Chapter XXXI: Part III: The Metamorphoses of Insects (2)

Among Orthoptera Acrydians moult five times; _Diapheromera femorata_ but twice (Riley); a katydid (_Microcentrum retinervis_) moults four times (Comstock). _Mantis religiosa_, according to Pagenstecher, moults seven times, having eight stages, including that before the amnion is cast, but the first “moult” being an exuviation of the amnion, the number of stages is seven. Cockroaches (_Periplaneta americana_) are said by Marlatt to “pass through a variable number of moults, there being sometimes as many as seven.”

In the Homoptera there are, in general, from two to four moults; thus in Typhlocyba there are five stages, and in Aphis at least three, and in Psylla four during the nymphal state. Psocus has four. Riley states that the nymph of the female coccid, _Icerya purchasi_, sheds its skin three times, and that of the male twice. Notwithstanding its slow growth, Riley says, the 17–year Cicada moults oftener than once a year, and the number of larval stages probably amounts to 25 or 30 in all. The bed-bug sheds its skin five times; and with the last moult appear the minute wing-pads characteristic of the adult. In _Conorhinus sanguisuga_ there are “at least two larval stages and pupal stages.” (Marlatt.)

In the dragon-flies moulting occurs, Calvert thinks, many times, since the rudiments of wings are said by Poletaiew to only appear in odonate nymphs after the third or fourth moult.

In the May-fly, Chloëon, the number of ecdyses is 20. The neuropterous _Ascalaphus_ (Helecomitus) _insimulans_ of Ceylon moults three times before pupating. Among the Mecoptera Felt has shown that _Panorpa rufescens_ moults seven times.

In Coleoptera the normal or usual number is not definitely known; Meloë moults five times, but this is a hypermetamorphic insect; _Tribolium confusum_ has been carried by Mr. Chittenden through seven moults. _Phytonomus punctatus_, the clover-leaf weevil, moults three times, according to Riley, who has observed that _Dermestes vulpinus_ passes through seven larval stages.

In the breeding jars, with plenty of food and a constant temperature
of from 68° to 78° F., the larvæ cast their 1st skin in from four to
nine days, the great majority moulting at seven days. Under the same
conditions the 2d skin was cast at from four to seven days, the
majority moulting at six days; the 3d skin at from three to six
days, the majority moulting at five days; and the 4th skin at from
three to six days, the majority moulting at five days; the 5th skin
at from five to seven days, and the 6th skin at six days. There are
thus seven larval stages. (Report for 1885, p. 260.)

Riley has ascertained that by rearing isolated larvæ of _Tenebrio
molitor_, one after being kept nearly a year had moulted 11 times,
when it died. A second larva, hatched June 5, had moulted 12 times
by June 10 of the following year, (1877), when it also died. Of _T.
obscurus_ three larvæ were reared to the imago state. One moulted 11
times by Aug. 30 of the same year, pupated Jan. 20, 1877, and
finally became a beetle Feb. 7, 1877. The other two both moulted 12
times, and reached the imago stage Feb. 18 and March 9,
respectively. “All were, as nearly as possible, under like
conditions of food and surroundings, and in all cases the moult that
gave the pupa is not considered among the larval moults.”

Two larvæ of the museum pest (_Trogoderma tarsale_) were kept by
Riley in a tight tin box with an old silkworm cocoon. “They were
half-grown when placed in the box. On Nov. 8, 1880, there were in
the box 28 larva skins, all very much of a size, the larva having
apparently grown but little. The skins were removed and the box
closed again as tightly as possible. Recently, or after a lapse of
two years, the box was again opened and we found one of the larvæ
dead and shrivelled up; but the other was living and apparently not
changed in appearance. There were 15 larva skins in the box. He
could not tell when the one larva died, but it is certain that
within a little more than three and a half years, two larvæ shed not
less than 43 skins, and that one larva did not, during that time,
appreciably increase in size. We know of no observations which
indicate the normal or average length of life, or number of moults
in either Tenebrio or Trogoderma, but it is safe to assume from what
is known, in these respects, of allied species, that in both the
instances here referred to, but particularly in the case of
Trogoderma, development was retarded by insufficient nutrition, and
that the frequent moulting and slow growth resulted therefrom, and
were correlated.”[99] Further observations such as these are greatly
needed.

Of the Siphonaptera the common cat and dog flea (_Pulex serraticeps_) moults three times before pupating. (Howard.)

In Lepidoptera the usual or average number of moults is four, but the number varies considerably, the greatest number yet known occurring in _Phyrrarctia isabella_, which, Dr. Dyar informs me, moults 10 times.

From Dyar’s observations it appears that there are usually five larval stages, but six and seven stages are not infrequent, while there are seven in _Seirarctia echo_, eight in _Ecpantheria scribonia_, Scepis, and Apatelodes, and nine and ten in arctians, while the European _Nola centonalis_ moults nine times, other species of this genus shedding their skins six times. (Buckler.) (Psyche, v, pp. 420–422.) _Callosamia promethea_ appears, as a rule, to moult but three times. _Orgyia antiqua_ was found by Hellins to moult from three to five times. Riley found that in _O. leucostigma_ the males moult four times, the female four, but sometimes five times, while Dyar states that in _O. gulosa_ the male larvæ moult three or four times, the female always four times; in _O. antiqua_, however, there are six stages, and in the female seven. Lithocolletis, Chambers thinks, as a rule, moults eight times, and Comstock thinks that _L. hamadryadella_ casts its skin seven or eight times.

In the blow-fly (Calliphora) Leuckart and Weismann have inferred at least two moults, while Weismann suspected that there are as many as four. In _Musca domestica_ we have observed that the larva moults three times; in Œstridæ there are three larval stadia. (Brauer.) In Corethra there are four larval moults, and Miall thinks there are probably as many in Chironomus. Passing to the phytophagous Hymenoptera, there are three moults or four larval stages in _Nematus erichsonii_, but Dyar informs us that less than four stages in saw-fly larvæ is very rare, that he has only one record of less than five, and that that is doubtful; “five for nematid, six and seven for others, is certainly the rule. The highest I have is the indication of 11 stages for _Harpiphorus varianus_, but this again is an inference only, and attended with doubt.” (Can. Ent., xxvii, p. 208.) In Bombus we have observed five different sizes of larvæ, and hence suppose the least number of ecdyses is five, while we are disposed to believe that this insect, as well as wasps and bees, in general shed their skins as many as eight times during their entire existence.

