INSECTS IN RELATION TO OTHER ANIMALS 235 



through the nostrils and pupate in the soil. The horse bot-fly (Gastro- 

 , philus equi) glues its eggs to the hairs of horses, especially on the fore 

 legs and shoulders, whence the larvae are licked off and swallowed; once 

 in the stomach, the bots fasten themselves to its lining, by means of 

 special hooks, and withstand almost all efforts to dislodge them; though 

 when the bots have attained their growth they release their hold and 

 pass with the excrement to the soil. Bots of the genus Hypoderma form 

 tumors on cattle and other mammals, domesticated or wild. The ox- 

 warble (H. lineata, Fig. 213, 7) reaches the cesophagus of its host in the 

 same manner as the horse bot, according to Curtice, but then makes 

 its way into the subcutaneous tissue and causes the well-known tumors 

 on the back of the animal; when full grown the bots squirm out of these 

 tumors and drop to the ground, leaving permanent holes in the hide. 



Parasitism in General. Parasitic insects evidently do not consti- 

 tute a phylogenetic unit, but the parasitic habit has arisen independently 

 in many different orders. These insects do, hpwever, agree superficially, 

 in certain respects, as the result of what may be termed convergence of 

 adaptation. Thus a dipterous larva, living as an internal parasite, in 

 the presence of an abundant supply of food, has no legs, no eyes or anten- 

 nae, and the head is reduced to a mere rudiment, sufficient simply to 

 support a pair of feeble jaws; the skin, moreover, is no longer armor-like 

 but is thin and delicate, the body is compact and fleshy, and the diges- 

 tive system is of a simplified type. The same modifications are found in 

 hymenopterous larvae, under similar food-conditions, except that the 

 head undergoes less reduction. The various external parasites lack 

 wings, almost invariably, and the eyes, instead of being compound, 

 are either simple or else absent. In some special cases, as in a few 

 dipterous parasites of birds and bats, the wings are present, either 

 permanently or only temporarily, enabling the insects to reach their 

 hosts. 



This so-called parasitic degeneration, widespread among animals in 

 general and consisting chiefly in the reduction or loss of locomotor and 

 sensory functions in correlation with an immediate and plentiful supply 

 of food, results in a simplicity of organization which is to be regarded 

 not as a primitive condition but as an expression of what is, in one 

 sense, a high degree of specialization to peculiar conditions of life. 

 This exquisite degree of adaptation to a special environment, however, 

 sacrifices the general adaptability of the animal, makes it impossible 

 for a parasite to adapt itself to new conditions; and while parasitism 

 may be an immediate advantage to a species, there are few parasites 



