108 THE ENTOMOLOGIST. 



yellow ; the head-shield and the fore feet are reddish, or choco- 

 late brown. 



After the last skin-change the hybrid larvae acquire an alto- 

 gether different dress from sordida, corresponding to the great 

 mutual modifications. Many were fox-red-haired (like the larvae 

 of mendica) ; in the majority the hairs were dark brown or 

 brown-gray ; some were light gray. In all the light, yellow- 

 bordered longitudinal stripes (dorsal and lateral) were as sharply 

 marked as in sordida, the last segments were reddish yellow 

 laterally ; the two last marks appear to be characteristic of these 

 hybrid larvae. 



In general, they do not occupy a position exactly in the 

 middle between the progenitors, but much nearer to the larvae of 

 sordida, only some examples were nearer to those of mendica 

 (var. rustica) than those of sordida. 



A GUIDE TO THE STUDY OF BRITISH WATERBUGS 



(AQUATIC RHYNCHOTA). 



By G. W. Kirkaldy. 



Plate I. 

 (Continued from p. 8.) 



Polymorphism. 



The phenomenon of Polymorphism — or the occurrence, in the 

 same species, of two or more different imaginal forms— obtains 

 to a very large extent among certain Heteroptera, and is asso- 

 ciated with the greatest alteration in structure in the semi- 

 aquatic species. Although there are more noticeable differences 

 in some exotics — e.g. the remarkable Ptilomera laticaudata 

 (Hardwicke) and the still more extraordinary Metrocoris stall 

 (Dohrn) — both from the Oriental Eegion, — nevertheless a very 

 fair example is afforded by the British Velia rivulorum (Fabr.) = 

 currens (Fabr.), a species well distributed over the British Isles. 

 Plate i., fig. 1, represents the head, thorax, etc., of the ordinary 

 apterous form ; fig. 2, the same parts of the rarer macropterous 

 form. In the former the pronotum is short (not completely 

 concealing the mesonotum in the middle, and leaving the 

 metanotum totally exposed), rather flat, gently rounded at the 

 sides, the base slightly rounded. In the latter the pronotum is 

 produced posteriorly over the mesonotum (which it conceals 

 except near the postero-lateral angles) and a part of the metano- 

 tum, is irregularly angular laterally, and is somewhat convex. 

 The powerful elytra and alae require strong working and support- 

 ing muscles, which are contained inside the prothorax ; when, 

 however, the flight-organs are altogether absent, or present only 



