, the blastoderm-cells now forming
a continuous layer; 5, cross-section of the egg with blastodermic
disk, also showing the disposition of the endodermal cells; 6,
cross-section of the blastodermic disk, with the myoblast cells
(_mb_) already formed; 7, cross-section through the thorax of the
embryo, the body-cavity extended into the limbs.
]
LETTERING.
_abc_, amœboid blastodermic cells.
_bc_, blastoderm-cells.
_bl_, blastoderm.
_en_, endodermal cells.
_M″_, cavity of the myoblast.
_mb_, myoblast cells.
_N_, nerve-furrow.
_P_, primitive groove.
_pd_, primitive disk.
[Illustration:
FIG. 505 _concluded_.—Later stages in the embryology of the
mole-cricket: 8, longitudinal section of the embryo; the
yolk-pyramids (_yp_) form a common inner yolk-mass (_y_). 9, section
through the heart; _H_, cavity of the heart; the two halves of the
heart-sinuses having united dorsally, ventrally they are still open
and are bounded by the walls of the mesenteron. 10, cross-section of
an embryo, showing the blood-lacunæ separated on the back by the
dorsal organ (_do_); the intestinal fasciated layer
(_Darmfaserblatt_) has not completely enclosed the yolk. 11, embryo
completely segmented, with the rudiments of the appendages, labrum
(_lab_), and nervous ganglia (_pc-ng_). 12, a more advanced embryo,
showing the stomodæum (_st_) indicated as a frontal protuberance.
13, section through the recently hatched larva, showing the cells of
the mesenteron or chyle-stomach, and the cellular layer on the front
surface, also the proventriculus or crop.
]
LETTERING.
_ant_, antenna.
_ar_, arterial sinus.
_bl_, blastoderm.
_bla_, abdominal vesicles.
_cr_, proventriculus, or crop.
_dm_, ventral diaphragm.
_do_, dorsal organ.
_d pm_, dorsal diaphragm.
_ent_, enteric layer.
_f_, fat-body.
_g_, ventral ganglion.
_H_, _ht_, heart.
_l_, lacuna.
_m_, mouth.
_md_, mandible.
_m.en_, mesenteron.
_mx′_, 1st maxilla.
_mx″_, labium, or 2d maxilla.
_ml_, leaf-like portion of mesenteron.
_oe_, œsophagus.
_pc_, procerebrum.
_pm_, proctodæum.
_sg_, subœsophageal ganglion.
_sm_, stomodæum.
_tg_, thoracic ganglion.
_vm_, ventral muscle.
_y_, yolk.
_yp_, yolk-pyramids.
_I_, 1st pair of feet.
_II_, 2d pair of feet.
_III_, 3d pair of feet.
—After Korotneff.
The origin of numerous division-nuclei as the offspring of the first has
been observed to take place in the eggs of those insects (Aphides,
Cecidomyia, and Cynips) which have a slight amount of yolk. Yet in the
large, ordinary eggs of insects with an abundance of yolk there is no
doubt, say Korschelt and Heider, that these numerous division-nuclei,
which soon after the process of oviposition are scattered within the egg
between the yolk-spheres, and are enveloped by a star-shaped
protoplasmic layer, and which constitute the formative elements of the
blastoderm,—there is no doubt but that they have practically arisen
through indirect nuclear division from the first division-nucleus.
The process of formation of the blastoderm in ordinary eggs with
abundant yolk was first observed by Bobretsky in the eggs of a moth
(Porthesia) and Pieris, also by Graber, and more recently by Blochmann
in Musca, and by Heider in Hydrophilus.
In the earliest stage observed by Bobretsky there first appear after
fertilization a few (the smallest number four) cell-like, minute amœboid
masses of protoplasm, each with a distinct nucleus. A few (one at least)
of these bodies gradually pass out of the centre of the yolk to the
surface of the egg (Fig. 504, _A_, _n_), these becoming larger and
rounder, and from one or two of these nuclei (_B_, _bc_) the blastoderm
originates (_C_, _bl_). Those nuclei remaining in the yolk increase in
number and afterwards become the nuclei of rounded masses of
yolk-granules, forming the so-called yolk-spheres which Bobretsky
regards as true cells.
To the few blastoderm cells situated on the upper end of the egg are
added others which continue to pass from the yolk to the periphery, and
then the blastoderm spreads out farther and farther from the upper end
of the egg until finally it covers or envelops the whole yolk. This
layer of cells is called the _blastoderm_.
As to the origin of the primitive amœboid cells, Bobretsky is in doubt,
but is disposed to think that they are the result of the subdivision of
the germinative vesicle or nucleus of the ovarian egg-cell. In this
connection may be quoted the observations of Graber, who states that an
examination of the ovarian cell at an early period has revealed the
presence, in the centre of the yolk, of a number of amœboid cells, which
appear to have been formed by the division of the germinal vesicle.
These “primary embryonic cells” have a relatively large nucleus and a
number of nucleoli. Several may be seen to unite with one another by
means of their pseudopodia, and they may also be observed to undergo
division. With this account may be compared the results obtained by
Korotneff in his work on the embryology of the mole-cricket (Fig. 505).
[Illustration:
FIG. 506.—Four successive stages in the formation of the blastoderm of
_Calliphora vomitoria_ (the figures represent segments of
cross-sections through the fly’s egg): _A_, the nuclei of the
division-cells have arranged themselves parallel with the outer
surface of the egg. _B_, the division-cells fused with the
“keimhautblastem.” _C_, the outer surface becomes furrowed by
indentations; all the nuclei of the blastoderm-cells in process of
division. _D_, the blastoderm-cells form a high cylinder-epithelium:
_b_, “keimhautblastem”; _bz_, blastoderm-cells; _d_, nutritive yolk;
_dz_, yolk-cell; _fz_, so-called division-cell; _i_, inner
“keimhautblastem.”—After Blochmann, from Korschelt and Heider.
]
The result of these and of later observations, especially those of
Blochmann on Musca, and those of Heider on Hydrophilus, show that the
division-nuclei lie near the centre of the egg, along the longitudinal
axis (Fig. 507, _A_). Each of these nuclei is enveloped by a star-shaped
mass of protoplasm, and on the whole resembles a wandering amœboid cell.
These isolated masses of protoplasm are all connected by a fine network
of rays, which unite to form within the yolk a syncytium. Afterwards, in
the later stages, these division-cells, as they may be, though somewhat
incorrectly, regarded, move nearer the periphery and arrange themselves
into a plane parallel with the surface (Figs. 506, _A_, 507, _B_).
