INSECTS 



blood is not known, but the saliva of some flies 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 rest. To horses, cattle, and wild mammals, how- 

 ever, these flies are extremely annoying pests, and, where 

 abundant, they must make the lives of animals 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 (Fig. 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 fly injected into the wound 

 dissolves the muscles of the victim, and the predigested 

 solution is then completely sucked out. 



As was shown in Chapter VIII, on metamorphosis, 

 whenever the adult form of an insect is highly specialized 

 for a particular kind of life, it is usually found 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 flies belong to that large group of insects which do 

 not 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 not only wingless but 

 legless as well (Fig. 171). The legs, however, as the wings, 



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