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- 
ne, 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 1s 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 larve, 
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 1s 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. Ichneumonidee, 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 