Continuing to divide, they reach the surface and fuse with the
peripheral protoplasmic layer (Figs. 506, _B_, 507, _C_). Then follows
the division into single cell-territories (Figs. 506, _B_, 507, _C_),
corresponding to the division-nuclei, through the appearance of furrows
which pass in from the outer surfaces of the egg into the interior and
gradually penetrate the entire “keimhautblastem.” In this way the
surface of the egg is covered with an epithelium (blastoderm). In many
insects the so-called inner “keimhautblastem” (Fig. 506, _D_, _i_) is
formed by the separation of a layer of protoplasm which contains larger
granules and are accumulated between the blastoderm and the upper
surface of the central nutritive yolk-mass. By the addition of this
plasmic layer the cells of the blastoderm increase in height, and now
form a cubical or cylinder epithelium, which continuously envelops the
surface of the egg. (Korschelt and Heider.)
_e._ Formation of the first rudiments of the embryo, and of the
embryonic membranes
[Illustration:
FIG. 507.—Formation of the blastoderm in Hydrophilus: _b_, completed
blastoderm; _d_, yolk; _f_, so-called division-cells; _k_,
“keimhautblastem”; _z_, yolk-cells.—After Heider, from Korschelt and
Heider.
]
The embryo first arises as a whitish streak or band-like thickening on
the ventral side of the egg, and is variously called the “primitive
streak,” “primitive band,” “germinal band,” or “embryonal streak.” In
most cases the primitive band is divided at regular intervals by
transverse furrows, indicating the limits of what are to be the body
segments.
Cross-sections (Fig. 509) show that the band is composed of several
layers, _i.e._ an outer layer (ectoderm) and an inner layer which
comprises the endoderm and mesoderm, and so long as these two layers are
not sharply differentiated from one another, this second layer may be
called, with Kowalevsky, “the inner lower layer, or ento-mesoderm”
(Figs. 508, 509, _B_, _C_, _u_).
It is characteristic of insects, only rarely occurring in other
arthropods (_e.g._ the scorpion), that the primitive streak is not
situated on the surface of the egg, but becomes overgrown by a folded
structure (Fig. 508, _af_) rising from its edges, the amnion-fold, so
that it appears somewhat depressed or sunken in under the upper surface
of the yolk. While the amnion-folds are extending from all sides over
the primitive band, there becomes formed under it, by the invagination
of the outer surface of the egg, a cavity, the amnion-cavity (_ah_),
which, when the amnion-fold has completely overgrown the primitive band
and united together (Fig. 509, _C_), appears completely closed from
without.
[Illustration:
FIG. 508.—Two schematic median sections through an insect-embryo to
represent the development of the embryonal membranes. In _A_ the
primitive streak is not wholly overgrown by the amnion-fold. In _B_
the amnion-folds have united with each other and completely
overgrown the primitive streak: _a_, fore, _b_, hind, egg-pole; _v_,
ventral side; _d_, dorsal side; _af_, amnion-folds; _ah_,
amnion-cavity; _am_, amnion; _do_, yolk; _ec_, ectoderm; _k_,
head-end, _k′_, hinder-end, of the primitive streak; _s_, the part
of the serosa arising from the amnion-fold; _s′_, the part of the
serosa arising from the unaltered blastoderm; _u_, lower
layer.—After Korschelt and Heider.
]
=Formation of the embryonic membranes.=—The amnion-folds finally
completely overgrow the primitive band (Fig. 509, _B_ and _C_), and form
the embryonal membranes. The primitive band is seen after its completion
to be overgrown by a double cellular epithelial membrane. The outer of
these two membranes, that which arises from the outer leaf or layer of
the amnion-fold, is the _serosa_ (Figs. 508, _B_; 509, _C_, _s_; 510).
This passes continuously into the unchanged part of the blastoderm,
which has no part in the formation of the primitive band and
germ-layers, and which covers the outer surface of the yolk. Thus the
_serosa_, which is usually held to include this portion also of the
blastoderm, forms a closed sac which covers the whole surface of the
egg, with one part extending over the surface of the yolk, and the other
over the primitive band (Fig. 510).
[Illustration:
FIG. 509.—Diagrammatic cross-section through three successive stages
of the primitive streak, and growing embryonal membranes of
insect-embryos. _A_, formation of the ventral plate and of the
gastrula invagination (_g_). _B_, upward growth of the amnion-folds
(_af_). _C_, complete overgrowth of the primitive band through the
amnion-folds: _v_, ventral side; _d_, dorsal side; _af_,
amnion-folds; _ah_, amnion-cavity; _am_, amnion; _bl_, blastoderm;
_bp_, ventral plate; _do_, yolk; _ec_, ectoderm; _s_, serosa; _u_,
under or inner layer.—After Korschelt and Heider.
]
The inner of the two layers, called the _amnion_ (Fig. 509, _am_), is
more closely connected with the embryo. The amnion and ectoderm of the
primitive band together form a completely closed sac, whose lumen forms
the amniotic cavity. Originally connected with the serous membrane, it
splits off from the primitive band about the time the appendages begin
to bud out, and continues to closely envelop the body and appendages, as
seen in Fig. 509. Both of these membranes are, before the time of
hatching, either absorbed, or, as in Lepidoptera, retained. The amnion
is retained until after hatching in the locust, etc. In certain
Coleoptera the serosa is retained, and the amnion is absorbed (Fig.
532), while in Chironomus and the Trichoptera the serosa is absorbed,
and the amnion retained, with the egg-shell or chorion. Hence we have
eight layers in the winged insects[81] during embryonic life:
[Illustration:
FIG. 510.—Surface view of fresh serosa from an Œcanthus, treated with
acetic carmine; the blastoderm completely formed, × 500: _p_, polar
body; _rf_, radiating fibres; _nls_, nuclear substance; _nlm_,
nuclear membrane.—After Ayers.
]
1. Exochorion. (Remains of the epithelium of the ovarian follicle.)
2. Chorion. (Egg-shell or cuticle secreted in the ovarian
follicle.) 3. Vitelline membrane. (Primary egg-membrane.
Yolk-skin or membrane.) 4. Serous or outer germ-membrane.
(Serosa.) } 5. Amnion or inner germ-membrane. } Derived from the
blastoderm. 6. Ectoderm. } } 7. Mesoderm. } Embryo. } 8.
Endoderm. } }
In the embryo of Xiphidium and Orchelimum Wheeler has found and
described with much detail a membranous structure which he calls the
_indusium_. “The organ,” he says, “appears to have been retained by
the Locustidæ, and completely lost by the embryos of other winged
insects.” It arises in Xiphidium, as a simple circular thickening of
the blastoderm, between and a little in front of the procephalic
lobes (Figs. 511, 512, _A-E_), and afterwards spreads over nearly
the whole surface of the egg, leaving the poles uncovered, as in
Fig. 513, where it is divided into two further membranes, the inner
and outer indusium, the former lying in contact with the amnion.
After this the serosa “is excluded from taking any part in the
development of the embryo; both its position and function are now
usurped by the inner indusium.”
