INSECTS IN RELATION TO OTHER ANIMALS 279 



constitute 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 com- 

 pact 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 com- 

 pound, 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, sacrifices the general adaptability of the animal, 

 makes it impossible for a parasite to adapt itself to new con- 

 ditions ; and while parasitism may be an immediate advantage 

 to a species, there are few parasites that have attained any 

 degree of dominance among animals. Ichneumonidse, to be 

 sure, are remarkably dominant among insects, but here the 

 parasitic adaptations are limited for the most part to the larval 

 stage and the adults may be said to be as free for new adapta- 

 tions as are any other Hymenoptera. 



Scavenger and Carrion Insects. Not a few families of 