The honey-bee, Cheshire thinks, since he has found the old and ruptured pellicles, probably moults six times before it spins its cocoon, or passes into the semipupa condition. (Bees and Bee-keeping, p. 20.)

As to the cause of the great number of moults in the arctians and in the beetles experimented with by Riley, it would seem that cold and the lack of food during hibernation were the agents in arctians, and starvation or the lack of food in the case of the beetles, such cause preventing growth, though the hypodermis-cells retained their activity.

=Reproduction of lost limbs.=—Here might be discussed the subject of the renovation or renewal of maimed or lost limbs, or the reparation of other injuries. As is well known, the cœlenterates, echinoderms, and worms under certain circumstances multiply by self-division, or if artificially mutilated, the parts are gradually restored by cell-proliferation or histogenesis. It is so with the antennæ and legs of crustaceans as well as the digits and tail of salamanders. The experiments first made by Le Pelletier[100] on spiders, and later by Heineken,[101] and others after him, on different spiders, as well as on Orthoptera and Hemiptera (Blatta, Reduvius, etc.), have proved that antennæ and legs and other external parts which have been injured or shortened, or entirely cut off in young individuals, are replaced at the next, or after successive moults, though generally in diminished size. This does not usually occur in adult life, and the process of reparation of lost parts is apparently due to the active growth of the cells of the parts affected during the process of moulting, when the histolysis of the maimed or diseased parts is succeeded by the rapid development of new cells, not only of the hypodermis, but also of the more specialized tissues within. And this tends to prove that such histolysis and making over of the muscles and other structures within occur especially in all metamorphic insects, and also in ametabolous forms, though the process has been most thoroughly examined in the Diptera, where these changes are more marked.

Gonin has found that the thoracic legs of the caterpillar correspond only to the tarsi of the imago (Fig. 608). It results, he says, from this fact that in accordance with the observations of Réaumur (which were wrongly interpreted by Newport and Künckel D’Herculais) that the amputation of the legs of the larva does not involve the entire leg, but only the extremity of the leg of the imago.

=Formation of the cocoon.=—While the larvæ of many insects, as those of the butterflies, suspend themselves before transforming, and spin no cocoon, or dig into the earth for protection and to secure an immunity from too great changes of temperature, a large proportion of the larvæ of metabolous insects which lead an inactive pupal life, line their earthen cells with silk, or spin a more or less elaborate case of silk, called the _cocoon_. We have seen that the inactive pupa of the male scale-insects is covered by the scale itself, or even in one case the insect forms a true cocoon of fibres of wax. The aquatic larvæ of the Neuroptera and Coleoptera creep out of the water, and by the movements of their bodies make a rude earthen cell in the bank, while that of Donacia spins a dense, leathery cocoon (Fig. 567) in the earth. The larvæ of the Embiidæ are protected by a cocoon, which they renew at each moult. Coniopteryx spins an orbicular cocoon, the Hemerobiidæ a spherical, dense, whitish one. The Trichoptera transform within their larval cases, which thus serve as cocoons, as do certain case-bearing Lepidoptera, notably the Psychidæ.

FIG. 567.—Cocoon (natural size) of _Donacia proxima_.
]

FIG. 568.—Cocoon and larva of _Lucanus dama_.
]

The pupa of certain leaf-eating beetles (Chrysomelidæ), as well as the Coccinellidæ, Dermestidæ, Hister, etc., are usually protected by the cast larval skin, which is retained, forming a rude shelter. While many beetles spin an oval cocoon (Gyrinus, Silphidæ), the wood-boring species make one of chips glued together, and that of Lucanus, which feeds on decayed wood, is lined with silk (Fig. 568). Anobium constructs a silken cocoon, interweaving the fine particles of its thin castings; the larvæ of weevils also usually spin silken cocoons.

FIG. 569.—Larva (_a_), puparium (_b_), and imago (_c_) of Sarcophaga,
enlarged.
]

FIG. 570.—_a_, _Erax bastardi_; _b_, pupa.—After Riley.
]

The larval skin of the coarctate Diptera is retained as a protection for the soft-bodied pupa within, the old larval skin separating from the integument of the semipupa. To this cocoon-like covering of the coarctate pupa we have restricted the term _puparium_, originally used by Kirby and Spence to designate the pupa. The puparium is usually cylindrical or barrel-shaped, rounded at each end.

FIG. 571.—Puparium of _Hypoderma bovis_: _a_, side; _b_, ventral view,
showing exit hole of adult; _c_, cap which splits off for exit of
fly.—After Clark, from Osborn, Bull. 5, Div. Ent. U. S. Dept. Agr.
]

In the _Diptera cyclorhapha_, or common house and flesh flies, etc., the puparium remains in vital connection, by means of four tracheæ, with the enclosed pupa, which escapes from the case through a curved seam or lid at the anterior end and not by a slit in the back, as do the orthoraphous families, represented by the horse-fly (Tabanidæ, Asilidæ, Fig. 570), etc., where in some cases the obtected pupa remains within the loose envelope formed by the old larval skin, which Brauer calls a false puparium. The dry, hard puparium is burst open at the cephalic end when the fly emerges, by means of the frontal vesicle, which is distended with fluid (Fig. 571).

The exact mode of spinning the cocoon by caterpillars has been carefully observed by L. Trouvelot in the case of the polyphemus silkworm.

