[Illustration: After Marlatt, _Ent. Bull._ 4, n. s. (U.S. Dept. Agr.).
FIG. 20.--a, Bed-bug (_Cimex lectularis_, Linn.); newly hatched young
from beneath; b, from above; d, egg, magnified; c, foot with claws; e,
serrate spine, more highly magnified.]
[Illustration: From Mally, _Ent. Bull._ 24 (U.S. Dept. Agr.).
FIG. 21.--e, f, Owl moth (_Heliothis armigera_); a, b, egg, highly
magnified; c, larva or caterpillar; d, pupa in earthen cell.]
After hatching or birth an insect undergoes a process of growth and
change until the adult condition is reached. The varied details of this
post-embryonic development furnish some of the most interesting facts
and problems to the students of the Hexapoda. Wingless insects, such as
spring-tails and lice, make their appearance in the form of miniature
adults. Some winged insects--cockroaches, bugs (fig. 20) and earwigs,
for example--when young closely resemble their parents, except for the
absence of wings. On the other hand, we find in the vast majority of the
Hexapoda a very marked difference between the perfect insect (imago) and
the young animal when newly hatched and for some time after hatching.
From the moth's egg comes a crawling caterpillar (fig. 21, c), from the
fly's a legless maggot (fig. 25, a). Such a young insect is a _larva_--a
term used by zoologists for young animals generally that are decidedly
unlike their parents. It is obvious that the hatching of the young as a
larva necessitates a more or less profound transformation or
metamorphosis before the perfect state is attained. Usually this
transformation comes with apparent suddenness, at the penultimate stage
of the insect's life-history, when the passive pupa (fig. 21, d) is
revealed, exhibiting the wings and other imaginal structures, which have
been developed unseen beneath the cuticle of the larva. Hexapoda with
this resting pupal stage in their life-history are said to undergo "a
complete transformation," to be metabolic, or holometabolic, whereas
those insects in which the young form resembles the parent are said to
be ametabolic. Such insects as dragon-flies and may-flies, whose young,
though unlike the parent, develop into the adult form without a resting
pupal stage are said to undergo an "incomplete transformation" or to be
hemimetabolic. The absence of the pupal stage depends upon the fact that
in the ametabolic and hemimetabolic Hexapoda the wing-rudiments appear
as lateral outgrowths (fig. 22) of the two hinder thoracic segments and
are visible externally throughout the life-history, becoming larger
after each moult or casting of the cuticle. Hence, as has been pointed
out by D. Sharp (1898), the marked divergence among the Hexapoda, as
regards life-history, is between insects whose wings develop outside the
cuticle (Exopterygota) and those whose wings develop inside the cuticle
(Endopterygota), becoming visible only when the casting of the last
larval cuticle reveals the pupa. Metamorphosis among the Hexapoda
depends upon the universal acquisition of wings during post-embryonic
development--no insect being hatched with the smallest external
rudiments of those organs--and on the necessity for successive castings
or "moults" (ecdyses) of the cuticle.
[Illustration: After Howard, _Insect Life_, vol. vii.
FIG. 22.--Nymph of Locust (_Schistocera americana_), showing
wing-rudiments.]
_Ecdysis._--The embryonic ectoderm of an insect consists of a layer of
cells forming a continuous structure, the orifices in it--mouth,
spiracles, anus and terminal portions of the genital ducts--being
invaginations of the outer wall. This cellular layer is called the
hypodermis; it is protected externally by a cuticle, a layer of matter
it itself excretes, or in the excretion of which it plays, at any rate,
an important part. The cuticle is a dead substance, and is composed in
large part of chitin. The cuticle contrasts strongly in its nature with
the hypodermis it protects. It is different in its details in different
insects and in different stages of the life of the same insect. The
"sclerites" that make up the skeleton of the insect (which skeleton, it
should be remembered, is entirely external) are composed of this
chitinous excretion. The growth of an insect is usually rapid, and as
the cuticle does not share therein, it is from time to time cast off by
moulting or ecdysis. Before a moult actually occurs the cuticle becomes
separated from its connexion with the underlying hypodermis. Concomitant
with this separation there is commencement of the formation of a new
cuticle within the old one, so that when the latter is cast off the
insect appears with a partly completed new cuticle. The new instar--or
temporary form--is often very different from the old one, and this is
the essential fact of metamorphosis. Metamorphosis is, from this point
of view, the sum of the changes that take place under the cuticle of an
insect between the ecdyses, which changes only become externally
displayed when the cuticle is cast off. The hypodermis is the immediate
agent in effecting the external changes.
