On Aquatic Carnivorous Coleoptera or Dytiscidce. 197 



sculpture of the two sexes ; thus in Dytiscus parallelo^rammus (No. 416) the female 

 differs from the male in that the punctuation is very much finer and less deep and 

 the surface is dull, and on examination with a considerable power of the microscope 

 it is seen that the dulhiess arises from the surface being covered with a very dense 

 minute sculpture of a peculiar character, but a good deal similar to that which is 

 seen on a larger scale in Meladema coriacea (No. 978). In another species of 

 Coelambus (Dytiscus impressopunctatus No. 409) the phenomena are quite different ; 

 here the female usually resembles the male in sculpture except that its punctuation 

 is a little denser and finer : there occurs however very rarely a second form of the 

 female (Dytiscus lineellus Gyll.) very different from the male, the surface being 

 much more finely punctured and dull, and on examination with a considerable 

 power of the microscope it is seen that the dullness arises from the surface being 

 covered with an extremely fine obsolete sculpture of a reticulate nature such as is 

 seen in Agabus. In the large genus Hydroporus the females frequently differ in 

 sculpture from the males in a more or less conspicuous manner, and the difference 

 is sometimes dimorphic {vide Hyphydrus memnonius No. 558). In the Agabini 

 we find in the genus Agabus that sexual differences of sculpture are of frequent 

 occurrence : the differences is sometimes slight as in the case of Dytiscus guttatus 

 (No. 670) where the female has the sculpture a little coarser than in the male ; or 

 it may be very considerable as in A. lecontei and A. griseipennis (No. 731 and 

 732), where the male is less but the female is more reticulate than is usual in the 

 genus ; or the sexual differences of sculpture may be variable as in the cases of 

 Dytiscus congener (No. 706), and Dytiscus bipustulatus (No. 751), and in these 

 cases the variations seem to be, to some extent at any rate, dependent on the 

 locality the specimens inhabit. 



In the genus Copelatus a difference in the sculpture of the two sexes is very fre- 

 quent, and the differences are in some species very great, in others only slight, and are 

 quite as frequently found on the surface of the prothorax as on the wing-cases ; 

 they usually consist of very short linear impressions, or fine, rather irregular scratches, 

 dimorphism of the females is occasional, and when it exists is strongly marked, the 

 female being either similar to the male in sculpture, or having a large area of the 

 surface covered with an additional sculpture (vide Copelatus neglectus No. 841). 

 It should be remembered that this genus is unique among the Dytiscidae by 

 reason of the existence in it of a remarkable sculpture common to the two sexes : 

 this non-sexual sculpture is of quite a similar nature to the sexual sculpture of other 

 genera : the impressions on the thorax of Colymbetes sulcipeunis (No. 895) and 

 the regular grooves on its elytra are just such as would (from what exists in the 

 females of other genera) make one suppose them to be a female sexual sculpture, 

 and yet they are equally developed in both male and female ; it is further interest- 

 ing to note, that the females of the species I am alluding to (and of other Copelati) 



have an additional sexual sculpture consisting of some extremely fine irregular 



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