“When fully grown, the worm, which has been devouring the leaves so
voraciously, becomes restless and crawls about the branches in
search of a suitable place to build up its cocoon; before this it is
motionless for some time, holding on to the twig with its front
legs, while the two hind pair are detached; in this position it
remains for some time, evacuating the contents of the alimentary
canal until finally a gelatinous, transparent, very caustic fluid,
looking like albumen, or the white of an egg, is ejected; this is a
preparation for the long catalepsy that the worm is about to fall
into. It now feels with its head in all directions, to discover any
leaves to which to attach the fibres that are to give form to the
cocoon. If it finds the place suitable, it begins to wind a layer of
silk around a twig, then a fibre is attached to a leaf near by, and
by many times doubling this fibre and making it shorter every time,
the leaf is made to approach the twig at the distance necessary to
build the cocoon; two or three leaves are disposed like this one,
and then fibres are spread between them in all directions, and soon
the ovoid form of the cocoon distinctly appears. This seems to be
the most difficult feat for the worm to accomplish, as after this
the work is simply mechanical, the cocoon being made of regular
layers of silk united by a gummy substance. The silk is distributed
in zigzag lines of about one-eighth of an inch long. When the cocoon
is made, the worm will have moved his head to and fro, in order to
distribute the silk, about 254,000 times.

“After about half a day’s work, the cocoon is so far completed that
the worm can hardly be distinguished through the fine texture of the
wall; then a gummy resinous substance, sometimes of a light-brown
color, is spread all over the inside of the cocoon. The larva
continues to work for four or five days, hardly taking a few minutes
of rest, and finally another coating is spun in the interior, when
the cocoon is all finished and completely air tight. The fibre
diminishes in thickness as the completion of the cocoon advances, so
that the last internal coating is not half so thick and so strong as
the outside ones.” (Amer. Naturalist, i, p. 86.)

The mode of spinning the cocoon of an ichneumon (Microgaster) parasitic on Philampelus has been well described by John P. Marshall, as follows:—

FIG. 572.—Microgaster larvæ; spinning their cocoons: _a_, enlarged
view of 5.—After Marshall.
]

The first appearance of the parasite is represented in Fig. 572, 1.
A warty excrescence appears on the back of the caterpillar, which
slowly emerges until it is seen to be a larva enclosed in a delicate
transparent membrane, as represented in 2. This it soon succeeds in
bursting, and, rising to its full length, balances itself a moment
as in 3, then, bending double, it ejects from its mouth a glairy
liquid, which instantly changes to silk, and fastens the posterior
end to the skin of the caterpillar, as shown in 4, side view. It now
begins to spin its cocoon by attaching a silken thread to the silky
mass by which it had previously fastened itself to the caterpillar,
and forming a series of loops of uniform size, first from right to
left, and then back again from left to right, as represented in the
front view, 5, and better in the enlarged view, _5^a_, the arrow
heads showing the direction in which the head of the larva moved
while forming the loops. The ends of the series, numbered 1, 2, 3,
4, are fastened to the edges of the ventral side of the body, which
thus serves as a measure of the width of the cocoon, and also acts
as a support for the frail fabric in the first stages of spinning.
After the larva has fastened the fabric as far up on its ventral
surface as it can, conveniently, it then begins to spin free, as
shown in the side view, 6, where it is represented as just
completing the first half of its cocoon, which resembles in form a
slipper. This accomplished, the larva ceases to spin for the time
being, bends its head, as in 7, towards its ventral surface, and
pushes the half cocoon free from its body. The form of the silken
fabric enables it to stand unsupported, while the larva, sliding its
head down to the base, holds on firmly until it swings its posterior
end into the toe of the slipper.

Figure 572, 8, shows it in the act of changing end for end, and in 9
the larva is seen erect, beginning at the base to complete the other
half of its cocoon; 10 shows the larva contracting its body as it
spins upward for about half the length of the cocoon, when it again
changes end for end, as shown in 11, where it is beginning at the
upper part to unite the two sides, finally enclosing itself as
represented in 12.

It may now be seen, under the microscope, through the meshes of its
cocoon actively engaged in lining the interior with layers of very
fine silk ejected from its mouth in great abundance. One half of the
cocoon is first lined by a forward and back movement of its head,
and then reversing its position, it lines the other half in a
similar manner.

In one case the larva was disengaged from the skin of the
caterpillar, after beginning its cocoon. It, however, began again,
and spun a portion while lying on the table. This was removed, when
it began a third time, and completed its cocoon.

In about 10 days the insect made its appearance through a hole in
the upper end, as represented in 13. The top was eaten off in a
perfect circle and hung by a few threads, so as to resemble a lid as
it was thrown back.

One caterpillar observed had between 300 and 400 cocoons on its back
and sides, and another was dissected after more than 30 larvæ had
escaped, and 130 were discovered in the soft integuments of the
back.

The figures from 1 to 13 are magnified five diameters, but in order
to observe the spinning of the cocoon a power of 50 is required.
(Amer. Naturalist, xii, pp. 559, 560.)

Certain differences observed by W. A. Buckhout in a Microgaster
parasitic on the different species of Macrosila, are referred to in
the same volume, p. 752.

FIG. 573.—Body of larva of Lithocolletis. swollen and filled with
cocoons of Copidosoma, enlarged.
]

While those chalcidid larvæ which feed internally on their host, as
a rule, transform into naked, more or less coarctate pupæ, Howard
states that the larvæ of Copidosoma, Bothriothorax, Homalotylus, and
perhaps others, which are much crowded within their host, cause a
marked inflation of the body of the latter (Figs. 573, 574). The
nature of this cocoon-like cell, and how it is produced, is unknown.
“Its structure shows it not to be silk, nor yet the last larval skin
of the parasite, and whether it is an adventitious tissue of the
host-larva or a secretion of the parasite, or is explicable upon
other grounds, I cannot say.”

The silken cocoon of an aphidiid ichneumon has been found by Miss
Murtfeldt, and also by Dr. Riley, under a rose aphid in which it had
lived, and referred by Howard to the genus Praon (Fig. 575).

=Sanitary conditions observed by the honey-bee larva, and admission of air within the cocoon.=—Cheshire has observed that after the larva of the honey-bee has spun its cocoon or silken lining of its cell, it observes the following means of preserving cleanliness. The food given to the larva, especially during the latter part of the growing period, contains much pollen, the cases of the grains of which consist of cellulose, which is indigestible.

