Guide to the study of insects, and a treatise on those injurious and beneficial to crops: for the use of colleges, farm-schools, and agriculturists online

tail are folded back upon the body of the embryo, the head
especially being sunk backwards down into the yolk-mass.

In a succeeding stage, as we have observed in the embryo of
Diplax, a Dragon-fly (Fig. 57), the head is partially sketched





123



Fig. 58.

out, with the rudiments of the limbs and mouth-parts ; and the
sternites, or ventral walls, of the thorax and of the two basal
rings of the head appear. The anterior part of the head, in-
cluding the so-called u procephalic lobes" overhangs and con-

FIG. 57. Side view of embryo. The procephalic lobes are not shown. 1, antennae;
2, mandibles; 3, maxillae; 4, second maxillae (labium); 5-7, legs. These numbers
and letters are the same in all the figures from 57-60. The under-side (sternum)
of six segments are indicated. FIG. 58. Ventral view of the same.



56 THE CLASS OF INSECTS.

ceals the base of the antennae. It is probable that more
careful observation would have shown the end of the abdomen
folded back upon the dorsal region, as usual at this period in
the embryos of those insects whose embryology has been
studied.

The antennae, mandibles, and maxillae form a group by them-
selves, while the second maxillae (or labium) are very much
larger and turned backwards, being temporarily grouped with
the legs.

There are traces only of the two basal sterna of the abdo-
men. This indicates that the basal abdominal segments grow
in succession from the base of the abdomen, the middle ones
appearing last. The post-'abdomen (Fig. 59 A) has probably
been developed synchronous with the procephalic lobes, as it is
in all insect and crustacean embryos yet observed. As stated
by Zaddach, these two lobes in their development are exact
equivalents; antero - posterior symmetry is very clearly de-
marked, the two ends of the body at first looking alike. But
in this stage, after the two ends of the body have been evolved
from the primitive cell-layer, development in the post-abdomi-
nal region is retarded, that of the head progressing with much
greater rapidity.

In the next stage (not figured) the yolk is completely walled
in, though no traces of segments appear on the back or side of
the embryo. The reA r olution of the embryo has taken place ;
the post-abdomen being curved beneath the body, and the back
presenting outwards.

The rudiments of the eyes appear as a darker, rounded mass
of cells indistinctly seen through the yolk-granules, and situ-
ated at the base of the antennae. They consist of a few epithe-
lial cells of irregular form, the central one being the largest.

The second maxillae are a little over twice the length of the
first maxillae and are grouped with the legs, being curved back-
wards. They are, however, now one-third shorter than the an-
terior legs. The second maxillary sternum is still visible.

The tip of the abdomen (or post-abdomen) consists of four
segments, the terminal one being much the larger, and ob-
scurely divided into two obtuse lobes.

The abdominal sternites are now well marked, and the ner-



THE DEVELOPMENT OF INSECTS.



57



32 E




A 7



vous cord is represented by eight or nine large oblong-square
(seen sideways) ganglia, which lie contiguous to each other.

The formation of the eyes, the post-abdomen, the sternites,
and median portion of the nervous cord seems nearly synchro-
nous with the closing up of the dorsal walls of the body, though
the division of the tegument into segments has not apparently
taken place over the yolk-mass.

The succeeding stage (Fig. 59) is signalized by the appear-
ance of the rudiments of tne intestine,
while the second maxillae are directed
more anteriorly.

In form the body is ovate-cylin-
drical, and there is a deep constric-
tion separating the post -abdomen
from the anterior part of the abdo-
men.

The terminal (eleventh) ring is
immensely disproportioned to its size in the embryo just pre-
vious to hatching (see Fig. 61, where it forms a triangular piece

situated between its appendages,
the anal stylets). At a later
period of this stage two more ab-
dominal segments have been added,
one to the end of the main body
of the abdomen, and another to
the post- abdomen. They have
been apparently interpolated at the
junction of the post-abdomen to
the abdomen proper. Should this
observation be proved to be correct, it may then be considered
as a rule that, after reaching a certain number of segments, all
additional ones are interpolated between the main body of the
abdomen and its terminal segment or segments. This is the
law of increase in the number of segments in Worms, and in
Myriapods (lulus, according to Newport's observations), in
Arachnids (Claparede), and Crustacea (Rathke).