[Illustration: Adapted from Koerschelt and Herder and Lowne.
FIG. 23.--Diagram showing position of imaginal buds in larva of fly. I.,
II., III., the three thoracic segments of the larva; 1, 2, 3, buds of
the legs of the imago; h, bud of head-lobes; f, of feeler; e of eye; b,
brain.]
The study of the physiology of ecdysis in its simpler forms has
unfortunately been somewhat neglected, investigators having directed
their attention chiefly to the cases that are most striking, such as
the transformation of a maggot into a fly, or of a caterpillar into a
butterfly. The changes have been found to be made up of two sets of
processes: histolysis, by which the whole or part of a structure
disappears: and histogenesis, or the formation of the new structure.
By histolysis certain parts of the hypodermis are destroyed, while
other portions of it develop into the new structures. The hypodermis
is composed of parts of two different kinds, viz. (1) the larger part
of the hypodermis that exists in the maggot or caterpillar and is
dissolved at the metamorphosis; (2) parts that remain comparatively
quiescent previously, and that grow and develop when the other parts
degenerate. These centres of renovation are called imaginal disks or
folds. The adult caterpillar may be described as a creature the
hypodermis of which is studded with buds that expand and form the
butterfly, while the parts around them degenerate. In some insects
(e.g. the maggots of the blowfly, _Calliphora vomitoria_) the imaginal
disks are to all appearance completely separated from the hypodermis,
with which they are, however, really organically connected by strings
or pedicels. This connexion was not at first recognized and the true
nature of imaginal disks was not at first perceived, even by Weismann,
to whom their discovery in Diptera is due. In other insects the
imaginal disks are less completely disconnected from the superficies
of the larval hypodermis, and may indeed be merely patches thereof.
The number of imaginal disks in an individual is large, upwards of
sixty having been discovered to take part in the formation of the
outer body of a fly. With regard to the internal organs, we need only
say that transformation occurs in an essentially similar manner, by
means of a development from centres distributed in the various organs.
The imaginal disks for the outer wall of the body, some of them, at
any rate, include mesodermal rudiments (from which the muscles are
developed) as well as hypodermis. The imaginal disks make their
appearance (that is, have been first detected) at very different
epochs in the life; their absolute origin has been but little
investigated. Pratt has traced them in the sheep-tick (_Melophagus_)
to an early stage of the embryonic life.
_Histolysis and Histogenesis._--The process of destruction of the
larval tissues was first studied in the forms where metamorphosis is
greatest and most abrupt, viz. in the Muscid Diptera. It was found
that the tissues were attacked by phagocytic cells that became
enlarged and carried away fragments of the tissue; the cells were
subsequently identified as leucocytes or blood-cells. Hence the
opinion arose that histolysis is a process of phagocytosis. It has,
however, since been found that in other kinds of insects the tissues
degenerate and break down without the intervention of phagocytes. It
has, moreover, been noticed that even in cases where phagocytosis
exists a greater or less extent of degeneration of the tissue may be
observed before phagocytosis occurs. This process can therefore only
be looked on as a secondary one that hastens and perfects the
destruction necessary to permit of the accompanying histogenesis. This
view is confirmed by the fate of the phagocytic cells. These do not
take a direct part in the formation of the new tissue, but it is
believed merely yield their surplus acquisitions, becoming ordinary
blood-cells or disappearing altogether. As to the nature of
histogenesis, nothing more can be said than that it appears to be a
phenomenon similar to embryonic growth, though limited to certain
spots. Hence we are inclined to look on the imaginal disks as cellular
areas that possess in a latent condition the powers of growth and
development that exist in the embryo, powers that only become evident
in certain special conditions of the organism. What the more essential
of these conditions may be is a question on which very little light
has been thrown, though it has been widely discussed.