FIG. 574.—Coccinellid larva infested by _Homalotylus obscurus_,
enlarged.
]

FIG. 575.—Cocoon of Praon under the body of a dead Aphis,
enlarged.—This and Figs. 573 and 574 after Howard, from Insect Life.
]

FIG. 576.—Pupation of Proctotrupes in the body of a larva of a beetle,
representing a case mentioned by Dr. Sharp, where the parasites have
pupated on the outside of the host, a pair of each attached to
nearly each segment of the body of their host.—After Sharp.
]

“These cases, with other refuse matters, collect in quantity within
the bowel, which becomes distended, since it has no opening. The
imprisoned larva, having little more than enough room for turning,
must be freed of these objectionable residua.... In a word, the
larva turns its head upon its stomach, and pushes the former towards
the base of the cell until its position is reversed, the tail being
outwards; and, thus placed, it laps up all residue of food,
especially from its old clothes previously referred to, until they
are dried, and practically occupy no space. It now throws up its
stomach and bowel, with all their contents, and without detaching
them from its outer skin, which is moulted as before, but in this
instance to be pressed against the cell, so as to form for it an
interior lining. The dejectamenta of the bowel in this way lie
between the cast skin and cell-wall (as seen at _e_, Fig. 577), and
so the larva remains absolutely unsoiled. It now turns its head and
resumes its old position, joining its cocoon to the edges of its
last cast skin, so that its habitation is relined, it is cleansed,
and air can still pass to it through the imperceptible openings left
by the bees in the sealing. This point is of radical importance,
since breathing is carried on pretty rapidly during the latter part
of its subsequent transformations, the absorbed oxygen permitting
then of a production of heat, and causing also considerable
diminution in weight.”

FIG. 577.—Larva and pupa of honey-bee in their cell: _SL_,
spinning-larva; _N_, pupa; _FL_, young feeding larva; _co_, cocoon;
_sp_, spiracles; _t_, tongue; _m_, mandible; _an_, antenna; _w_,
wing; _ce_, compound eye; _e_, excrement; _ex_, exuvium.—After
Cheshire.
]

As to the passage of air into the bee’s cocoon, Cheshire states that before the cocoon can be built, a cover, technically called sealing, is put over the larva by its nurses. These covers are made of pollen and wax, and are pervious to the air. They are more convex and regular in form than those sealing in the honey.[102]

THE PUPA STATE

The word _pupa_ is from the Latin meaning baby. Linnæus gave it this name from its resemblance to a baby which has been swathed or bound up, as is still the custom in Southern Europe. The term _pupa_ should be restricted to the resting inactive stage of the holometabolous insects.

Lamarck’s term _chrysalis_ was applied to the complete or obtected
pupa of Lepidoptera and of certain Diptera, and _mumia_, a mummy, to
the pupæ of Coleoptera, Trichoptera, and most Hymenoptera. Latreille
(1830) also restricted the term pupa to the “oviform nymph,” or
puparium, of Diptera. Brauer applies the term _nymph_ to the pupa of
metabolous insects.

FIG. 578.—Pupa obtecta: _a_, of Sesia, with its cocoon-cutter on the
head; _b_, of _Tortrix vacciniivorana_.
]

The typical pupa is that of a moth or butterfly, popularly called a chrysalis. A lepidopterous pupa in which the appendages are more or less folded close to the body and soldered to the integument, was called by Linnæus a _pupa obtecta_; and when the limbs are free, as in Neuroptera, Mecoptera, Trichoptera, and the lepidopterous genus Micropteryx it is called a _pupa libera_ (Fig. 579). When the pupa is enclosed in the old larval skin, which forms a pupal covering (puparium), the pupa was said by Linnæus to be _coarctate_. The pupa of certain Diptera, as that of the orthoraphous families, is nearly as much obtected as that of the tineoid families of moths, especially as regards the appendages of the head; the legs being more as in _pupæ liberæ_ (Fig. 580).

FIG. 579.—Pupa libera of neuropterous insects _a_, _Corydalus
cornutus_; _b_, Sialis; _c_, Hemerobius.
]

The male Coccid anticipates the metabolous insects in passing through a quiescent state, when, as Westwood states, it is “covered by the skin of the larva, or by an additional pellicle.” The body appears to be broad and flat, the antennæ and fore legs resting under the head, while the two hinder pairs of legs are appressed to the under side of the body. There is but a slight approach to the pupa libera of a metabolous insect.

Riley states that the male larva of _Icerya purchasi_ forms a cocoon
waxy in character, but lighter, more flossy, and less adhesive than
that of the female egg-cocoon. It melts and disappears when heated,
proving its entirely waxy nature. When the mass has reached the
proper length, the larva casts its skin, which remains in the hind
end of the cocoon, and pushes itself forward into the middle of the
cocoon. The pupa (Fig. 581) is of the same general form and size as
the larva. All the limbs are free and slightly movable, so that they
vary in position, though ordinarily the antennæ are pressed close to
the side, as are the wing-pads; the front pair of legs are extended
forward. “If disturbed, they twist and bend their bodies quite
vigorously.” The pupa state lasts two or three weeks. A similar pupa
is that of _Icerya rosæ_. (Riley and Howard.)

FIG. 580.—Pupa obtecta of Diptera: _a_, Ptychoptera; _b_, _Tabanus
atratus_; _c_, _Proctacanthus philadelphicus_; _d_, _Midas
clavatus_.
]

FIG. 581.—Pupa libera of _Icerya purchasi_, ventral view.—After
Riley, Insect Life.
]

The metamorphosis of _Aspidiotus perniciosus_ is of interest. The
male nymph differs much after the first moult from the female,
having large purple eyes, while the female nymph loses its eyes
entirely. It passes into what Riley terms the _pro-pupa_ (Fig. 582,
_b_), in which the wing-pads are present, while the limbs are short
and thick. The next stage is the “true pupa” (Fig. 582, _c_, _d_),
in which the antennæ and legs are much longer than before. There is
no waxy cocoon, but only a case or scale composed of the shed larval
skin, i.e. “with the first moult the shed larval skin is retained
beneath the scale, as in the case of the female; with the later
moultings the shed skins are pushed out from beneath the scale,” and
when they transform into the imago they “back out from the rear end
of their scale.”

