INSECTS IN RELATION TO OTHER ANIMALS 21 7 



glues its eggs to the hairs of horses, especially on the fore legs and shoul- 

 ders, 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. 211, /) reaches the oesophagus of its host in the same manner as the 

 horse bot, according to Curtice, but then makes its way into the sub- 

 cutaneous 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, however, 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 digestive 

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

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

 head usually 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, however, 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, sacri- 

 fices 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 that have 

 attained any degree of dominance among animals. Ichneumonidae, to 