Much consideration has been given to the nature of metamorphosis in
insects, to its value to the creatures and to the mode of its origin.
Insect metamorphosis may be briefly described as phenomena of
development characterized by abrupt changes of appearance and of
structure, occurring during the period subsequent to embryonic
development and antecedent to the reproductive state. It is, in short, a
peculiar mode of growth and adolescence. The differences in appearance
between the caterpillar and the butterfly, striking as they are to the
eye, do not sufficiently represent the phenomena of metamorphosis to the
intelligence. The changes that take place involve a revolution in the
being, and may be summarized under three headings: (1) The
food-relations of the individual are profoundly changed, an entirely
different set of mouth-organs appears and the kind and quantity of the
food taken is often radically different. (2) A wingless, sedentary
creature is turned into a winged one with superlative powers of aerial
movement. (3) An individual in which the reproductive organs and powers
are functionally absent becomes one in which these structures and powers
are the only reason for existence, for the great majority of insects die
after a brief period of reproduction. These changes are in the higher
insects so extreme that it is difficult to imagine how they could be
increased. In the case of the common drone-fly, _Eristalis tenax_, the
individual, from a sedentary maggot living in filth, without any
relations of sex, and with only unimportant organs for the ingestion of
its foul nutriment, changes to a creature of extreme alertness, with
magnificent powers of flight, living on the products of the flowers it
frequents, and endowed with highly complex sexual structures.
[Illustration: After Westwood, _Modern Classification_.
FIG. 24.--Campodeiform Larva of a Ground-Beetle (_Aepus marinus_).
Magnified.]
[Illustration: After Howard, _Ent. Bull._ 4, n. s. (_U.S. Dept. Agr._).
FIG. 25.--Vermiform Larva (maggot) of House-fly (_Musca domestica_).
Magnified. b, spiracle on prothorax; c, protruded head region; d,
tail-end with functional spiracles; e, f, head region with mouth hooks
protruded; g, hooks retracted; h, eggs. All magnified.]
_Forms of Larva._--The unlikeness of the young insect to its parent is
one of the factors that necessitates metamorphosis. It is instructive,
further, to trace among metabolic insects an increase in the degree of
this dissimilarity. An adult Hexapod is provided with a firm,
well-chitinized cuticle and six conspicuous jointed legs. Many larval
Hexapods might be defined in similar general terms, unlike as they are
to their parents in most points of detail. Examples of such are to be
seen in the grubs of may-flies, dragon-flies, lacewing-flies and
ground-beetles (fig. 24). This type of active, armoured larva--often
bearing conspicuous feelers on the head and long jointed cercopods on
the tenth abdominal segment--was styled campodeiform by F. Brauer
(1869), on account of its likeness in shape to the bristle-tail
_Campodea_. As an extreme contrast to this campodeiform type, we take
the maggot of the house-fly (fig. 25)--a vermiform larva, with soft,
white, feebly-chitinized cuticle and without either head-capsule or
legs. Between these two extremes, numerous intermediate forms can be
traced: the grub (wireworm) of a click-beetle, with narrow elongate
well-armoured body, but with the legs very short; the grub of a chafer,
with the legs fairly developed, but with the cuticle of all the
trunk-segments soft and feebly chitinized; the well-known caterpillar of
a moth (fig. 21, e) or saw-fly, with its long cylindrical body, bearing
the six shortened thoracic legs and a variable number of pairs of
"pro-legs" on the abdomen (this being the eruciform type of larva); the
soft, white, wood-boring grub of a longhorn-beetle or of the saw-fly
_Sirex_, with its stumpy vestiges of thoracic legs; the large-headed but
entirely legless, fleshy grub of a weevil; and the legless larva, with
greatly reduced head, of a bee. The various larvae of the above series,
however, have all a distinct head-capsule, which is altogether wanting
in the degraded fly maggot. These differences in larval form depend in
part on the surroundings among which the larva finds itself after
hatching; the active, armoured grub has to seek food for itself and to
fight its own battles, while the soft, defenceless maggot is provided
with abundant nourishment. But in general we find that elaboration of
imaginal structure is associated with degradation in the nature of the
larva, eruciform and vermiform larvae being characteristic of the
highest orders of the Hexapoda, so that unlikeness between parent and
offspring has increased with the evolution of the class.