FIG. 582.—_Aspidiotus perniciosus_, development of male insect: _a_,
ventral view of larva after first moult; _b_, the same, after
second moult (pro-pupa stage); _c_ and _d_, true pupa, ventral and
dorsal views. All greatly enlarged.—After Riley.
]

The pupæ of Coleoptera and of Hymenoptera, though there is, apparently, no near relationship between these two orders, are much alike in shape, and, as Chapman pertinently suggests, those of both orders are helpless from their quiescence, and hence have resorted for protection to some cocoon or cell.

But it is quite otherwise with the pupæ of Lepidoptera and Diptera, which vary so much in adaptation to their surroundings, and hence afford important taxonomical and phylogenetic characters. This, as regards the Lepidoptera, was almost wholly overlooked until Chapman called attention to the subject, and showed that the pupæ had characters of their own, of the greatest service in working out the classification, and hence the phylogeny, of the different lepidopterous groups. We have, following the lead of Chapman, found the most striking confirmation of his views, and applied our present knowledge of pupal structures to dividing the haustellate Lepidoptera into two groups,—Paleolepidoptera and Neolepidoptera.

The pupæ of the Neuroptera, Coleoptera, and Hymenoptera differ structurally from the imago, in the parts of the head and thorax being less differentiated. Thus in the head the limits or sutures between the epicranium and clypeus, and the occiput and gula, are obscurely marked, while the tergal and pleural sclerites of the imago are not well differentiated until the changes occurring just before the final ecdysis.

It is easy, however, to homologize the appendages of the pupæ with those of the imago of all the holometabolous orders except in the case of the obtected pupa of the Lepidoptera (and probably of the obtected dipterous pupæ), where the cephalic appendages are soldered together.

That the appendages of the lepidopterous pupa are, as generally supposed, merely cases for those of the imago has been shown by Poulton to be quite erroneous. He says: “If we examine a section of a pupal antenna or leg (in Lepidoptera), we shall find that there is no trace of the corresponding imaginal organ until shortly before the emergence of the imago. In the numerous species with a long pupal period, the formation of imaginal appendages within those of the pupa is deferred until very late, and then takes place rapidly in the lapse of a few weeks. This also strengthens the conclusion that such pupal appendages are not mere cases for the parts of the imago, inasmuch as these latter are only contained within them for a very small proportion of the whole pupal period.” On the other hand, Miall and Hammond claim that there is a strong superficial contrast as to the formation of the imaginal organs, between Lepidoptera and tipularian Diptera, the appendages, wings, and compound eyes being substantially those of the imago. “With the exception of the prothoracic respiratory appendages and the tail-fin, there is little in the pupa of Chironomus which does not relate to the next stage.”

The exact homology of the “glazed eye” of the lepidopterous pupæ and of the parts under the head, situated over the maxillæ, is difficult to decide upon, and these points need farther examination. In the dipterous pupa it is interesting to observe that the halteres are large and broad, which plainly indicates that they are modified hind wings. The number and arrangement of the spiracles is different in pupæ from those of the larva and imago.

FIG. 583.—_Simulium piscicidium_: _a_, larva; _b_, _c_, _d_, pupa;
_e_, thoracic leg; _f_, row of bristles at end of body. _A_, _S.
pecuarum_, pupa; _a_, _b_, _c_, adminicula.—After Riley.
]

There are also secondary adaptive structures peculiar to the pupa, which are present and only of use in this stage. These are the thoracic, spiracular, or breathing appendages of the aquatic Diptera (Fig. 583), the various spines situated on the head or thorax, or on the sides, or more often at the end of the abdomen, besides also the little spines arranged in more or less circular rows around the abdominal segments, the cocoon-breaker, and the cremaster of many pupæ.

In the pupa of certain Diptera, there is a terminal cremaster-like spine, as in that of _Tipula eluta_ (Fig. 584), _Tabanus lineola_ (Fig. 585), besides adminicula or locomotive spines like those of lepidopterous pupæ (Fig. 580, _a_, _b_, _c_).

FIG. 584.—Pupa of _Tipula eluta_.
]

FIG. 585.—Pupa of _Tabanus lineola_.—This and Fig. 584 after Hart.
]

FIG. 586.—Pupa of _Galerita lecontei_, and of _Adelops hirtus_ (_a_,
_b_, _c_).—After Hubbard.
]

The pupæ of Coleoptera are variously spined or hairy (Fig. 586). Those of Hydrophilus and of Hydrobius are provided with stout spines on the prothorax and abdomen which support the body in its cells, so that, as Lyonet first showed, though surrounded on all sides by moist earth, it is kept from contact with it by the pupal spines; other pupæ of beetles, such as that of the plum weevil, which is also subterranean, possess similar spines. The abdomen of many coleopterous pupæ, such as those of Carabidæ, end in two spines, to aid them in escaping from their cells in wood or in the earth; others have stiff bristles, and others spines along each side of the abdomen (Fig. 586). All these structures are the result of a certain amount of activity in what we call quiescent pupæ, but most of these are for use at the end of pupal life, at the critical moment when by their aid the insect escapes from its cocoon or subterranean cell, or if parasitic, bores out of its host.

If we are to account for the causes of their origin, we are obliged to infer that they are temporary deciduous structures due to the need of support while the body is subjected to unusual strains and stresses in working its way out of its prison in the earth, or its cell within the stems and trunks of plants and similar situations. They are pupal inheritances or heirlooms, and well illustrate the inheritance of characters acquired during a certain definite, usually brief, period of life, and transmitted by the action of synchronous heredity.

The pupæ of certain insects are quite active, thus that of Raphidia, unlike that of Sialis, before its final ecdysis regains its activity and is able to run about. (Sharp, p. 448.)