The next stage (Fig. 60), is characterized by the different ia-




C123



FIG. 59. An embryo much farther advanced, c, clypeus; E, eye; A, bi-lobed
extremity of the abdomen; i, the rudiments of the intestines.



"58



THE CLASS OF INSECTS.



tion of the head into the rudiments of the ophthalmic ring, and
the supraclypeal piece, and clypeus, together with the approx-
imation of the second pair of maxillae, which, when united, form
the labium, the extremities of which are now situated in the
middle of the body.

The antennae now extend to the middle of the labium, just
passing beyond the extremities of the mandibles and maxillae.
The oesophagus can now be seen going from the mouth-opening
situated just beneath the labium. It curves around just behind
the eyes. There are at this period no appearances of movable
blood-disks or of a dorsal vessel.

The abdomen is now pointed at the extremity and divided
into the rudiments of the two anal stylets, which form large,
acute tubercles. The yolk-mass is now almost
entirely inclosed within the body walls, form-
ing an oval mass.

Another embryo, observed July 27th, had
T reached about the same stage of growth. The
front of the head, including the antennary
segment, is farther advanced than before. The
entire head is divided into two. very distinct
regions ; i. e. one before the mouth-opening
(the preoral region, including the ocellary, or
first and second segments ; the ophthalmic, or
third segment, and antennary, or fourth seg-
ment of the head) ; and the other behind the
mouth (postoral, consisting of the mandibular,
or fifth segment, the first maxillary, or sixth
segment, and the second maxillary, or labial, being the seventh
and last cephalic ring.

At a later period the embryo is quite fully formed, and is
about ready to leave the egg. The three regions of the body
are now distinct. The articulations of the tergum are present,
the yolk-mass being completely inclosed by the tergal walls.

FIG. 61. The embryo taken from the egg, but nearly ready to hatch. T, the
dotted line crosses the main trachea, going through the yolk-mass, now restricted
to the thoracic region. At x, the tracheae send off numerous branches around an
enlargement of the intestine (colon), where the blood is aerated; better seen in fig.
62. The abdomen consists of eleven segments, the last being a minute triangular
piece.




Fig. 61.



THE DEVELOPMENT OF INSECTS.



59



The body is so bent upon itself that the extremities of the
second maxilla? just overlap the tip of the abdomen.

The two limbs of the labium are now placed side by side,
with the prominent spinous appendage on the outer edges of
the tip. These spines are the rudiments of the labial palpi.

The general form of the embryo at a still later period (Fig.
61), on being taken from the egg and straightened out, re-
minds us strikingly of
the Thysanura, and, in
these and other re-
spects, tend to prove
that the Podurse and
Lepismae, and allied
genera, are embryonic,
degraded forms of Neu-
roptera, and should
therefore be considered
as a family of that sub-
order. Seen laterally,
the body gradually ta-
pers from the large
head to the pointed ex-
tremity. The body is
flattened from above
downwards. At this
stage the appendages
are still closely ap-
pressed to the body.

Just before the ex-
clusion of the embryo,
the legs and mouth-
parts stand out freer Fig. 62.
from the body. The labium, especially, assumes a position at
nearly right angles to the body. The antennae, mandibles,
and maxillae have taken on a more definite form, being like

FIG. 62. The larva just hatched and swimming in the water. N, ventral cord or
nervous ganglia; D, dorsal vessel, or "heart," divided into its chambers. The
anal valves at the end of the abdomen, which open and shut during respiration, are
represented as being open. Both of the dotted lines cross the trachete. x, net-
work of the tracheae, surrounding the cloaca.




60



THE CLASS OF INSECTS,




Fig. 63.



that of the young larva, and stand out free from the body.
The head is much smaller in proportion to the rest of the
body, and bent more upon the breast.

The Larva (Fig. 62)
when hatched is about
five hundredths of an
inch in length. The
head is now free and
the antennae stand out
free from the front.
The thorax has greatly
diminished in size,
while the abdomen has
become wider, and the
limbs very long; and
the numerous minute tubercles, seen in the preceding stage,
have given origin to hairs. The dorsal vessel can now, for the
first time, be seen. When in motion, the resemblance
to a spider is most striking. The flow of blood to
the head, and the return currents through the lacunar
or venous circulation along the side of the body were
easily observed. The vessels were not crowded with
blood disks, the latter being few in number, only one
Fig. 64. or two passing along at a time. Two currents, pass-
ing in opposite directions, were observed in the legs.

