194 Oa Aquatic Carnii'orous Coleoptera or Dytiscidw. 



indefinite spots of pale colour on the vertex of the head. The upper and under- 

 surfaces of an individual are frequently very different in colour, the one being often 

 pallid while the other is entirely or in great part dark {e.g., Dytiscus rceselii, 

 Cybister, No. 1,169) : it is, I believe, always the case, when such discrepancy pre- 

 vails, that the upper surface is darker than the under, and the latter is never much 

 variegate as the upper surface sometimes is. As a rule the colouration of the two 

 sexes of one species is nearly or quite similar, but to this there are a few singular 

 and interesting exceptions ; the colour of the ventral segments in Hydroporus bistri- 

 gatus (Bidessus, No. 299) is red in the female, black in the male ; in other species 

 of Bidessus the same fact is repeated but not in so conspicuous a manner ; and we 

 find it again among the Hydroporini in Coelambus (flaviventris, pallidulus, &c.) and 

 in Hydroporus (Dytiscus halensis, No. 528) and again among the Colymbetides 

 in Lancetes (Colymbetes prsemorsus. No. 916). In all these cases it is the males 

 that have the ventral segments dark ; the repetition of this character in a few 

 widely separated members of the family is a very interesting fact and should engage 

 the attention of those who are interested in the questions of sexual colouration and 

 selection. Indeed the whole family of the Dytiscidae is one that seems peculiarly 

 adapted for throwing light on the questions of development and correlations of 

 colour in the insecta, owing to the comparative simplicity of the facts ; most of the 

 other families of insects show such a variety of colour and pattern that these are 

 quite bewildering to any one who wishes to comprehend their meaning ; but the 

 facts in the Dytiscida3 are less complex, and appear to be capable of being 

 generalized in such a manner as to lead us to hope we may some day understand 

 them. 



In the Sculpture of the surface we find that the Dytiscidae show less vaiiety than 

 the other families of Coleoptera : to this rule there is however one most interesting 

 exception in the fact that difference — and frequently extreme difference— in the 

 sculpture of the sexes is of very common occurrence. Leaving this for a while out 

 of the question, we may note that in the larger and in the more highly developed 

 forms the surface is remarkably smooth and polished, and punctuation is nearly 

 absent. In these smooth species there exist, however, with remarkable constancy 

 three longitudinal series of fine punctures on each wing-case ; the series are about 

 equidistant from one another and from the suture and outer margin, but frequently 

 the external one of the three is much more indistinct than the others ; the persistence 

 of these punctures throughout the family is remarkable, and seems to indicate that 

 they have some considerable functional importance. In the coarsely sculptured and 

 punctate species, they are more difficult to trace than they are in the smoother 

 species, not only on account of the roughness of the surface, but probably also 

 because they are really less developed. Each of the })unctures in these series bears 

 a very fine hair, and sometimes the series can only be detected by means of these 

 hairs, which can be distinguished as forming a series, although the punctures them- 



