198 On Aquatic Carnivorous Coleoptera or Dytiscide. 
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. 907) 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 development 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 Rhantus 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 with 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 Colymbetes dahuricus* the 
sculpture of the female presents very exceptional peculiarities, it is decidedly fmer 
than in the male, but it is more irregular, owing to the transverse scratches, bemg 
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 whether the specimens of this species I here allude to are really the sexes 
of one and the same species. 