FIG. 63. Side view of the head of the larva of Diplax before the first moult, c,
deciduous tubercles terminating in a slender style; their use is unknown; they
have not been observed in the full-grown larva, e, the compound eyes. 1, the
three jointed antennae, the terminal joint nearly three times as long as the two
basal ones. 2, the mandibles, and also enlarged, showing the cutting edge divided
into four teeth. 3, maxillae divided into two lobes : d, the outer and anterior lobe,
2-jointed, the basal joint terminating in two setae ; and a, the inner lobe concealed
from view, in its natural position, by the outer lobe, d. 4, the base or pedicel of
the second maxillre, or labium, the expanded terminal portion being drawn sepa-
rately; d and o, two movable stout styles representing, perhaps, the labial palpi;
the lobe to which they are attached is multidentate, and adapted for seizing
prey; on the right side the two styles are appressed to the lobe, x represents,
perhaps, the ligula; but we have not yet studied its homologies carefully: this
part is attached to a transversely linear piece soldered to the main part of the
labium. y, the llth abdominal ring, with its pair of conical anal styles, z, the
last tarsal joint and pair of long slender claws.

FIG. 64. The pupa of Diplax, having rudimentary rings, in which the eyes are
much larger, and the legs much shorter than in the recently hatched larva; in-
troduced to be compared with the young larva. Figs. 57-64, original.




TRANSFORMATIONS OF THE INSECT. Cl

On review it will be seen how remarkable are the changes in
form of the insect before it is hatched, and that all are the
result of simple growth. We have seen that the two ends of
the body are first formed, and that the under side of the body
is formed before the back ; that the belly is at first turned out-
wards, and afterwards the embryo reverses its position, the
back presenting outwards. All the appendages are at first
simple protrusions from the body-walls, and new segments are
interpolated near the tip of the abdomen. These changes take
place very rapidly, within a very few days, and some of the
most important and earlier ones in a few hours. We can now
better understand that the larva and pupa stages are the result
of a similar mode of growth, though very marked from being
in a different medium, the insect having to seek food and act
as an independent being.

TRANSFORMATIONS OF THE INSECT. We have seen that
during the growth of the embryo, the insect undergoes remark-
able changes of form, the result of simple growth. The meta-
morphoses of the animal within the egg are no less marked
than those which occur after it has hatched. It will also be
seen that the larva and pupa stages are not always fixed, defi-
nite states, but only pauses in the development of the insect,
concealing beneath the larva and pupa skins the most impor-
tant changes of form.

The process of hutching. No other author has so carefully
described the process of hatching as Newport, who observed
it in the larva of Meloe. "When the embryo larva is ready
for its change, the egg-shell becomes thinned and concave on
that side which covers the ventral surface of the body, but is
much enlarged, and is more convex on the dorsal, especially
towards the head. The shell is then burst longitudinally along
the middle of the thoracic segments, and the fissure is ex-
tended forwards to the head, which then, together with the
thoracic segments, is partially forced through the opening, but
is not at once entirely withdrawn. The antennae, parts of the
mouth, and legs are still inclosed within separate envelopes,
and retain the larva in this covering in the shell. Efforts are
then made to detach the posterior segments of the body, which



62 THE CLASS OF INSECTS.

are gradually released, and with them the antennae, palpi, and
legs, and the larva removes itself entirely from the shell and
membranes. In this process of evolution the young Meloe
throws off two distinct coverings : first, the shell with its lining
membrane, the analogue of the membrane in which, as I have
elsewhere shown,* the young Myriapod is inclosed, and re-
tained several days after the bursting of the ovum, and which
represents in the Articulata, not the allantois, but apparently
the amnion, of Vertebrata ; next, the first, or foetal deciduation
of the tegument, analogous probably to the first change of skin
in the Myriapod, after it has escaped from the amnion, and
also to the first change which the young Arachnidan invariably
undergoes a few days after it has left the egg, and before it
can take food. This tegument, which, perhaps, may be analo-
gous to the vernix caseosa of Vertebrata, thrown off at the
instant of birth, is left by the young Meloe with the amnion
in the shell ; and its separation from the body, at this early
period, seems necessary to fit the insect for the active life it
has commenced." (Linn. Trans, xx. p. 006, etc.)