_Hypermetamorphosis._--Among a few of the beetles or Coleoptera (q.v.),
and also in the neuropterous genus _Mantispa_, are found life-histories
in which the earliest instar is campodeiform and the succeeding larval
stages eruciform. These later stages, comprising the greater part of the
larval history, are adapted for an inquiline or a parasitic life, where
shelter is assured and food abundant, while the short-lived, active
condition enables the newly-hatched insect to make its way to the spot
favourable for its future development, clinging, for example, in the
case of an oil-beetle's larva, to the hairs of a bee as she flies
towards her nest. The presence of the two successive larval forms in the
life-history constitutes what is called hypermetamorphosis. Most
significant is the precedence of the eruciform by the campodeiform type.
In conjunction with the association mentioned above of the most highly
developed imaginal with the most degraded larval structure, it indicates
clearly that the active, armoured grub preceded the sluggish
soft-skinned caterpillar or maggot in the evolution of the Hexapoda.
_Nymph._--The term nymph is applied by many writers on the Hexapoda to
all young forms of insects that are not sufficiently unlike their
parents to be called larvae. Other writers apply the term to a "free"
pupa (see _infra_). It is in wellnigh universal use for those instars of
ametabolous and hemimetabolous insects in which the external
wing-rudiments have become conspicuous (fig. 27). The mature dragon-fly
nymph, for example, makes its way out of the water in which the early
stages have been passed and, clinging to some water-plant, undergoes the
final ecdysis that the imago may emerge into the air. Like most
ametabolic and hemimetabolic Hexapoda, such nymphs continue to move and
feed throughout their lives. But examples are not wanting of a more or
less complete resting habit during the latest nymphal instar. In some
cicads the mature nymph ceases to feed and remains quiescent within a
pillar-shaped earthen chamber. The nymph of a thrips-insect
(Thysanoptera) is sluggish, its legs and wings being sheathed by a
delicate membrane, while the nymph of the male scale-insect rests
enclosed beneath a waxy covering.
_Sub-imago._--Among the Hexapoda generally there is no subsequent
ecdysis nor any further growth after the assumption of the winged state.
The may-flies, however, offer a remarkable exception to this rule. After
a prolonged aquatic larval and nymphal life-history, the winged insect
appears as a sub-imago, whence, after the casting of a delicate cuticle,
the true imago emerges.
_Pupa._--In the metabolic Hexapoda the resting pupal instar shows
externally the wings and other characteristic imaginal organs which have
been gradually elaborated beneath the larval cuticle. It is usual to
distinguish between the free pupae (fig. 26, b)--of Coleoptera and
Hymenoptera, for example--in which the wings, legs and other appendages
are not fixed to the trunk, and the obtect pupae (fig. 21, d)--such as
may be noticed in the majority of the Lepidoptera--whose appendages are
closely and immovably pressed to the body by a general hardening and
fusion of the cuticle. In the degree of mobility there is great
diversity among pupae. A gnat pupa swims through the water by powerful
strokes of its abdomen, while the caddis-fly pupa, in preparation for
its final ecdysis, bites its way out of its subaqueous protective case
and rises through the water, so that the fly may emerge into the air.