Hence in an egg of the Locustidæ Wheeler distinguishes, passing from
within outward in a median transverse section of the egg, the
following envelopes:
1. The chorion. 2. The blastoderm-skin-like cuticle secreted by
the serosa. 3. The serosa. 4. The outer indusium. 5. A layer
of dark granular secretion (probably some urate). 6. The
cuticle secreted by the inner indusium. 7. The inner indusium.
8. The amnion. While envelopes 1–7 invest the whole egg; layer
8, the amnion, covers only the embryo.
[Illustration:
FIG. 511.—Diagrams illustrating the movements and envelopes of the
embryo of Xiphidium: _A_, after the closure of the amnioserosal
folds. _B_, during the embryo’s passage to the dorsal surface.
_C_, just after the straightening of the embryo on the dorsal
surface; _ind_, indusium afterwards forming _ind^1_, the inner,
and _ind^2_, the outer indusium; _ch_, chorion; _sr_, serosa;
_am_, amnion; _gb_, germ-band; _v_, yolk; _bl. c_, blastoderm
membrane.
]
Wheeler further suggests that the so-called micropyle of the
Collembola (Anurida), which has been homologized with the “dorsal
organ” of Crustacea, is a possible homologue of the indusium, as
also the “primitive cumulus” of spiders, and the “facette” or
“cervical cross” of Pentastomids described by Leuckart and also by
Stiles.
=The gastrula stage.=—The primitive band invaginates so as to give the
opportunity for the formation of the inner layer. This invagination,
which at a certain stage is established along the whole length of the
primitive band, forms a median furrow and may be regarded as the
gastrula-invagination of insects. The lower (inner) layer thus arising
afterwards spreads out under the entire primitive band (Fig. 509, _B_
and _C_), the edges of which become bordered by the growing amnion-fold.
(Korschelt and Heider.)
In certain forms the primitive band arises from several separate
rudiments which afterwards unite. Thus in Musca and Hydrophilus the
anterior and posterior ends develop first, and in Hydrophilus the
procephalic lobes originate independently of the rest of the band.
In the Aphides, also, according to Will, these lobes arise
independently, afterwards uniting with the primitive band.
[Illustration:
FIG. 512.—Diagrams illustrating the movements and envelopes of the
embryo of Xiphidium: _D_, the stage of the shortened embryo on the
the dorsal yolk. _E_, embryo returning to the ventral surface. _F_,
embryo nearly ready to hatch; _ch_, chorion; _b. lc_, blastoderm
membrane; _sr_, serosa; _ind^1_, outer indusium; _ind^2_, inner
indusium; _ind^2 + am_, inner indusium and amnion fused; _am_,
amnion; _ind^1 c_, cuticle of the inner indusium; _ind^2 s_,
granular secretion of the inner indusium; _am. s_, amniotic
secretion; _v_, yolk; _cl_, columella; _gb_, primitive band.
]
=Division of the embryo or primitive band into body-segments.=—Meanwhile
the primitive band grows at the expense of the yolk, spreading out more
and more over its surface, until in certain cases (Coleoptera, Diptera,
Siphonaptera, and Trichoptera) it lies like a broad ribbon over the
yolk, so that the two ends nearly meet on the dorsal side. By this time
it becomes divided by transversely impressed lines into segments, which
correspond to those of the larva and adult. The first of these segments
is divided into two broad and flaring flaps, which are called the
procephalic lobes. It becomes the antennal segment.
[Illustration:
FIG. 513.—Two stages in the spreading of the indusium. _A_, lateral
view of egg just after the arrival of the embryo on the dorsal yolk.
_B_, lateral view of the egg with the indusium nearly reaching the
poles. _C_, same egg seen from the dorsal surface.
]
The mouth (_stomodæum_) now develops, and is situated at the
anterior,[82] and the rectum (_proctodæum_,) at the posterior pole, or
end of the primitive band.
[Illustration:
FIG. 514.—Median section of the egg of _Anurida maritima_: _do_,
“micropyle”; _bld_, blastoderm.—This and Figs. 511–513, after
Wheeler.
]
In Blatta, Hydrophilus, the Trichoptera, and the Lepidoptera the
hindermost part of the primitive band is turned in ventrally (Figs. 534,
_C_).
The preceding account of the relations of the primitive band to the
yolk does not apply to all insects, since there are variations which
appear to depend on the form of the egg, and on the amount and
distribution of the yolk-masses. In certain Coleoptera, the
primitive band sinks down and thus becomes immersed into the yolk.
In Donacia (Kölliker and Melnikow) and Hydrophilus (Heider), and in
the Chrysomelidæ and Attelabus, a weevil, as we have observed, the
primitive band rests on the outside of the yolk, but in _Telephorus
fraxini_ it is immersed. In the Hemiptera it is immersed (Fig. 516),
but there is much variation in this respect, the degree of immersion
being most marked in the Coccidæ (Aspidiotus), and least so in
Corixa. Besides the position of the primitive band, there are in
Odonata and Hemiptera differences in the origin of the primitive
band itself and of the embryonic membranes.
[Illustration:
FIG. 515.—Ventral view of five developmental stages of Hydrophilus:
_a_ and _b_, places at which the blastopore contracts; _af_, edge
of the amnion-fold; _af′_, caudal fold; _af″_, paired head-fold of
the amnion; _an_, antenna; _es_, last segment; _g_, pit-like
invagination (first indication of the amniotic cavity); _k_,
head-lobes; _r_, furrow-like invagination; _s_, portion of the
primitive streak covered by the amnion.—After Heider, from Lang.
]
[Illustration:
FIG. 516.—Embryo of the louse: _am_, serosa; _db_, amnion; _as_,
antenna; _vk_, clypeus.—After Melnikow.
]
Korschelt and Heider divide the early embryo of insects into two
types:
1. Into those with a superficial primitive band; viz., where there
is no passage of yolk-elements into the space between the amnion and
serosa. The primitive band has in such cases a relatively
superficial position (Figs. 508, 509, 521, 535). Examples are
certain Orthoptera (Blatta, Œcanthus, Mantis, Gryllotalpa), also
certain Hemiptera (Corixa), certain Coleoptera, and the Trichoptera,
Diptera, and Hymenoptera.
2. Into those with an immersed primitive band, with the space
between the serosa and amnion filled with yolk (Figs. 517, 518,
534). Examples are the orthopterous Stenobothrus, Odonata, many
Hemiptera (the Pediculina and Pyrrhocoris), the Coleoptera already
mentioned, and Lepidoptera.
It should be observed, however, that these differences are of little
phylogenetic or taxonomic value, since genera of the same order,
notably the Coleoptera, differ as to the position of the primitive
band, so also two orders so nearly allied as the Trichoptera and
Lepidoptera.
