Molting

“ It is important to understand the process of molting as it takes place in the grasshopper, because the processes of metamorphosis, such as those which accomplish the transformation of a caterpillar into a butterfly, differ only in degree from those that accompany the shedding of the skin between any two stages of the grasshopper's life. The principal growth of the insect is made during those resting periods preceding the molts. ”

“ In some individuals a gray patch on the nape remained but emerging hair was not apparent; perhaps the molt had been halted just prior to completion. The progressing band of emerging hair is narrow in most specimens but in some up to one-fifth of the circumference of the body has hair at the same degree of emergence. Subsequent molts, both from postjuvenal to adult pelage and between adult pelages, are less regular in point, or points, of origin, width of progressing molt, and amount of surface molting at one time. ”

“ I also found all voles molting to be between eight and 12 weeks old but found none so large as 38 grams without full adult pelage. This may have been, in part, due to the dry weather delaying or inhibiting growth. Because of the small size of the sample and the influence of the unusual weather conditions, no conclusions concerning normal molting were drawn from the data described below. They are presented only as a description of a small sample drawn from a single population at one time. ”

“ These seven molting individuals make up nearly 17 per cent of 42 individuals that had completed the juvenal to postjuvenal molt. In November, 80 per cent of individuals (92 of 115) that had previously obtained their postjuvenal or adult pelage were molting. These mice were in age-categories 3, 4, and 5. Some of the individuals in category 3 were developing new hair beneath a relatively unworn bright pelage that I judge to be an adult pelage rather than a postjuvenal pelage. ”

“ With the exception of a synoptic collection presented to the Departamento de la Fauna Silvestre, Dirección General de Caza, México, D.F., all of the specimens mentioned are in the museum at the University of Kansas.
Unless otherwise indicated, specimens taken were not molting. For birds undergoing postnuptial or postjuvenal molt, the degree of advancement of the molt is indicated by the numbers of primaries and rectrices that are new or partly ensheathed. Information on the condition of the testes refers to the left testis unless otherwise noted. ”

“ Adult male in molting plumage may be distinguished from the adult female by the greater brilliancy of the blue and metallic green on the wing, the plain dark upper tail-coverts, and the general darker color of the entire plumage ”

“ This plumage fades somewhat during migration and the body plumage is mostly all molted before October. In their first winter plumage young birds can be distinguished from adults only by the wing coverts and a few retained scapulars and tertials. At the first prenuptial molt they become practically adult.
Adults have a complete postnuptial molt in the summer and fall, molting the body plumage in July and August and the wings and tail in November or later. The prenuptial molt in March and April involves only the body plumage. ”

“ The larva strength ens this capping by a loose webwork of silk within, extending down the side but slightly and attached at its edges to the last skin cast by the molting larva. This skin, extremely delicate and pressed closely against the inside of the cell, forms the lining of its sides and bottom. ”

“ Their development is retarded by cold weather or lack of food, and they remain alive in the larval state, in such conditions and particularly in a dry atmosphere, for an almost indefinite period molting frequently and feeding upon their cast skins. Under normal conditions, however the skin is cast about six times, and there are, probably, in the North, not more than two annual generations. When the larva reaches full growth the yellowish pupa is formed within the last larval skin. ”

“ Then a new skin forms underneath and the old skin is cast off. This process of casting off the old skin is called molting, and is repeated four times during the one, two, three or more weeks of larval life.
PUPA
With the fourth molt the active feeding larva changes to the still active but non-feeding pupa (Fig. 59) . The head and thorax are closely united and a close inspection will reveal the head, antennæ, wings and legs of the adult mosquito folded away beneath the pupal skin. ”

“ As the changes of structure are necessarily confined to the molting periods, the stages of growth coincide with the stages of change in organization, and there is none of the indefiniteness which often characterizes the different larval stages of animals with a more continuous metomorphosis. ”

“ Instead crabs must regularly shed their exoskeletons in order to increase in size. This process is known as molting, and individuals typically increase in size by about 15 to 25 per cent with each molt. ”

“ Molting commonly occurs from June to September, but there is no month of the year in which soft lobsters may not be caught.
(14) The male probably molts oftener than the female.
(15) In the adult female the molting like the spawning period is a biennial one, but the two periods are one year apart. As a rule, the female lays her eggs in July, carries them until the following summer, when they hatch; then she molts. Possibly a second molt may occur in the fall, winter, or spring, but it is not probable, and molting just before the production of new eggs is rare. ”

“ A period of about 11 weeks was formerly supposed to be necessary for the complete maturity of the insect, but breeding experiments with this insect, conducted in this department in 1896, indicated that the life cycle is subject to great variation, being entirely dependent on warmth and food supply. Under favorable conditions of temperature and food it was found that there was an average period of about eight days between moltings and between the laying of eggs and their hatching, giving about seven weeks as the period under these conditions from egg to adult insect. ”

“ The entire pupal stage is divided bi the last molting of the young into a propupal period, still occupying the loosened cuticula of the insect in its last adolescent stage, and a true pupal period, which is that between the shedding of this last skin of the young and the final molt which discloses the matured insect. All insects that undergo a metamorphosis may be divided, therefore, into two classes according as the trans- formation from the young into the adult is direct or is COlnpleted in an intervening pupal stage, lnsects of the first class are said to have ineomplete metamorphosis ”