Some pupae are thus more active than some nymphs; the essential
character of a pupa is not therefore its passivity, but that it is the
instar in which the wings first become evident externally. The division
of the winged Hexapoda into Exopteryga and Endopteryga is thus again
justified.
[Illustration: From Chittenden, _Bull._ 4 (n.s.) _Div. Ent. U.S. Dept.
Agr._
FIG. 26.--a, Saw-toothed Grain-Beetle (_Silvanus surinamensis_); b,
pupa; c, larva, magnified--; d, feeler of larva.]
If we admit that the larva has, in the phylogeny of insects, gradually
diverged from the imago, and if we recollect that in the ontogeny the
larva has always to become the imago (and of course still does so)
notwithstanding the increased difficulty of the transformation, we
cannot but recognize that a period of helplessness in which the
transformation may take place is to be expected. It is generally
considered that this is sufficient as an explanation of the existence
of the pupa. This, however, is not the case, because the greater part
of the transformation precedes the disclosure of the pupa, which, as
L. C. Miall remarks, is structurally little other "than the fly
enclosed in a temporary skin." Moreover, in many insects with
imperfect metamorphosis the change from larva or (as the later stage
of the larva is called in these cases) nymph to imago is about as
great as the corresponding change in the Holometabola, as the student
will recognize if he recalls the histories of _Ephemeridae_, Odonata
and male _Coccidae_. But in none of these latter cases have the wings
to be changed from a position inside the body to become external and
actively functional organs. The difference between the nymph or false
pupa and the true pupa is that in the latter a whole stage is devoted
to the perfecting of the wings and body-wall after the wings have
become external organs; the stage is one in which no food is or can be
taken, however prolonged may be its existence. Amongst insects with
imperfect metamorphosis the nearest approximations to the true pupa of
the Holometabola are to be found in the sub-imago of _Ephemeridae_ and
in the quiescent or resting stages of Thysanoptera, _Aleurodidae_ and
_Coccidae_. A much more thorough appreciation than we yet possess of
the phenomena in these cases is necessary in order completely to
demonstrate the special characteristics of the holometabolous
transformation. But even at present we can correctly state that the
true pupa is invariably connected with the transference of the wings
from the interior to the exterior of the body. It cannot but suggest
itself that this transference was induced by some peculiarity as to
formation of cuticle, causing the growth of the wings to be directed
inwards instead of outwards. We may remark that fleas possess no
wings, but are understood to possess a true pupa. This is a most
remarkable case, but unfortunately very little information exists as
to the details of metamorphosis in this group.
_Life-Relations._--Only a brief reference can be made here to the
fascinating subject of the life-relations of the larva, nymph and pupa,
as compared with those of the imago. For details, the reader may consult
the special articles on the various orders and groups of insects. A
common result of metamorphosis is that the larva and imago differ
markedly in their habitat and mode of feeding. The larva may be aquatic,
or subterranean, or a burrower in wood, while the imago is aerial. It
may bite and devour solid food, while the imago sucks liquids. It may
eat roots or refuse, while the imago lives on leaves and flowers. The
aquatic habit of many larvae is associated with endless beautiful
adaptations for respiration. The series of paired spiracles on most of
the trunk-segments is well displayed, as a rule, in terrestrial
larvae--caterpillars and the grubs of most beetles, for example. In many
aquatic larvae we find that all the spiracles are closed up, or become
functionless, except a pair at the hinder end which are associated with
some arrangement--such as the valvular flaps of the gnat larva or the
telescopic "tail" of the drone-fly larva--for piercing the surface film
and drawing periodical supplies of atmospheric air. A similar
restriction of the functional spiracles to the tail-end (fig. 25, d) is
seen in many larvae of flies (Diptera) that live and feed buried in
carrion or excrement. Other aquatic larvae have the tracheal system
entirely closed, and are able to breathe dissolved air by means of
tubular or leaf-like gills. Such are the grubs of stone-flies, may-flies
(fig. 27) and some dragon-flies and midges. An interesting feature is
the difference often to be observed between an aquatic larva and pupa of
the same insect in the matter of breathing. The gnat larva, for example,
breathes at the tail-end, hanging head-downwards from the surface-film.
