198 On Aquatic Carnivorous Coleoptera or Dijtiscidce. 



scratches on the wing-cases : it might therefore be roughly said that in Copelatus 

 the males have gained a highly developed and pseudo sexual sculpture as ■well 

 as the females, but that the latter have in addition to this a minute sculpture of a 

 different character and confined to their sex. In the genus Coptotomus there 

 exists a minute difference in the punctuation of a portion of the wing-cases, the 

 punctures in the female being more elongate than in the male. And in Matus 

 (Colymbetes bicarinatus No. 1)07) the females have the upper surface less shining 

 than the males, owing to the existence of an extremely minute, almost impercep- 

 tible, sculpture, which appears to be slightly more developed in the former of the 

 two sexes. In the genus Rhantus sexual differences of sculpture are usually either 

 absent or very slight, but in some species become important. Thus in Dytiscus 

 bistriatus (No. 949) the female may on a very careful examination be seen to possess 

 a slight develojjment of a more coarse reticulate sculpture than the male on certain 

 spots situated along the lines of serial punctures ; and in Dytiscus pustulatus (No. 

 945) there are two small patches of a similar but deeper reticulation, one on the 

 outer, and the second on the middle of the two rows of serial punctures, some little 

 distance behind the base. In Pthantus anisonychus ? the sculpture on the outer and 

 basal portion of the wing case, is a good deal deeper than in the male ; and in 

 Rhantus discedens (No. 938) a deep coarse sculpture exists in the female on the 

 part just mentioned ; while in Dytiscus notatus (No. 947) the whole of the wing- 

 case, except a small portion at the outer and apical part, is covered Avith such 

 sculpture : in this latter species however the female is sometimes smooth like the 

 male, and individuals occur in which the sculpture is intermediate in extent between 

 these two extremes. The peculiar sculpture of the genus Colymbetes exhibits some 

 very interesting sexual peculiarities ; although in several species no appreciable 

 difference between the sexes in this respect can be detected, yet in others very 

 notable differences exist ; in Colymbetes sculptilis (No. 968) the peculiar transverse 

 scratches of this genus are placed closer to one another in the female than they are 

 in the male ; while in Dytiscus dolabratus (No. 971) and in D. striatus (No. 

 972) the female peculiarity is that the scratches are very much deeper than in the 

 male ; in Colymbetes exaratus (No. 966) the strice or scratches are a little deeper 

 and a little closer together in the female than they are in the male, and moreover 

 in the former, the basal portion of the wing-case is rendered very dull, by the 

 development of an excessively minute sculpture (consisting of very small longitudinal 

 corrugations) which is quite absent in the other sex : in Colj'mbetes dahuricus* the 

 sculpture of the female presents very exceptional peculiarities, it is decidedly finer 

 than in the male, but it is more irregular, owing to the transverse scratches, being 

 less elongate, and so running much more frequently into one another, and the 

 surface is duller than in the male owing to the greater development of an exces- 



* There is some little doubt whotlier the specimens of this species I here allude to are really the sexes 

 of one and the same species. 