=Differences between the invaginated and overgrown primitive
band.=—In respect to the mode of origin of the primitive band and
its relative position, there are two opposite types, though
connected by transitional forms. In the one case the primitive band,
_i.e._ its ventral portion, the “ventral plate” (Fig. 518, _b_, _p_)
is pushed in or invaginated in the interior of the egg; in the other
case it becomes overgrown by the folds of the amnion arising from
its edges.
[Illustration:
FIG. 517.—Primitive streak of a lepidopter in cross-section: _ah_,
amniotic cavity; _am_, amnion; _c_, cœlomic cavity; _do_,
nutritive yolk, divided into single nucleated masses; _ec_,
ectoderm; _m_, mesoderm; _pr_, primitive thickenings of the
ventral nervous cord; _s_, serosa.—Combined figure after those of
Brobretsky and Hatschek, from Korschelt and Heider.
]
[Illustration:
FIG. 518.—Five diagrammatic median sections representing the growth
of a dragon-fly (Calopteryx). _A-C_, development of the primitive
streak (_k_, _k′_) by invagination. _D_, the amnion-fold (_af_),
growing over the head-end of the primitive streak. _E_, closing of
the opening of the amnion-cavity (_ah_): _v_, ventral, _d_, dorsal
side; _a_, fore, _b_, hind end of egg; _bl_, blastoderm; _bp_,
ventral plate; _do_, yolk; _k_, head-end, _k′_, caudal end, of the
primitive streak; _kh_, germinal thickening or initial point of
invagination; _s_, serosa.—After Brandt, from Korschelt and
Heider.
]
In insects with an overgrown primitive band, the band at the
beginning is generally short and always situated on the ventral side
of the egg, with the head-end looking forward, and remains in this
position throughout embryonic life. There is no revolution of the
embryo. The embryonal membranes arise through the formation of
folds.
[Illustration:
FIG. 519.—Three embryonic stages of Calopteryx: _am_, amnion; _g_,
edge of the ventral plate; _ps_, germ of primitive band; _se_,
serosa.—After Brandt, from Balfour.
]
[Illustration:
FIG. 520.—Three farther stages of growth of Calopteryx. _B_ and _C_
show the inversion of the embryo: _a_, opening of the
amniotic-cavity, out of which the embryo emerges; _ab_, abdomen;
_am_, amnion; _at_, antenna; _md_, mandible; _mx^1_, _mx^2_, 1st
and 2d maxillæ; _œ_, œsophagus; _p^1_, _p^2_, _p^3_, legs; _se_,
serosa; _v_, anterior end of the primitive streak.—After Brandt,
from Balfour.
]
In insects with an invaginated primitive band, of which the Odonata
afford examples, the first rudiment of the primitive band is in the
form of a ventral plate of slight extent passing ventrally in the
hinder half of the egg, in whose posterior section a process of
invagination (Fig. 518, _A_, _kh_), soon occurs. The cavity of this
invagination is the first indication of the amnion-cavity (Fig. 518,
_B_, _ah_), while its wall in its thickened ventral part (_K′_) is
concerned in the formation of the primitive band, and, in its dorsal
thin part, in the formation of the amnion (_B_, _C_, _am_).
=Revolution of the embryo where the primitive band is
invaginated.=—At first the head-end of the embryo is directed
towards the posterior end of the egg, as in dragon-flies (Fig. 518).
Also that surface of the primitive band which afterwards faces the
ventral, is at first turned towards the dorsal side of the egg. In
order to bring the primitive band into the later relations, there
must occur the process of revolution, or turning, of the embryo. The
somewhat advanced embryo of the Odonata, after the appearance of the
head and thoracic appendages, undergoes a rotating motion around its
transverse axis, and at the same time turns out of the amniotic
cavity (Fig. 520, _B_). This process is so managed that near the
head-region, the amnion and serosa, there closely situated to each
other, are fused together, and at this place tear or burst open.
Through this rent (_a_), in the same place in which the original
invagination-opening was situated, the amniotic cavity again opens,
and through the opening thus formed first the head and then the
succeeding segments of the primitive band (Fig. 520, _B_) pass out,
and remain there while the head passes on to the anterior pole of
the egg on the ventral side, the embryo thus assuming a position
like that of other insects. (Kowalevsky.)
In the parasitic Hemiptera (Pediculina), according to Melnikow, the
opening in the membranes near the head remains permanent, and the
embryo becomes everted through it, while the yolk, enclosed in the
continuous membrane formed by the amnion and serous membrane, forms
a yolk-sac on the dorsal surface. The same process occurs in
Mallophaga, and also in Œcanthus, as described by Ayers (Fig. 521).
Generally as soon as the embryo passes out of the amniotic cavity
the latter soon becomes smaller and finally completely disappears.
[Illustration:
FIG. 521.—Revolution of the embryo of Œcanthus (diagrammatic): _a_,
fore, _b_, hind end of egg; _am_, amnion; _d_, dorsal, _v_,
ventral side of egg; _k_, primitive streak; _r_, dorsal plate
(originating by the contraction of the serosa (_s_)).—After Ayers,
from Korschelt and Heider.
]
As the embryo grows, and the sides grow up and the back closes over,
the contents of the yolk-sac are soon taken up and absorbed in the
intestinal cavity, which communicates with it.
In Phyllodromia, according to Wheeler, the process of revolution is
“hurried through by the embryo from the beginning of the 16th to the
end of the 17th day.” Several successive stages are represented in
Fig. 522. In the 15th day the embryo still occupies the middle of
the ventral surface of the egg. Soon the envelopes (amnion and
serosa, _as_) rupture, an irregular slit being formed, and soon the
egg and embryo are as seen in Fig. 522, _B_, the embryo standing out
free from its envelopes on the yolk, and the edges of its dorsal
growing walls (_b_) are distinctly marked. The tail now lies at the
caudal end of the egg (Fig. 522, _C_). By the 17th day the walls
have closed in the median dorsal line, and the embryo has grown in
length to such an extent as to bring its head to the cephalic pole
(Fig. 522, _E_).
Korschelt and Heider consider, since the primitive band of the
chilopod myriopods (Geophilus) is curved in at the middle and sinks
into the interior of the yolk, that in insects the invaginated
primitive band is the ancestral or primitive one, the overgrown
primitive band being derived from it. The overgrown primitive band
by its position may also be better insured against certain
mechanical attacks, perhaps also against the danger of drying up.
[Illustration:
FIG. 522.—Embryo of Phyllodromia, 15 days old; revolution about to
begin. The stages in revolution are represented, after the rupture
of the amnion and serosa, in _A_ to _E_, which are from embryos 16,
16½, 16¾, and 17 days old respectively: _as_, amnion and serosa;
_s_, edge of serosa; _b_, dorsal growing body-wall; _d.o_, dorsal
organ; _x_, clear zone covered with scattered amniotic nuclei.—After
Wheeler.
