AND FLIES
or buried in mud or any other sort medium, so long as it keeps one end of the body out for breathing. The rat-tailed maggot (Fig. I72), which is the larva of a large fly that looks like a drone bee, has taken a special advantage of its respiratory system; for the rear end of its body, bearing the posterior spiracles, is drawn out into a long, slender tube. The creature, which lives in foui water or in mud, can by this contrivance hide itself beneath a floating object and breathe through its tail, the tip of which may come to the surface of the water at a point some distance away. The end of the tail is provided with a circlet of radiating hairs surrounding the spiracles, which keeps the tip of the tail afloat and prevents the water from entering the breathing apertures. The great disparity of structure between the larva of a tir and the adult necessarily involves much reconstruc- ti'on during the period of transformation, and probably the inner processes of metamorphosis are more intensive in the more highly specialized Diptera than in any other group of insects. The pupa of an insect, as we have seen in Chapter VIII (page 254), is very evidently a preliminary stage of the adult, the larval characters being usually discarded with the last molt of the larva. The pupa ofmost files, however, while it has the general structure of the adult fly (Fig. ?82 A, F), retains the special respiratory scheme of the larva and at least a part of the larval breathing organs. The fact that the larvae breathe through special spiracles located on the back suggests that the primitive fly larvae lived in water or in sort mud, and that it was through an adaptation to such an environment that the lateral spiracles were closed and the special dorsal spiracles de- veloped. The retention by many fly pupae of the larval method of breathing and of at least a part of the larval respiratory organs, though their habitat would hot seem necessarily to demand it, suggests, furthermore, that the
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INSECTS