_a._ The pupa considered in reference to its adaptation to its
surroundings and its relation to phylogeny

The form of the pupa is a very variable one, as even in Lepidoptera it is not entirely easy to draw the line between a pupa libera and a pupa obtecta (Fig. 578); and though the period is one of inactivity, yet when they are not in cocoons or in the earth in subterranean cells, their form is more or less variable and adapted to changes in their surroundings. Even in the obtected pupa of butterflies, there is, as every one knows, considerable variability of shape and of armature, which seems to be in direct adaptability to the nature of their environment. Scudder has well shown that in certain chrysalids, such as those of the Nymphalidæ, which are variously tuberculated, and hang suspended by the tail, and often hibernate, these projections serve to protect the body. All chrysalids with projections or ridges on different parts of the body, being otherwise unprotected, move freely when struck by gusts of wind, hence “the greater the danger to the chrysalis from surrounding objects, the greater its protection by horny tubercles and roughened callous ridges.” The greater the protection possessed in other ways, as by firm swathing or a safe retreat, the smoother the surface of the body and the more regular and rounded its contours. The tendency to protection by tubercles is especially noticeable in certain South American chrysalids of nymphalid butterflies. This response to the stimuli of blows or shocks is also accompanied by a sensitiveness to the stimulus of too strong light.

Previously Scudder[103] had made the important suggestion that the smooth crescent-shaped belt of the “glazed eye” or “eyepiece” of chrysalids is, as an external covering of the eye, midway between that of the caterpillar and the perfect insect, and he asks: “May it not be a relic of the past, the external organ of what once was? And are we to look upon this as our hint that the archaic butterfly in its transformations passed through an _active_ pupal stage, like the lowest insect of to-day, when its limbs were unsheathed, its appetite unabated?” etc. Scudder also shows that “the expanded base of the sheath covering the tongue affords protection also to the palpi which lie beneath and beside the tongue.”

All this tends to show the importance of studying the structure of the pupa, in order to ascertain how the pupal structures have been brought about, with the final object of discovering whether the pupæ of the holometabolic insects are not descended from active nymphs, and if so, the probable course of the line of descent.

_b._ Mode of escape of the pupa from its cocoon

FIG. 587.—Pupa of _Micropteryx purpuriella_, front view: _md_,
mandibles; _mx. p_, maxillary palpus, end drawn separately; _mx.′
p_, labial palpi; _lb_, labrum.
]

“In all protected pupæ,” as Chapman says, “the problem has to be faced, how is the imago to free itself from the cocoon or other envelope protecting the pupa.” In the Coleoptera and Hymenoptera the imago becomes perfected within the cocoon or cell, as the case may be, and as Chapman states, “not only throws off the pupal skin within the cocoon, but remains there till its appendages have become fully expanded and completely hardened, and then the mandibles are used to force an outlet of escape,” and he calls attention to the fact that “in many cases, even in some entire families, they are of no use whatever to the imago except in this one particular,” and he cites the Cynipidæ as perhaps the most striking instance of this circumstance.

In those Neuroptera which spin a silken cocoon, _e.g._ the Hemerobiidæ, the Trichoptera, and in Micropteryx (Fig. 588), the jaws used by the pupa for cutting its way out of the cocoon are even larger in proportion than in the pupa of caddis-flies (Fig. 588), being of extraordinary size.

FIG. 588.—Mandibles (_md_) of _Micropteryx purpuriella_,
enlarged.—Author _del._ _A_, pupal head of a hydropsychid
caddis-fly, showing the large mandibles.—After Reaumur, from Miall.
]

In Myrmeleon the pupa pushes its way half out of the cocoon, and then remains, while the imago ruptures the skin and escapes (Fig. 589, _a_).

Thus in the Neuroptera and Trichoptera we have already established the more fundamental methods of escape from the cocoon, which we see carried out in various ways in the more generalized or primitive Lepidoptera.

The most primitive method in the Lepidoptera of escaping from the cocoon seems to be that of Micropteryx.

FIG. 589.—Larva of Myrmeleon with (_a_) its cocoon and cast pupa-skin.
]

“In this genus,” says Chapman, “though it is nominally the pupa that
escapes from the cocoon, it is in reality still the imago, the imago
clothed in the effete pupal skin. To rupture the cocoon it uses not
its own jaws, but those of the pupal skin, energizing them, however,
in some totally different way from ordinary direct muscular action,
their movements being the result of the vermicular movements of the
pupa, acting probably by fluid pressure on the articular structure
of the jaws, by some arrangement not altogether different perhaps
from the frontal sac of the higher Diptera. In the Micropteryges the
jaws of the pupa not only rupture the cocoon, but appear to be the
most active agents in dragging the pupa through the opening in the
cocoon and through any superincumbent earth, being merely assisted
by the vermicular action of the abdominal segments, and we find in
accordance with this circumstance that the pupal envelope is still
very thin and delicate, and has little or no hardening or roughness
by which to obtain a leverage against the walls of the channel of
escape.” (Trans. Ent. Soc. London, 1896, pp. 570, 571.)

FIG. 590.—Pupa of Talæporia: _a_, cocoon-cutter; with vestiges of four
pairs of abdominal legs, and the cremaster.
]

Some sort of a beak or hard process, more or less developed, according to Chapman, adapted for breaking open the cocoon exists in nearly all the Lepidoptera with incomplete pupæ (_pupæ incompletæ_), except the limacodid and nepticulid section. “In all these instances the pupa emerges from the cocoon precisely as in the Micropteryges, that is, the moth it really is that emerges, but does so encased in the pupal skin. To achieve this object, it seems to have been found most efficient to have three, four, or five abdominal segments capable of movement, but to have the terminal sections (segments) soldered together.”

This cocoon-breaker, as we may call it, is especially developed in _Lithocolletis hamadryadella_. As described by Comstock, it forms a toothed crest on the forehead which enables it to pierce or saw through the cocoon.