]
_f._ Formation of the external form of the body
=Origin of the body-segments.=—As we have seen, the first traces of
segments appear very early, the primitive band being divided by
superficial transverse furrows into segments. This segmentation into
arthromeres (somites or metameres) can be observed in Hydrophilus and
Chalicodoma at a time when gastrulation begins (Figs. 515, 536). The
segmentation extends not only across the median portion of the primitive
band, through whose invagination the inner layer (endomesoderm) results,
but also to the lateral portions which become a part of the ectoderm of
the primitive band. These transverse furrows correspond to thinner
places in the epithelium, which in this stage forms the embryonal
rudiment. It thus happens that, in the forms named, after the end of
gastrulation not only the ectoderm, but also the endomesoderm, is
already segmented.
So early an appearance of segmentation as that observed in
Hydrophilus and Chalicodoma we must regard as a falsification of the
process of development due to heterochrony. We must consider the
conditions observed in other forms as the primitive ones, in which
(as, for example, in Lina and in Stenobothrus, according to Graber)
the gastrulation and separation of the ectoderm occurs in the still
unsegmented primitive band, the division into segments occurring in
later stages (Fig. 524). In these forms, then, the segmentation
affects the invaginated endomesoderm, as well as the ectoderm.
(Korschelt and Heider, p. 789.)
[Illustration:
FIG. 523.—Diagrammatic cross-section through three successive
stages of Gryllotalpa, showing the formation of the heart.
(Compare Fig. 505.) The germs of the glandular intestinal layer
(_darmdrüsenblatt_) are omitted. _A_, earliest stage; the
primitive streak extends from _*x_ to _y*_. The embryonal
membranes are torn and pressed against the back: _am_, edge of
the rent; _rp_, dorsal plate (serosa); _l_, lamella (amnion
turned up) standing in connection with the ectoderm of the
primitive streak. _B_, second stage; the primitive streak has
completely grown around the yolk; the dorsal organ is absorbed.
_C_, third stage, dorsal portion; the formation of the heart is
finished: _am_, vestige of the amnion-fold; _bs_, blood-sinus;
_dd_, rudiment of the dorsal diaphragm; _dv_, ventral diaphragm
(compare Fig. 505); _do_, yolk; _dz_, yolk-cells; _ec_,
ectoderm; _gr_, vascular groove (rudiment of the heart); _l_,
lamella of the upturned amnion; _lh_, definite body-cavity; _m_,
transverse muscle; _n_, nervous cord; _r_, heart; _rp_, dorsal
plate; _sp_, splanchnic; _so_, somatic layer of the mesoderm;
_us_, primitive segmental cavity; _*x_, _y*_, lateral
terminations of the primitive streak.—After Korotneff, from
Korschelt and Heider.
]
In the completely segmented primitive band may be distinguished two
regions of a peculiar appearance (Figs. 515, 527), one at the anterior,
and the other at the hinder end. The anterior, the primary head-section,
contains the mouth-opening, and is characterized by its lateral
expansions, or procephalic lobes. The other section, or posterior
section, the so-called anal segment or telson, contains the anus.
Between the two sections lies the segmented primary trunk-segment, which
in insects consists of 17 segments. Of these the three most anterior are
those destined to bear the mandibles and two pairs of maxillæ; the three
following are the thoracic, which are succeeded by 10 abdominal
segments, besides the 11th or telson (pygidium, or suranal plate).
It is now generally believed that there are primarily eleven
abdominal segments, while Heymons has detected twelve in the embryos
of Blattids and Forficula (see p. 162). In the later stages of
embryonic development the number of abdominal segments is
diminished, the 10th and 11th abdominal segments becoming fused. In
Hydrophilus and Lina this is the case, but according to Graber, in
the Lepidoptera there is a fusion of the 9th and 10th abdominal
segments, the llth remaining free.
According to Wheeler, in Doryphora, and also in Chalicodoma
(Carrière), between the primary head-region and the mandibular
segment is interpolated a rudimentary and transitory body-segment,
the premandibular segment. According to Carrière this segment
corresponds to a rudimentary pair of limbs, and also to a ganglion,
which participates in the formation of the œsophageal commissure
(see p. 51).
[Illustration:
FIG. 524.—Three embryonic stages of a leaf-beetle (Lina): _A_,
unsegmented primitive streak; in _B_ and _C_ the segmentation
becomes distinct on the lower layer (_u_). _B_, with the germs of
the gnathal segments (_k′-k‴_), and in _C_ the three thoracic
segments (_t’-t‴_), with the first two abdominal segments (_a_′,
_a″_): _bl_, blastopore; _kl_, head-lobes; _th_, extension of the
primitive streak into the thoracic region.—After Graber, from
Korschelt and Heider.
]
=The procephalic lobes.=—The head-lobes, or procephalic lobes, appear at
a very early period (Fig. 524, _kl_), before any traces of the segments
of the trunk region. Ayers has shown that in Œcanthus the primitive
band, in its earliest condition and before the appearance of the
head-lobes, is a simple oval plate or almond-shaped thickening near the
posterior end of the egg (Fig. 525, 1, 2). This plate is “soon divided
into two tolerably well-marked regions by the enlargement of the
head-end,” the first indication of the head-lobes (3). A depression next
forms in what is to be the middle of the forehead. “It indicates the
position of the future labrum, and forms the inner boundaries of the two
cephalic ganglia, which are developed on either side of this depression
at a much later stage.” Almost simultaneously with the appearance of
this depression, two lateral folds are formed in the trunk portion of
the band, which are the first indications of the maxillary and thoracic
regions, the abdominal portion not yet showing traces of a division into
segments (Fig. 525, 5). The thickened outer edges of the head-fold next
gradually grow in towards the median line (Fig. 525, 5), and bend
forward towards the region of the future mouth. The rounded angle made
by the posterior end of the head-fold is the first indication of the
antennæ. The embryo is now composed of four well-marked regions:
cephalic, maxillary, thoracic, and abdominal. The primitive band then
grows much longer, the primitive mouth and anus appear, and the
appendages bud out, and eventually the embryo revolves and appears on
the ventral side of the egg (Fig. 525, 6).
