1908] Habits of Insects as a Factor in Classification 81 



The bot flies illustrate remarkably well the degrees of specializa- 

 tion in the parasitic life, the sheep bot which lives in the frontal 

 sinus of the skull representing a much less important departure 

 from a non-parasitic form than the horse bot which lives within the 

 stomach and must be adapted not only to a special mode of 

 nutrition but to a particular limitation in the matter of respiration, 

 a feature which goes still further in the case of the ox bot with its 

 circuitous route from egg through alimentary tract to its final 

 resting place beneath the skin. We cannot conceive this latter 

 form of adaptation except as a derivation from the more simple 

 form of parasitism, and our classification in this group may well 

 take this into consideration. A particularly extreme form of 

 parasitism with the results of parasitic life is exhibited in the 

 sheep tick and its allies where there has been not only a striking 

 modification of the structures of the body, but a profound modifi- 

 cation in its mode of development. In this it shows an extremity 

 which is perhaps not exceeded by any other group of animals 

 though paralleled by the parasitic Stylopidse, and in every detail 

 of which we must recognize the effect of the parasitic life. The 

 other forms of parasitism such as the occupancy of the nest of 

 bees by flies or other species of bees which occur in a bewildering 

 number of intricate forms cannot be dwelt upon here. The re- 

 markable adaptations of such parasitic forms as the Ichneumons 

 and Chalcids in their adjustment to plant lice and scale insects, 

 and the egg parasites in their extreme adjustment to the com- 

 pletion of a life cycle within the minute Qgg of some other species 

 of insect, cannot fail to occur to all who have become at all familiar 

 with the complexities of insect life. 



I may perhaps be permitted to further illustrate this idea with 

 one other example drawn from a group which has been one of my 

 special studies. The Pediculid^ are, I will grant, a not very 

 popular division of insects and yet in some of their adaptations 

 and in the long course of parasitism which they seem to have 

 undergone, they give us some of the most positive evidences as to 

 the effects of the parasitic habit and also as to their course of 

 evolution. We may readily appreciate their long adoption of the 

 parasitic habit when we consider the wide di\'ergence they show 

 from other groups of insects, and the range of their hosts, and 

 yet we must assume beyond question that their establishment as a 

 parasitic group has been subsequent to the evolution of the group 

 of mammals of which they are exclusively parasites. 