“Each pupa first sawed through the cocoon near its juncture with the
leaf and worked its way through the gap, by means of the minute
backward-directed spines upon its back, until it reached the upper
cuticle of the leaf. Through this cuticle it sawed in the same way
that it did through the cocoon. The hole was in each case just large
enough to permit the chrysalis to work its way out, holding it
firmly when partly emerged. When half-way out it stopped, and
presently the skin split across the back of the neck and down in
front along the antennal sheaths, and allowed the moth to
emerge.”[104]

We have observed and figured the cocoon-breaker in Bucculatrix, Talæporia (Fig. 590, _a_), Thyridopteryx, and Œceticus, and rough knobs or slight projection answering the purpose in Hepialidæ, Megalopyge, Zeuzera, and in Datana.[105] See also the spine on the head of _Sesia tipuliformis_ (Fig. 578).

The imago of the attacine moths cuts or saws through its cocoon by means of a pair of large, stout, black spines (_sectores coconis_), one on each side of the thorax at the base of the fore wings (Fig. 591), and provided with five or six teeth on the cutting edge (_C_, _D_).

FIG. 591.—Cocoon-cutter of the Luna moth: front view of the moth with
the shoulders elevated and the rudimentary wings hanging down: _s_,
cocoon-cutter; _p_, patagium. _B_, represents another specimen with
fully developed wings: _ms_, scutum; _st_, scutellum of the
mesothoracic segment; _s_, cocoon-cutter, which is evidently a
modification of one of the pieces at the base of the fore wings; it
is surrounded by membrane, allowing free movement. _C_ and _D_,
different views of the spine, magnified, showing the five or six
irregular teeth on the cutting edge.
]

FIG. 592.—Larva and pupa of a wood-wasp (Rhopalum), enlarged: _h_,
temporary locomotive tubercles on head of pupa.—Trouvelot _del._
]

Our attention[106] was drawn to this subject by a rustling, cutting,
and tearing noise issuing from a cocoon of _Actias luna_. On
examination a sharp black point was seen moving to and fro, and then
another, until both points had cut a rough irregular slit, through
which the shoulder of the moth could be seen vigorously moving from
side to side. The hole or slit was made in one or two minutes, and
the moth worked its way at once out of the slit. The cocoon was
perfectly dry. The cocoon-cutter occurs in all the American genera,
in _Samia cynthia_, and is large and well marked in the European
_Saturnia pavonia-minor_ and _Endromis versicolora_. In _Bombyx
mori_ the spines are not well marked, and they are quite different
from those in the Attaci. There are three sharp points, being acute
angles of the pieces at the base of the wing, and it must be these
spines which at times perform the cutting through of the threads of
the cocoon described by Réaumur, and which he thought was done by
the facets of the eyes. It is well known that in order to guard
against the moths cutting the threads, silkraisers expose the cocoon
to heat sufficient to destroy the enclosed pupa. In Platysamia the
cocoon-cutters, though well developed, do not appear to be used at
all, and the pupa, like that of the silkworm and other moths
protected by a cocoon, moistens the silk threads by a fluid issuing
from the mouth, which also moistens the hairs of the head and
thorax, together with the antennæ. It remains to be seen whether
these structures are only occasionally used, and whether the
emission of the fluid is not the usual and normal means of egress of
the moth from its cocoon. Dr. Chapman remarks that throughout the
obtected moths “there are many devices for breaking through the
cocoon: specially constructed weak places in the cocoon, softening
fluid, applied by the moth, assisted by special appliances of
diverse sorts, such as in Hybocampa[107] and Attacus,” etc.

As to the fluid mentioned above, Trouvelot states that it is
secreted during the last few days of the pupa state, and is a
dissolvent for the gum so firmly uniting the fibres of the cocoon.
“This liquid is composed in great part of bombycic acid.” (Amer.
Naturalist, i, p. 33.)

The pupa of the dipterous genus Sciara (_S. ocellaris_ O. S.)
resembles a tineid pupa, and before transforming emerges for about
two-thirds of its length from the cocoon; the pupa-skin remaining
firmly attached in this position.[108]

Certain hymenopterous pupæ are provided with temporary deciduous
conical processes. Thus we have observed in the pupa of _Rhopalum
pedicellatum_ two very prominent acute tubercles between the eyes
(_h_, Fig. 592). As the cocoon is very slight, these may be of use
either in extracting itself from the silken threads or in pushing
its way along before emerging from the tunnel in the stem of plants.
(See also p. 611.)

_c._ The cremaster

Although this structure is in general confined to lepidopterous pupæ, and is not always present even in them, since it is purely adaptive in its nature, yet on account of its singular mode of development from the larval organs, and the accompanying changes in the pupal abdomen, it should be mentioned in this connection. The cremaster is the stout, triangular, flattened, terminal spine of the abdomen, which aids the pupa in working its way out of the earth when the pupa is subterranean, or in the pupa of silk-spinning caterpillars its armature of secondary hooks and curved setæ enables it to retain its hold on the threads of the interior of its cocoon after the pupa has partially emerged from the cocoon, restraining it, as Chapman well says, “at precisely that degree of emergence from the cocoon that is most desirable.” He also informs us that while in the “_pupæ incompletæ_ the cremaster is attached to an extensible cable, which always allows some emergence of the pupa, in the pupæ obtectæ there is no doubt but that in such cases as the Ichthyuræ, Acronyctæ, and many others, it retains the pupal case in the same position within the cocoon that the living pupa occupied; this is also very usually the case in the Geometræ and in the higher tineids (my pyraloids).”

In many of the more generalized moths there is no cremaster
(Micropteryx, Gracilaria, Prodoxus, Tantura, Talæporia, Psychidæ,
Hepialidæ, Zeuzera, Nola, Harrisina), though in Tischeria and
Talæporia (Fig. 590, but not in Solenobia) and Psychidæ, two stout
terminal spines perform the office of a cremaster, or there are
simply curved setæ on the rounded, unarmed end of the abdomen, as in
Solenobia.