[Illustration:
FIG. 525.—Early stages in the embryology of _Œcanthus niveus_. Fig. 1,
the youngest observed primitive band, the serosa not yet formed; 2,
longitudinal optic section (diagrammatic) of Fig. 1; 3, the
primitive band after the appearance of the head-fold, which is
indicated at this time by the more rapid growth and consequent
greater breadth of the lower end of embryo, x 25; 4, a young embryo
after the appearance of the primitive segment-folds, x 50; 5, a more
advanced embryo, with the antennal folds distinctly marked off; the
free ends of the primitive folds have united across the embryo
posterior to the antennal folds, x 50; 6, ventral view of the embryo
with the appendages budding out, x 25 (the embryo in this stage lies
dormant through the six colder months of the year): _am_, amnion;
_m_, micropylar end; _ch_, chorion; _gb_, primitive band; _bf_,
brain-fold; _yl_, yolk; _tf_, caudal fold; _kf_, head-fold
(procephalic lobe); _p.fd.t_, primitive thoracic fold; _p.fd.m_,
primitive maxillary fold; _p.fd.a_, primitive abdominal fold;
_ab.c_, abdominal constriction; _t.c_, thoracic constriction;
_at.l_, antennal lobe; _M_, mesoderm; _h.g_, head groove; _mo_,
mouth; _sk_ invagination of ectoderm to form head-apodeme; _md_,
rudiment of mandible; _m^1_, 1st, _m^2_, 2d maxilla; _T^1–T^5_,
legs: _ab.p_, 1st abdominal appendage; _ap_, other appendages; _tb_,
caudal expansion; _mf_ median furrow; _B_, primitive unpaired organ
(metastomum).—After Ayers.
]
These primitive regions of the primitive band, before the segments
are formed, are called by Graber _macrosomites_, and the secondary
segments into which they divide (which afterwards become the
body-segments), _microsomites_. The macrosomites are peculiar to
insects, and may be an inheritance from a hypothetical ancestral
form. With Korschelt and Heider, we should hardly share this view.
[Illustration:
FIG. 526.—Older embryo of Œcanthus with the appendages budded out,
those of the abdomen distinct: _abp_, first pair; _a.s_, anal
stylet; _pr_, proctodæum; _am_, amnion.—After Ayers.
]
Our observations on locusts show clearly (1) that the procephalic lobes
are the pleural portion of the first cephalic or antennal segment; (2)
that the antenna is an appendage or outgrowth of the procephalic lobes;
(3) that the eyes are a specialized group of epidermal cells of the
upper part of the procephalic lobes, and are not homologues of the
antennæ or of the appendages in general; and (4) it seems to follow from
a study of the relations and mode of development of the clypeus and
labrum, that they arise between the procephalic lobes, and probably
represent the tergal part of the antennal segment, forming the roof of
the mouth, _i.e._ closing in from above the pharynx.
In general the formation of the body-segments into the primitive band is
in succession from before to the hinder end. This successive appearance
has been observed by Graber in genera of different orders (Stenobothrus,
Lina, and Hylotoma). For example, in the beetle Lina, after the
appearance of the mandibular and two maxillary segments, appear the
three thoracic segments, together with the two anterior abdominal
segments, the other abdominal segments arising afterwards. In other
cases, the formation of segments seems to be simultaneous along the
entire length of the primitive band. An exception to the rule has been
noticed by Heider in Hydrophilus, as in this beetle the development of
the segments of the middle region appears somewhat delayed, while the
fore and hind parts of the primitive band are more rapid in development.
In Pieris, according to Graber, the thoracic segments are more rapidly
developed than the others; soon after, the gnathic segments (mandibles
and two pairs of maxillæ) appear, and finally the abdominal segments are
formed.
=Fore-intestine (stomodæum) and hind-intestine (proctodæum),
Labrum.=—The digestive canal of insects consists, as in other animals,
of three portions, the fore, mid, and hind gut or intestine. The next
change after the completion of the segments of the primitive band is the
development of the fore and hind intestine and the appendages. The
fore-intestine (stomodæum) arises as an invagination in the area of the
primary head-section, and the hind-intestine (proctodæum) in the
terminal section (Figs. 300 and 546).
In insects generally the formation of the fore-intestine occurs
earlier than that of the hind-intestine. An exception was discovered
by Graber and also by Voeltzkow in Muscidæ, where the proctodæum
appears earlier.
[Illustration:
FIG. 527.—Rudiments of the appendages of the embryo of Hydrophilus:
_an_, antenna; _md_, mandibles; _mx_{1}_, 1st, _mx_{2}_, 2d maxilla;
_vk_, clypeal region; _m_, mouth; _p^1-p^3_, legs; _p^4-p^9_,
rudiments of abdominal appendages, 1–9; _st_, stigma; _a_,
anus.—After Heider, from Lang.
]
Usually at the time of origin of the stomodæum a projection arises at
the anterior edge of the primary head-region, the so-called forehead
(Fig. 527, _vk_), which is the common rudiment of the clypeus and
labrum. In many cases (certain Coleoptera and Lepidoptera) these
rudiments first assume the form of paired hooks (see Figs. 83, 102, 104,
105, of Graber’s Keimstreif der Insekten, also Figs. 529 and 546), which
afterwards, by fusion in the median line, become single, though notched
in the middle; but in the more generalized Blatta and Mantis, as well as
in bees, the rudiment is single at the outset.
The view advanced by Patten, and also by Carrière, that the labrum
is a first pair of antennæ, is scarcely tenable, and we quite agree
with Korschelt and Heider in regarding the clypeo-labral region as
homologous with the upper lip of Crustacea, and, we may add, of
Merostomes and of Trilobites.
It should be observed that in many insects, in their earlier
embryonic state, directly behind the mouth arises, from paired
rudiments, what seem provisional lower lip structures (not to be
confounded with the 2d maxillæ of insects). This under lip structure
was first discovered by Bütschli in the bee (his inner or 2d
antennæ), and afterwards by Tichomiroff in Lepidoptera. Heider, in
his work on Hydrophilus, describes it as the “lateral mouth-lips,”
while, more recently, Nusbaum has observed it in Meloë. This under
lip structure may be regarded as analogous to the paragnaths of
Crustacea, although to attempt to homologize it with these seems
useless. (Korschelt and Heider.)
=Completion of the head.=—Sufficient attention has not been paid to this
subject by embryologists. The head is at first, dorsally, mostly
composed of the head-lobes, or antennal segment only, and the dorsal or
tergal portion of the oral appendages develop at a later period. We have
observed in the embryo of dragon-flies (Æschna) that the tergites of the
mandibles and first maxillæ are simultaneously fused with the
head-lobes, while the much larger tergal region of the 2d maxillæ
remains for some time separate from the anterior part of the head, and
is continuous with the thoracic segments, and it is only just before
hatching that this segment becomes fused with the rest of the head (Fig.
36). In a sense, the 2d maxillary segment when it is free from the head
reminds us of the foot-jaw, or 5th segment of chilopod myriopods (see
also p. 53).
_g._ The appendages
As we have seen, nearly or quite simultaneously all the limbs as a rule
bud out from each side of the median line of the primitive band. They
arise as saccular evaginations or outgrowths of the ectoderm, directed a
little backwards. They are at first filled with mesoderm cells, and in
the Orthoptera diverticula of the cœlom-sac are taken up into the
rudimental limbs, as in Peripatus and Myriopoda. (Graber, Cholodkowsky.)