The larva state. The larva (Latin larva, a mask) was so-
called because it was thought to mask the form of the perfect
insect. The larvae of Butterflies and Moths are called cater-
pillars; those of Beetles, grubs; and those of the two-winged
Flies (Diptera) maggots; the larvae of other groups have no
distinctive common names.

As soon as it is hatched the larva feeds voraciously, as if in
anticipation of the coming period of rest, the pupa state, for
which stores of fat (the fatty bodies) are developed for the
supply of fat globules out of which the tissues of the new
body of the pupa and imago are to be formed.

Most larvae moult, or change their skin, four or five times.
In the inactive thin-skinned larvae, such as those of Bees,
Wasps, and Gall-flies, the moults are not apparent ; as the
larva increases in size it out-grows the old skin, which comes
off in thin shreds. But in the active larvae, such as cater-
pillars, grasshoppers, and grubs, from the rapid deposition of
chitine in the outer layers of the skin, just before the change,

* Philosophical Transactions, Pt. 2, 1841, p. ill.



TRANSFORMATIONS OF THE INSECT. 63

it becomes hard mid dry, and too small for the growing in-
sect, and is then cast off' entire.

A series of bee-larvae can be selected showing a graduation
in size and form from the egg and recently hatched larva up to
the full-grown larva. In the caterpillar and other active larvae,
there are usually four or five stages, each showing a sudden and
marked increase in size. Newport states that the caterpillar
of sphinx ligustri moults six times, and at the last moult be-
comes a third larger than at any earlier period; the larva of
Arctia caja moults from five to ten times.

A few days before the assumption of the pupa state, the
larva becomes restless, stops eating, and deserts its food, and
usually spins a silken cocoon, or makes one of earth, or chips,
if a borer, and there prepares for the change to the pupa state.

During this semipupa period (lasting, in many insects, only
for a day or several days, but in some Saw-flies through the
winter) the skin of the pupa grows beneath that of the quies-
cent larva. While the worm-like larva exhibits no trire-
gional distinctions, the muscles of the growing pupa contract
and enlarge in certain parts so as to modify the larva form,
until it gradually assumes the triregional form of the adult
insect, with the differentiation of the body into a head, thorax,
and abdomen.

In a series of careful studies, abundantly illustrated with
excellent plates, Weismann has recently show r n that Swammer-
dam's idea that the pupa and imago skins were in reality
already concealed under that of the larva is partially founded
in truth. Swammerdam states, "I can point out in the larva
all the limbs of the future nymph, or Culex, concealed beneath
the skin," and he also observed beneath the skin of the larvae
of bees just before pupating, the antennae, mouth-parts, wings,
and limbs of the adult. (Weismann.)

During its transformations the pupa skin is developed from
the lypodermis, or inner layer of skin. This peals off, as it
were, from the inner layer of the old larva skin, which soon
dries and hardens, and is thrown off. Meanwhile the muscles
of the body contract and change in form, thus causing the origi-
nal segments of the larva to infold and contract at certain parts,
gradually producing the pupa form. If, during this period, the



64 THE CLASS OF INSECTS.

insect be examined at intervals, a series of slight changes of
form may be seen, from the larva to the imago state. In some
cases each change is accompanied by a moult, as in the "ac-
tive" Ephemera, where Lubbock counted twenty moults.

As a general rule, then, it may be stated that the body of
the larva is transformed into that of the imago ; ring answer-
ing to ring, and limb to limb in both, the head of the one
is homologous with that of the other, and the appendages of
the larva are homologous with the appendages of the imago.

Weismann has shown that in the larva of the Meat-fty, Musca
vomitoria, the thorax and head of the imago are developed
from what he calls "imaginal disks." These disks are minute
isolated portions of the hypodermis, which are formed in the
embryo, before it leaves the egg, and are held in place within
the body-cavity of the larva by being attached either to nerves
or tracheae, or both. After the outer layer of the larva skin
dries and hardens, and forms the cask-shaped puparium, the
use of which corresponds to the cocoon of moths, etc., these
imaginal disks increase in size so as to form the tegument of
the thorax and head. The abdomen of the Meat-fly, however,
is formed by the direct conversion of the eight hinder segments
of the body of the larva, into the corresponding segments of
the imago.