In the obtected Lepidoptera, for example in such a group as the
Notodontidæ, where the cremaster is present, though variable in
shape, it may from disuse, owing to the dense cocoon, be without the
spines and hooks in Cerura, or the cremaster itself is entirely
wanting in Gluphisia, and only partially developed in Notodonta. In
the butterflies whose pupæ are suspended (Suspensi), the cremaster
is especially well developed. Reference might here be made to the
temporary pupal structures in certain generalized moths, which take
the place of a cremaster, such as the transverse terminal row of
spines in Tinea, the two stout spines in Tischeria, and the dense
rough integument and thickened callosities of the pupal head and end
of abdomen of Phassus, which bores in trees with very hard wood;
also the numerous stout spines at the end and sides of the abdomen
in Ægerians. These various projections and spines, besides acting as
anchors and grappling hooks, in some cases serve to resist strains
and blows, and have undoubtedly, like the armature in the larvæ and
imagines of other insects, arisen in response to intermittent or
occasional pressure, stresses, and impacts.

=Mode of formation of the cremaster and suspension of the chrysalis in butterflies.=—We are indebted to Riley[109] for an explanation of the way the cremaster has originated, his observations having been made on species of over a dozen genera of butterflies (Suspensi).

He shows that the cremaster is the homologue of the suranal plate of the larva.[110] The preliminary acts of the larva have been observed by various authors since the days of Vallisneri, _i.e._ the larva hanging by the end of the abdomen, turning up the anterior part of the body in a more or less complete curve, and the skin finally splitting from the head to the front edge of the metathoracic segment, and being worked back in a shrivelled mass toward the point of attachment. The critical feat, adds Riley, which has most puzzled naturalists, is the independent attachment of the chrysalis and the withdrawal from and riddance of the larval skin which such attachment implies. Réaumur explained this in 1734 by the clutching of the larval skin between sutures of the terminal segments of the chrysalis, and this is the case, though the sutures act in a somewhat different way.

Before pupation the larva spins a mass or heap of silk, the shape of
which is like an inverted settee or a ship’s knee, and “one of the
most interesting acts of the larva, preliminary to suspension, is
the bending and working of the anal parts in order to fasten the
back of the (suranal) plate to the inside of the back of the settee,
while the crotchets of the legs are entangled in the more flattened
position or seat.”

In shedding the larval skin, the following parts are also shed, and
have some part to play in the act of suspension: _i.e._ 1st, the
tracheal ligaments (Fig. 593, _tl_), or the shed tracheæ from the
last or 9th pair of spiracles; 2d, the rectal ligament (Fig. 593,
_rl_), or shed intestinal canal; 3d, the Osborne or retaining
membrane (_membrana retinens_, Fig. 593, _mr_), which is the
stretched part of the membrane around the rectum and in the anal
legs, and which is intimately associated with the rectal ligament.

FIG. 593.—Shrunken larval skin of _Vanessa antiopa_, cut open from
the back and showing (_mr_) the retaining membrane, (_rl_) the
rectal ligament, and (_tl_) the tracheal ligaments.
]

The structures in the chrysalis are, first, the cremaster, with its
dorsal (Fig. 594, _dcr_) and ventral (_vcr_) ridges, and the
cremastral hook-pad (_chp_), said by Riley to be “thickly studded
with minute but stout hooks, which are sometimes compound or
furnished with barbs, very much as are some of our fishing-hooks,
and which are most admirably adapted to the purpose for which they
are intended.”

FIG. 594.—Ideal representation of the anal subjoint of _Vanessa
antiopa_, from behind, with the spines removed, and all parts
forced apart by pressure so as to show the homologies of the parts
in the chrysalis which are concerned in pupation: homologies
indicated by corresponding letters in Fig. 595, except that _r_
(the rectum) corresponds with _pr_ in Fig. 595.
]

FIG. 595.—Anal parts of chrysalis of _Vanessa antiopa_, just prior
to final extraction from shrunken larval skin: _c_, cremaster;
_chp_, cremastral hook-pad; _h_, one of the hooks, more enlarged;
_vcr_, ventral cremastral ridge; _dcr_, dorsal cremastral ridge;
_lr_, larval rectum; _pr_, pupal rectum; _rp_, rectal plate; _sr_,
sustentor ridges; _mr_, _membrana retinens_; _rl_, rectal
ligament; _tl_, tracheal ligament; the 11th or last
spiracle-bearing joint and the 12th joint being numbered.
]

Secondly, there are the other structures, viz., the sustainers
(_sustentors_), two projections which Riley states “homologize with
the soles (_plantæ_) of the anal prolegs, which take on various
forms (3), but are always directed forward so as easily to catch
hold of the retaining membrane.” These sustentors are, however, as
Jackson[111] has shown, and as we are satisfied, the vestiges of the
anal legs.

FIG. 596.—_A_, chrysalis of Terias. _B_, posterior end of chrysalis
of Paphia. _C_, posterior end of chrysalis of Danais. _E_, one of
the sustainers of Terias, greatly enlarged to show its hooked
nature. All the parts of subjoint lettered to correspond with Fig.
595.
]

Thirdly, the sustentor ridges, which, as Riley states, may be more
or less obsolete in some forms, in Paphia (Fig. 596, _B_) and
Limenitis form “quite a deep notch, which doubtless assists in
catching hold of the larval skin in the efforts to attach the
cremaster.”

FIG. 597.—Pupation of butterflies: _a_, attachment of larva of
_Danais archippus_; _p_, attachment of larva of _Paphia
glycerium_; _b_, ideal larva soon after suspension; _d_, ideal
larva a few hours later, the needle (_n_) separating the forming
membrane from the sustainers; _l_, ideal larva just before
splitting of larval skin, with retaining membrane loosened from
the sustainers and showing its connection both with the larval and
pupal rectum. In all the figures the joints of the body are
numbered; the forming chrysalis is shaded in transverse lines; the
intervening space between it and larval skin is dotted: _h_, is
the hillock of silk; _hl_, hooks of hind legs; _ap_, anal plate;
_lr_, larval rectum; _pr_, pupal rectum; _mr_, retaining membrane;
_c_, cremaster; _s_, sustainers.—This and Figs. 593–596 after
Riley.
]