As the antennæ, mouth-parts, legs, and abdominal appendages are all
alike at first, their strict homology with one another is thus
demonstrated. In insects never more than a single pair of limbs is known
to arise from one segment.
=The cephalic appendages.=—The antennæ evidently arise from the hinder
edge of the procephalic lobes (Fig. 527, _an_). As in Limulus, the first
pair of appendages are at first postoral (Fig. 528, _at_), afterwards
moving forward owing to changes in the relative proportions of the parts
of the head, and they are in all respects, in their development and
position in relation to the segment from which they arise, homologous
with the appendages succeeding them.
The occurrence of rudiments of a pair of preantennal appendages in
Chalicodoma which is claimed by Carrière, needs confirmation, as
other embryologists have not observed them.
The postoral appendages of the head are the mandibles and the 1st and 2d
maxillæ, besides the supposed premandibular segment already referred to
on pp. 50–54, which only temporarily exists.
The trophi or oral appendages are all alike at first, but soon differ in
shape, acquiring their characteristic form shortly before the embryo
leaves the egg. The mandibles of Œcanthus are said by Ayers at the time
of revolution of the embryo to be slightly bilobed, and in his Fig. 5,
Pl. 19, they are represented as deeply trilobed, but in general they are
undivided. The 1st maxillæ are at this time distinctly trilobed. The 2d
maxillæ are separate, and distinctly though unequally bilobed, becoming
united shortly before birth. In the embryos of dragon-flies they are at
an early date very large and long, and directed backwards, and are not
fused together until just before hatching, when the extraordinary
mask-shaped labium is fully developed.
[Illustration:
FIG. 528.—Two embryonic stages of the primitive streak of Melolontha.
_A_, younger stage, with rudiments of eight pairs of abdominal
appendages (_a^1–a^8_). _B_, older stage, the primitive band now
very broad: _a_, 1st abdominal appendage, in _B_ sac-like; _x_,
place of adhesive disc; _g_, brain; _l_, clypeo-labrum; _s_, lateral
cord of the ventral nervous cord; other lettering as in previous
figures.—After Graber, from Korschelt and Heider.
]
The distal parts of the labium, such as the ligula, palpifer, and palpus
are elaborated before the mentum and submentum. Many details as to the
final changes in the mouth-parts before hatching remain to be worked
out.
=The thoracic appendages.=—The three pairs of legs arise at the same
period and in the same manner in all insects; it is not until the end of
embryonic life that they become jointed, and that the claws and onychia
are developed. Especial attention has not yet been given to the details
of the development of the parts of the last joint of the tarsus.
In many forms the antennæ are the first to appear, the mandibles,
maxillæ, and legs appearing at a latter date, though simultaneously.
It is thus in Stenobothrus, Hydrophilus, and Melolontha. In Lina,
according to Graber, the mandibles precede the antennæ in
appearance. In the Libellulidæ, according to Brandt, the legs first
appear, then the jaws, and lastly the antennæ. This did not seem to
be the case in the embryos of Æschna observed by us, although our
observations were more superficial.
On the other hand, in those insects whose larvæ are footless, the
rudiments of the legs are retarded and aborted just before hatching
(fossorial Hymenoptera and Apidæ), or the rudiments of the legs are
not developed at all.
=The abdominal appendages.=—These appear soon after the thoracic limbs,
corresponding in most cases to the latter in shape and position, and
their position in the embryo is a matter of the greatest interest. Von
Rathke was the first embryologist to detect those of the first abdominal
segment, in his examination of the development of Gryllotalpa. Long
afterwards Bütschli detected them in the embryo of the honey-bee,
observing a pair on each segment. Patten observed them in Trichoptera;
Kowalevsky first perceived them in Lepidoptera, Tichomiroff confirming
his observations. Graber, Ayers, and Wheeler have observed them in
Orthoptera and Coleoptera, and the latter has detected them also in
Hemiptera and Neuroptera; and while they do not arise in the embryos of
Diptera and of Siphonaptera, they are to be looked for in any or all the
lower or more generalized orders.
As the result of these discoveries of polypodous embryos occurring in
all but the most specialized order (Diptera), it appears to be a
rational deduction that the winged insects have descended from insects
in which there were functional legs on each abdominal segment. Such an
ancestor was the forerunner of the Thysanura, in which abdominal
locomotive appendages still survive, though in a modified, more or less
aborted condition. This polypodous ancestral form was apparently allied
to Scolopendrella, which has a pair of functional legs on each abdominal
segment.
The subject, then, of polypodous embryo insects is one of special
interest, and has attracted much attention from Graber, Wheeler,
Haase, and others. That these are genuine, though transitory
appendages, is shown by the fact that certain pairs persist
throughout adult life. The embryology of the Thysanura when worked
out will throw much light on this subject, but we know that the
spring (elater) of Collembola (and possibly the collophore) and the
cerci of the winged insects are survivals of these limbs. That the
three pairs of appendages forming the ovipositor, or sting, are most
probably derived from these appendages is claimed by Wheeler (p.
167), and seems proved by the fact that Ganin and also Bugnion has
detected three pairs of imaginal disks in the embryo of parasitic
Hymenoptera. Hence the abdominal appendages may ultimately be found
to arise in nearly all cases from imaginal disks like those giving
origin to the cephalic and thoracic appendages.
As regards the Diptera, Pratt has observed that each of the three
thoracic and eight abdominal segments of the embryo brachycerous
Diptera (seen especially well in Melophagus) has two pairs of
imaginal disks, a dorsal and a ventral pair. He thinks there is no
doubt but that the ventral abdominal disks are homologous with the
rudimentary appendages which appear in the embryos of all other
insects, though not in the brachycerous dipters.
=Appendages of the first abdominal segment (pleuropodia).=—As early as
1844 Rathke observed in the embryo of the mole-cricket a pair of
appendages on the 1st abdominal segment, which he described as
mushroom-shaped bodies, and supposed to be embryonic gills. They are
called _pleuropodia_ by Wheeler, who, with Patten, Graber, and Nusbaum,
ascribes a glandular function to them, while Wheeler suggests that they
were odoriferous repugnatorial organs. In Blatta (Phyllodromia) they are
of large size, in Melolontha enormous (Fig. 528, _B_) and filled with
blood. Wheeler distinguishes as varieties, beside the mushroom-shaped
appendages of Gryllotalpa and Hydrophilus, the reniform (Œcanthus), the
broadly pyriform (Blatta), and the elongate pyriform (_Mantis
carolina_). In the European Mantis they are most limb-like, with a
digitiform continuation divided by a constriction into two sections.
(Graber.) In Meloë they assume the shape of a stalked cup. (Nusbaum.) In
the bee and in Lepidoptera the pleuropodia are not present, though the
temporary appendages on the succeeding segments appear; Carrière,
however, found them on the two first abdominal segments of very young
larvæ of the wall-bee (Chalicodoma).
