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Most of our best authorities are now convinced that the various larval and pupal stages of insects have thus been acquired through adaptation, and not through inheritance from some ancient form. The curious case of Sitaris a beetle which passes through certain unusual stages of development will illustrate how this might occur.
The external differences between the larval and pupal states are fixed for a large part of the year in most butterflies and moths, though even in this respect there is every possible variation, some moths or butterflies passing through their transformations in a few weeks, others requiring several months, while still others take a year, the majority of the moths living under ground in the pupa state for eight or nine months.
These insects belong among the orthoptera an order including species whose transformations are less complete than in other groups, and whose larval and pupal forms are very active, and closely resemble the imago.
The male, at the close of the pupal rest, escapes from one end of this singular covering, but the female makes it her dwelling for life; moving about with it at pleasure, and entrenching herself within it, when alarmed, by drawing together the purse-like aperture at the open end.
One character, which is unique among insects, is peculiar to Ephemerids; the adults issuing from the pupal envelope undergo still another moult in divesting themselves of a thin pellicle that covers the body, wings, and other appendages. This is what is called the subimago, and precedes the imago or perfect state of the insect.
About eight or ten days after hatching the larvæ spin delicate brownish cocoons in which they pass the pupal stage, issuing a few days later as the adult fleas. It will at once appear, then, that it is important to provide the cats and dogs with sleeping-places that can be kept clean.
A succeeding stage of this mite, which may be called the pupal, is considerably smaller than the larva and looks somewhat like the adult, the body having become shorter and broader.
As most insects emerge from the pupal state in a mature condition, it is doubtful whether the period of development can determine the transference of their characters to one or to both sexes.
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. Instead of the breathing-tube on the eighth segment of the abdomen as in the larva, the pupa has two trumpet-shaped tubes on the back of the thorax through which it now gets its air from above the surface.
It is also an interesting fact, as bearing on sexual selection, that the stridulating organs of certain male Orthoptera are not fully developed until the last moult; and that the colours of certain male dragon-flies are not fully developed until some little time after their emergence from the pupal state, and when they are ready to breed.
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