On Aquatic Carnivorous Coleoptera or Dytiscidce. 201 



their basal portion, with regularly placed elongate Impressions ; the largest of the 

 Dytiscidce, viz., Cybister giganteus No. 1117 and Megadytes ducalis No. 1118, have 

 no sexual sculpture whatever : in Homoeodytes we find the females of two species 

 to possess a most excessively fine sexual sculpture, consisting of extremely delicate, 

 short scratches, giving rise to a silky appearance on the basal part of the elytra. 



From this imperfect review of the sexual sculpture of the Dytiscidas it will be 

 gathered that much variety exists as to its character, and as to its degree of 

 development, and that the occurrence of two forms of the female of a single species 

 is not unfrequent in various portions of the family : there is considerable reason to 

 suppose that the development of the sexual sculpture is to some extent connected 

 with local and climatic conditions, and it may prove to be that it is of more frequent 

 occurrence in temperate and cold climates than in warmer ones : if it serves any, or 

 what purpose is still undetermined, but it is certainly amongst the most interesting 

 peculiarities of the Dytiscidse, and of considerable importance in respect to its 

 bearings on the questions of sexual variation and selection. 



The peculiar sculpture of the species of Copelatus is of much interest and well 

 deserving investigation. In some of the higher forms of the genus (Col. sulcipennis. 

 Lap. e.(j.) it appears as very deep and regular stria3 or grooves, to the number of 

 10, 11, or 12 on each wing-case, and extending neai'ly their whole length, but 

 becoming finer at the extremity, where usually some of them are more abbreviated 

 than others. In other species no trace of such striation can be detected ; but in 

 some of these cases the fine punctuation of the wing-cases assumes the form 

 of very short fine scratches, which while they may be very distinct in the female, 

 can sometimes scarcely be traced in the male (C. simplex, Clk.) The scratches 

 displayed in such cases frequently assume a different direction on the hinder portion 

 of the wing-case, becoming transverse there, when they have a longitudinal direction 

 in front (Celina australis No. 806) ; while in some other species only the transverse 

 posterior scratches exist (as in C ferrugineus). 



In some of the cases where the elytra are truly striate, i.e. display elongate 

 regular straight lines, it can be perceived that these are developed along the series 

 of impressed punctures which are so constant a feature of the Dytiscidae ; this is 

 not the case in those species, where the scratches are short, for they are not then 

 naore developed near the series of the punctures than they are elsewhere ; so it 

 would seem there are as regards the sculpture three ditFerent series of species, viz., 

 1, punctuation ordinary ; 2, punctuation elongate, and diffuse ; 3, punctuation 

 -elongate, but only along the lines of serial (and secondarily of the interserial) 

 punctures ; the punctuation of this latter category becoming in the most differentiated 

 cases, highly developed symmetrical strise. 



It seems impossible to believe that the development of this beautiful sculpture 

 can have been determined by the action of natural selection ; preserving those 

 individuals of a species in which it was more developed than in others, for iu 