“ The moltings take place in the tent, except the molt of the caterpillar to the pupa, and each molt renders the caterpillars inactive for the greater part of two davs. When most of them shed their skins at the same time there results an abrupt cessation of activity in the colony. By the rime the caterpillars reach maturity the discarded skins in a tent outnumber the caterpillars rive to one. ”

“ This moulting takes place several times, until the body is fully grown, when the cuticle formed at the last moult is retained by the now adult insect for the rest of its life.
The different stages through which an insect passes from egg to adult constitute its life history, or life cycle, and the relation of the latter to the seasons, its seasonal history. According to the species, a full twelve months or even more may be necessary for the complete life cycle, or the cycle may be repeated several times within the year; when the cycle occupies twelve months, the insect is single-brooded ”

“ Those species which drop to the ground to molt must again climb to some favorable point and wait for another host on which they may feed for a while. Then they drop to the ground for a second molt and if they are successful in gaining a new host for the third time they feed and develop until fully mature and the female is ready to lay her eggs. ”

“ The life of a winged insect, such as a butterfly, fly, or bee, may be divided into four stages: the embryo, or egg state, the larva, pupa, and imago,—the term metamorphosis being applied to the changes after birth, or post-embryonic stages of life. The transformations of the more specialized orders of insects involve wonderful changes of form, which are only paralleled in other types of animals by the metamorphoses of the echinoderms, of certain worms, and of the Crustacea, as well as by those of the frog. ”

“ Adults molt, usually completely but occasionally only partially, at least twice a year—once in spring (in May and June in Nebraskan specimens) from winter to summer pelage, and once in autumn (in October and November in Nebraskan specimens) from summer to winter pelage. Of the two molts, the one in spring is most easily discernible because the contrast in color between worn winter pelage and fresh summer pelage is considerably greater than that between worn summer pelage and fresh winter pelage, and because the progress of spring molt is seemingly more regular than that of autumn molt. ”

“ The first molt is the shedding of the embryonic skin, which, we have seen, takes place normally as soon as the young insect emerges from the earth. The grasshopper now lives uneventfully for about a week, feeding by preference on young clover leaves, but taking almost any green thing at hand. ”

“ The female’s feelers are thread-like.
In the Vapourer Moth (Fig. 228) , whose “looping” flight may often be observed even in the streets of towns during the day, the two [Pg 370] sexes are remarkably different from each other. The male (C) alone can fly; the female (D) is wingless, and is confined for the whole of her short adult life to the place where she emerged from the cocoon. Here she lays her eggs and then dies. Neither she nor her mate is capable of feeding.
Fig. 228.—Stages of Vapourer Moth. A, Larva (male) ; B, male pupa; C, male moth; D, female moth. ”

“ It is not so with the molt, for it is useless as food, and like the detached wings with which it is always associated, is light and easily floated by rains into streams and rivers, and thence carried by currents into larger bodies of water, where permanent deposits are accumulating. Both wings and molts are fairly resistant to decay, and thus more readily preserved than are the softer parts of the body. ”

“ In this stage the eggs are produced. When there is very little resemblance between the larva and the imago, and no pupal stage, the metamorphosis, or change, is said to be complete. When, as with the grasshopper, no very marked change takes place between the larva and the imago, there being no pupal stage, the metamorphosis is said to be incomplete. By studying the illustrations and specimens, and by thinking of your past observations of insects, determine which of the insects in the following list have a complete metamorphosis: beetle, house fly, grasshopper, butterfly, cricket, wasp. ”

“ It is scarcely possible to account for such a succession of change in plumage in so short a time, except by supposing that a change in the constitution of the bird, produced by captivity and a want of natural food, had caused obstruction to the usual course of moulting, and that the autumnal change had been retarded, and was scarcely effected before the spring moulting commenced. ”

“ In the former of the ways mentioned, the generation of insects is effected where by metamorphosis a worm is born from an egg; or out of a putrescent material, the drying of a moist substance or the moistening of a dry one, rudiments are created, from which, as from a caterpillar grown to its full size, or from an aurelia, springs a butterfly or fly already of a{335} proper size, which never attains to any larger growth after it is first born; this is called metamorphosis. But the more perfect animals with red blood are made by epigenesis, or the superaddition of parts. ”

“ Outstanding features in a life-cycle involving metamorphosis are that growth takes place only in the larval state, and that the insect parades through life in different guises—​egg, larva, pupa, and adult—​each with its own peculiarity of habit and form, although the adult and pupa resemble one another much more than do the adult and larva ”

“ Conversely, if the new muscles are not perfected at the rime of the last normal molt, the insect must have an extra molt later in order to give the muscles a functional connection with the body wall. Thus Poyarkoff would explain the origin of the pupal stage in the lire cycle of the insect. His theory has much to commend it, for, as Poyarkoff shows in an analysis of the various processes accompanying metamorphosis, none of the cha.nges in any of the organs other than the muscles would seem to necessitate the production of a new cutic- ula and thus involve an added molt. ”

“ The female of this elegant fly, when about to lay her eggs, builds up a tubular nest of cells with fine mud, which she makes by mingling and kneading road-dust with her saliva. Each tube consists of several cells, separated by transverse partitions of the mortar; and in each, before she closes it up, she lays a single egg, which she then covers with spiders which are to constitute the food of the grub when hatched, and to last it during the whole period of its larval growth. ”