Their cellular structure is peculiar, and they are either formed by
evagination or invagination, those of the latter type being subspherical
and solid. Those of the former type have a cavity communicating by means
of a narrow duct through the peduncle with the body-cavity (Blatta). No
tracheæ, nerves, or muscles enter them, though blood-corpuscles have
been seen in the cavities. “In some species the pleuropodia produce a
secretion from the ends of their enlarged cells. This secretion may be a
glairy albuminoid substance (Cicada, Meloë), a granular mass
(Stenobothrus), a bundle of threads (Zaitha), or a thick, striated,
cuticula-like mass (Acilius).” They attain their greatest size during
the revolution of the embryo, and they are “mere rudiments of what were
probably in remote ages much larger and more complex organs.” (Wheeler.)
Lameere has observed that in Phyllodromia the first pair of
abdominal appendages, after becoming of considerable size, undergo
an enlargement at their free end, become detached, and fall into the
amnion.
Wheeler also calls attention to the homology of these pleuropodia
with the 1st abdominal appendages of Campodea, shown by Haase to be
originally glandular, but with at present a respiratory function. In
the embryos of later, higher orders of insects, these appendages are
in size and shape similar to those of the succeeding segments. (See
also p. 164.)
[Illustration:
FIG. 529.—Primitive band of _Bombyx mori_, showing the temporary legs
on abdominal segments 2–11: _A_, early stage, in which the abdominal
legs _al^2–al^10_ appear. _B_, later stage, when they are very faint
and all except _al^3–al^6_ and _al^10_ are about to disappear. _C_,
the persistent abdominal legs _al^3–al^6_ and _al^10_; _st^2_,
_st^9_, the 2d and 3d pair of stigmata; _sgl_, silk duct.—After
Tichomiroff.
]
=Are the abdominal legs of larval Lepidoptera and phytophagous
Hymenoptera true limbs?=—The presence of these abdominal legs in the
embryos of Sphinx (Kowalevsky), of _Bombyx mori_ (Tichomiroff), and both
_Bombyx mori_ and _Gastropacha quercifolia_ (except those of the first
segment), as well as in Hylotoma, which has 11 pairs of such appendages,
has suggested that the prop or prolegs of caterpillars and saw-fly larvæ
are survivals of these outgrowths, and not secondary, adaptive
structures. Opinions on this point vary. Balfour, and also, more
recently, Cholodkowsky, hold that the prolegs are survivals of the
embryonic appendages. Graber cautiously, after a lengthy and interesting
discussion, says that the question cannot be, in the present state of
our knowledge, solved. He, however, seems inclined to believe that the
prolegs are not merely secondary structures, and that the rudiments of
limbs may remain for a long time in a latent state before their final
development. Korschelt and Heider are disposed to regard the abdominal
appendages of Lepidoptera and Hymenoptera as true limbs, referring to
Balfour’s statement that in the Crustacea there are different examples
of the loss and later appearance of limbs, such as the loss of the
mandibular palpi of the zoëa of decapods, and the loss in the zoëa of
appendages in the Erichthus form of the Squilla larva corresponding to
the third pair of maxillipedes and first two pairs of legs of Decapoda,
and which are afterwards reproduced; similar cases occurring in the
Acarina. In the wasps and bees also, as is well known, the imaginal
disks of the thoracic appendages appear, the legs themselves being
suppressed in the larva (the imaginal disk probably existing in an
indifferent state), to reappear in the pupa and imago. It does not,
however, necessarily follow that the numerous pairs of hooked ventral
tubercles of certain dipterous larvæ (Ephydra) are true appendages.
It seems to us that it is a strong argument for the view that these
prolegs are survivals of primitive limbs, that from similar embryonic
paired outgrowths on different segments arise the spring of Podurans,
the anal cerci, and three pairs of appendages forming the ovipositor,
and the anal legs of the Corydalus larva, as well as those of
caddis-worms; at least five abdominal segments throughout the class of
insects as a whole bearing appendages in the adult.
On the other hand the view of Haase, that the prolegs of caterpillars
are secondary, adaptive characters, is supported by the fact of the
rapidity with which two pairs on the 3d and 4th segments nearly
disappear in the larvæ of certain Noctuidæ (Catocala, etc.), a reduction
evidently due to disuse.
=The tracheæ.=—The tracheal system arises as ectodermal invaginations on
one side of the appendages, appearing soon after the latter. The
earliest condition of the tracheal invagination is seen in section at
Fig. 539, _E_, _tr_; as it deepens, it sends off diverticula or tracheal
branches, while the narrow mouth of the invagination forms the stigma.
The cup-like cavities situated serially one behind the other, and
arising from the single tracheal invaginations, become at the end or
bottom of the cup elongated along the length of the body and fused
together at their ends; then the two longitudinal stems of the system
arise, by a breaking through at the place where the original
invagination had become fused, thus forming a continuous tube, the
lumina opening into each other. (Bütschli.)
The cuticular tracheal intima is differentiated late in embryonic life.
The entrance of the air is accomplished in part before the embryo
hatches, the air being derived from the tissues and fluids of the body.
The farther development of the tracheal branches is due to the
progressive formation of diverticula. The branches thus arising are
intercellular formations. On the other hand, the finest twigs are
intercellular structures. However, as Schaeffer states, the
differences between the two modes of formation are not important.
Wheeler mentions the existence of “two pairs of very indistinct
tracheal openings in the 10th and 11th somites” of the abdomen of
Doryphora (Fig. 546, _t_{19}_, _t_{20}_), and Heider believes that
they exist in Hydrophilus.
The tracheal invaginations as a rule begin to appear after the
appendages commence to bud out. An exception is met with in the bee
(Apis), where the tracheal ingrowths are seen before the rudiments of
the legs. Most of the tracheal invaginations appear simultaneously. Only
rarely do we see an indication of their successive development from
before backwards. Thus in Hydrophilus, Graber observed that the
mesothoracic stigmata appeared somewhat earlier than those of the other
segments.
_h._ Nervous system
The rudiments of the nervous and tracheal systems essentially contribute
to the building up of the relief of the primitive band of insects. The
nervous system is the earliest to appear, being indicated very early, in
fact before the appendages begin to grow out. The first traces of the
nervous system are two ridges extending along the primitive band, the
depression between them being called the primitive furrow. At an early
period the segmentation is observed in the primitive ridges, while
widened spaces (the rudiments of the ventral ganglia) alternate
segmentally with the narrow places which are the incipient longitudinal
commissures (Fig. 527, _A_, _g_).
The primitive ridges extend anteriorly into the head-lobes; this part
must be regarded as the rudiment of the œsophageal commissure. The
rudiments of the brain are from their first appearance directly
connected with the ventral chain of ganglia.[83]