INSECTS
pupae of the ancestors of such species lived in the same medium as the larvae.
If our supposition is correct, we may see a reason for the apparent exception in the flies to the general rule that the pupa presents the adult structure and discards the peculiarly larval characters. The pupae of some flies whose
Fig. 173. Larva (A) and pupa (B) of a horsefly, Tabanus punctifer (about 1½ times natural size)
An, anus; H, head; PSp, posterior spiracle; Sp, spiracle
larvae live in the water, however, revert at once to the adult system of lateral spiracles (Fig. 173 B, Sp). With such species, the larva comes out of the water just before pupation time and transforms in some place where breathing is possible by the ordinary respiratory organs. This is the general rule with other insects whose larvae are aquatic.
The order of the Diptera is a large one, and we might go on indefinitely describing interesting things about flies in general. Such a course, however, would soon fill a larger book than this; hence, since we are already in the last chapter, a more practical plan will be to select for special consideration a few species that have become closely associated with the welfare of man or of his domesticated animals. Such species include the mosquitoes, the house fly, the blowfly, the stable fly, the tsetse fly, the flesh flies, and related forms.
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