A GUIDE TO THE STUDY OF BKITISH WATERBUGS. 109 



vestigially, the thorax is much smaller and simpler, on account 

 of the more or less rudimentary condition of the alar muscles. 

 This distinction is even more remarkable in Hebrus, Mesovelia, 

 and Hydroessa, but not usually so marked, though sufficiently 

 noticeable, in the British species of Gerris. 



Not only may the same species be dimorphic (comprehending 

 macropterous and apterous forms), but even polymorphic, through 

 the occurrence also of brachypterous individuals ; * in the last, 

 the elytra may be only slightly shorter than the fully-winged 

 form, and the alse be present and functional, or the elytra may 

 be represented merely by short pads, the alae being absent, or 

 there may be various intermediate degrees between these. In 

 this respect, Gerris lacustris (Linn.) has provided me with more 

 variations — at least six, from apterous to macropterous — than 

 any other species, t It is noteworthy that in the brachypterous 

 forms, the two abbreviated elytra are not always equal in length 

 and development (in the same specimen). 



It is not difficult to understand something of the causes that 

 underlie this remarkable variety of structure. 



The intermediate and posterior pairs of legs afford to the 

 " surface-skaters " an ample means of progression on the streams 

 and sheets of more or less still water which they frequent, J and 

 the absence of wings is (considering their environment) not only 

 an advantage, but, I apprehend, necessarily ensues from the 

 operation of Natural Selection in tending to weed out the macro- 

 pterous and brachypterous forms. § Under certain circumstances, 

 however, the results of Natural Selection will prove deleterious 

 to the species, by producing only apterous individuals, so that 

 local (perhaps even total) extermination of the species will 

 ensue when the conditions of their habitat are altered for a 

 sufficiently prolonged period of time. Upon large sheets of 

 water, with many inlets and other places favourable to the 

 flourishing of these species, winged forms will be superfluous, 

 and will, in course of time, only appear rarely. In small wayside 

 pools or streamlets which dry up periodically, it is obvious that 

 the apterous inhabitants will die out unless their habitat is 

 sufficiently near to a larger stream or pond ; a severe drought 

 occurring over a fairly large area, prolonged sufficiently to dry 



* pax.(0q (mdkros), long; fifd%vq (brachus), short; a (a) privat. 



f See Miall's ' Natural Hist, of Aquatic Insects,' fig. 103, No. 1, and 

 fig. 104, No. 8, where a macropterous Gerris thoracicus and a brachypterous 

 G. lacustris ( '? ) are figured. 



I So far as nay own observations go — and I am not aware of any published 

 researches that tend to invalidate thern — the Hebrides and Gerritlce always 

 escape from danger by the employment of their legs, never by flight, the 

 wings being used only for migration. 



§ F. B. White was undoubtedly wrong in supposing that the apterous 

 represent the primitive forms. 



