VESTIGIAL WINGS AMONG INSECTS. 183 



a locality would more readily remove them from their proper 

 environment than in the lowlands, and consequently from condi- 

 tions favorable for the growth of their offspring. This would 

 favor wingless forms. 



As wingless insects are rarer in cold than in warm regions, the 

 cold does not seem to be a possible factor. 



The wingless condition of desert forms has never, I think, been 

 satisfactorily explained, although it is the expression of a well- 

 marked tendency. 1 



A secluded environment very often induces the atrophy of 

 wings. This is seen most strikingly among myrmecophilous and 

 cave insects. There can be but little doubt that in such cases 

 wings are nearly always an inconvenience or even danger. Thus 

 we find that all blind insects are also wingless, wings being 

 evidently detrimental to safety when unguided by vision. 



The influence of external parasitism apon the wings is so well 

 recognized that it need only be recalled in the present connection. 



From the fact previously alluded to, that the loss of wings is 

 at first very rapid and then suddenly becomes extremely slow 

 when they have reached a very vestigial condition, it can be 

 readily seen that attempts to ascertain the phylogenetic age of a 

 certain wingless type must be very difficult except in a very few 

 cases. Added to this the great differences in plasticity among 

 the widely separated groups with which we have to deal add 

 further difficulties. Nevertheless, by comparing the various cases 

 considered in the second portion of this paper, it is readily seen 

 that the loss of wings must be very readily brought about, for 

 nearly every apterous or subapterous form has a closely related 

 form living under apparently nearly the same conditions, which 

 is winged. Moreover, the loss of wings is usually accompanied 

 by only slight changes in the external morphology of the insect. 2 



1 In this connection it may be mentioned not as a full explanation, but as a fact 

 which may bear upon this question ; that in wingless coleoptera such as Eleodes and 

 allied forms the tight-fitting and immovable elytra must prevent to a great extent the 

 evaporation of water from the body and thus enable them better to withstand long and 

 severe droughts. The fact that Eleodes also occurs in certain moist regions can be 

 understood when we remember that arid regions seem to be the centers of distribution 

 for such forms. 



2 In many apterous coleoptera a correlated change is the shortening of the meta- 

 thoracic or wing-bearing segment. 



