Page:Insects - Their Ways and Means of Living.djvu/382



blood is not known, but the saliva of some files is said to prevent coagulation of the blood. Some of the smaller horseflies will give us an unsolicited sample of their biting powers, and in shaded places along roads they often make themselves most vexatious to the foot traveler just when he would like to sit down and enjoy a quiet test. To horses, cattle, and wild mammals, how- ever, these files are extremely annoying pests, and, where abundant, they must make the lives of animais almost unendurable; for the sole means of protection the latter have against the painful bites of the flies is a swish of the tail, which only drives the insects to make a fresh attack on some other spot. There is another family of "biting" flies, known as the robber flies, or Asilidae (]Tig. 167), the members of which attack other insects. They are strong flyers and take their victims on the wing, even bees falling prey to them. The robber flies have no mandibles, and the strong, sharp- pointed hypopharynx appears to be the chief piercing implement. The saliva of the tir injected into the wound dissolves the muscles of the victim, and the predigested solution is then completely sucked out. As was shown in Çhapter VIII, on metamorphosis, ? whenever the adult form of an insect is highly specialized for a particular kind of lire, it is usually round that the young is also specialized but in a way of its own to adapt it to a manner of living quite different from that of its parent. This principle is particularly true of the flies, for, if the adult flies are to be regarded as in general the most highly evolved in structure of all the adult insects, there can be no doubt that the young fly is the most highly specialized of all the insect larvae. The files belong to that large group of insects which do hot have external wings in the larval stage, but with the flies the suppression of the body appendages includes also the legs, so that their larvae are hot only wingless but legless as well (Fig. 7I). The legs, however, as the wings,

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