But the pupa hangs from the surface by means of paired respiratory
trumpets on the prothorax, the dorsal thoracic surface, where the
cuticle splits to allow the emergence of the fly, being thus directed
towards the upper air.
[Illustration: From Miall and Denny (after Vayssiere), _The Cockroach_,
Lovell Reeve & Co.
FIG. 27.--Nymph of May-fly (_Chloeon dipterum_), with wing rudiments (a)
and tracheal gill-plates (b, b). Magnified--. (The feelers and legs are
cut short.)]
A marked disproportion between the life-term of larva and imago is
common; the former often lives for months or years, while the latter
only survives for weeks or days or hours. Generally the larval is the
feeding, the imaginal the breeding, stage of the life-cycle. The extreme
of this "division of labour" is seen in those insects whose jaws are
vestigial in the winged state, when, the need for feeding all behind
them, they have but to pair, to lay eggs and to die. The acquisition of
wings is the sign of developed reproductive power.
_Paedogenesis._--Nevertheless, the function of reproduction is
occasionally exercised by larvae. In 1865 N. Wagner made his classical
observations on the production of larvae from unfertilized eggs
developed in the precociously-formed ovaries of a larval gall-midge
(Cecidomyid), and subsequent observers have confirmed his results by
studies on insects of the same family and of the related _Chironomidae_.
The larvae produced by this remarkable method (paedogenesis) of
virgin-reproduction are hatched within the parent larva, and in some
cases escape by the rupture of its body.
_Polyembryony._--Occasionally the power of reproduction is thrown still
farther back in the life-history, and it is found that from a single egg
a large number of embryos may be formed. P. Marchal has (1904) described
this power in two small parasitic Hymenoptera--a Chalcid (_Encyrtus_)
which lays eggs in the developing eggs of the small moth _Hyponomeuta_,
and a Proctotrypid (_Polygnotus_) which infests a gall-midge
(Cecidomyid) larva. In the egg of these insects a small number of nuclei
are formed by the division of the nucleus, and each of these nuclei
originates by division the cell-layers of a separate embryo. Thus a mass
or chain of embryos is produced, lying in a common cyst, and developing
as their larval host develops. In this way over a hundred embryos may
result from a single egg. Marchal points out the analogy of this
phenomenon to the artificial polyembryony that has been induced in
Echinoderm and other eggs by separating the blastomeres, and suggests
that the abundant food-supply afforded by the host-larva is favourable
for this multiplication of embryos, which may be, in the first instance,
incited by the abnormal osmotic pressure on the egg.
_Duration of Life._--The flour-moth (_Ephestia kuhniella_) sometimes
passes through five or six generations in a single year. Although one of
the characteristics of insects is the brevity of their adult lives, a
considerable number of exceptions to the general rule have been
discovered. These exceptions may be briefly summarized as follows: (1)
Certain larvae, provided with food that may be adequate in quantity but
deficient in nutriment, may live and go on feeding for many years;
(2) certain stages of the life that are naturally "resting stages" may
be in exceptional cases prolonged, and that to a very great extent; in
this case no food is taken, and the activity of the individual is almost
_nil_; (3) the life of certain insects in the adult state may be much
prolonged if celibacy be maintained; a female of _Cybister roeselii_ (a
large water-beetle) has lived five and a half years in the adult state
in captivity. In addition to these abnormal cases, the life of certain
insects is naturally more prolonged than usual. The females of some
social insects have been known to live for many years. In _Tibicen
septemdecim_ the life of the larva extends over from thirteen to
seventeen years. The eggs of locusts may remain for years in the ground
before hatching; and there may thus arise the peculiar phenomenon of
some species of insect appearing in vast numbers in a locality where it
has not been seen for several years.