Accompanying this change in the integument there is a
destruction of all the larval system of organs ; this is either
total or effected by the gradual destruction of tissues. Now
we see the use of the "fatty body;" this breaks up, setting
free granular globules of fat, which, as we have seen in the
embryo, produces by the multiplication of cells the new tissues
of the pupa. ' Thus the larva-skin is cast aside, and also the
softer organs within, but the formation of new tissues keeps
even pace with the destruction of the old, and the insect pre-
serves its identity throughout. The genital glands, however,
are indicated even in the embryo, and are gradually developed
throughout the growth of the insect, so that this histolysis, or
destruction of tissues, is not wholly complete. The quiescent
pupa-state of Musca is long-continued, and its vitality is latent,
the acts of respiration and circulation being almost suspended.
(Weismann. )



TRANSFORMATIONS OF THE INSECT. G5

In the metamorphosis of Corethra, a Mosquito-like Fly, which
is active both in the larva and pupa states, "the segments of
tlif larva are converted directly into the corresponding seg-
ments of the body of the imago, the appendages of the head
into the corresponding ones of the head of the imago ; those
of the thorax arc produced after the last moult of the larva
as ilfverticula of the hypodermis round a nerve or trachea,
from the cellular envelope 'of which the formation of tissue in
the interior of the appendages issues. The larval muscles of
the abdominal segments are transferred unchanged into the
imago ; the thoracic muscles peculiar to the imago, as also
some additional abdominal muscles, are developed in the last
larval periods from indifferent cellular cords which are indi-
cated even in the egg. The genital glands date back to the
embryo, and are gradually developed ; all the other systems of
organs pass with little or no alteration into the imago. Fatty
body none or inconsiderable. Pupa-state short and active."
(Weismann.)

As the two types are most clearly discriminated by the
presence or absence of true imaginal disks, Weismann suggests
that those insects which undergo a marked metamorphosis
might be divided into Insecta discota (or Insects with imaginal
disks), and those without, into Insecta adiscota.

The metamorphosis of Corethra may prove to be a type of
that of all insects which are active in their preparatory stages ;
and that of Musca typical of all those that are quiescent in the
pupa-state, at least the Lepidoptera and those Diptera which
have a coarctate * pupa, together with the Coleoptera and those
Neuroptera in which the metamorphosis is complete, as Phry-
ganea, Hemerobius, etc.

The transformations of the Humble-bee are easily observed
by taking a nest after the first brood have matured, when we
shall find individuals in all stages of development from the
larva to the imago state. The figures below show four stages,
but in reality there is every gradation between these stages.

*The larvae of some of the higher Diptera spin a slight cocoon, while the true
flies, such as the Muscidae and Syrphidae, etc., change to pupae within the larva
skin which contracts into a cylindrical "puparium" corresponding in use to the
cocoon; such pupae are called "coarctate."
5



66



THE CLASS OF INSECTS.



Fig. 64 shows what we may call the semipupa, concealed by
the larval skin. There are eleven pairs of stigmata, three
thoracic and eight abdominal. The head of the semi-pupa
lies under the head (a) and prothoracic ring (b). The basal
ring of the abdomen (c), or fourth ring from the head, is un-
changed in form. This figure also will suffice to represent



g a





Fig. 65.



771




Fig. 67.

the larva, though a little more produced anteriorly than in
its natural form.

In another stage (Fig. 65) of the semi-pupa, the larval skin
is entirely sloughed off, the two pairs of wing-pads lying paral-
lel, and very equal in size, like the wings of Neuroptera. The
thoraco- abdominal ring, or propodeum (c), is distinguished by
its oblong spiracle (n), essentially differing from those on
the abdomen. At this point the body contracts, but the head



TRANSFORMATIONS OF THE INSECT. 67

and thorax together are yet, as still more in the previous
stage, much smaller than in the pupa, and there is still a con-
tinuous curve from the tip of the abdomen to the head. (0,
antenna; /*, lingua, maxilla;, and palpi; i, fore-legs; j, mid-
dle legs ; A 1 , meso-scutum ; /, meso-scutellum ; ?i, spiracle of
the .propodeum.)

In a succeeding stage (Fig. 66) of the semi-pupa, the head
and thorax together nearly equal in size the abdomen, and the
propodeum (c) has become entirely transferred to the thorax.
The head has become greatly enlarged ; the rings are very un-
equal, the hinder pair are much smaller, and overlaid by the
anterior pair ; the three terminal pair of abdominal rings, so
large in Fig. 65, have been absorbed, and partially inclosed in
the cavity of the abdomen ; and there has been a farther dif-
ferentiation of the ring into the sternite ((/), pleurite (e), and