194 On Aquatic Carnivorous Coleoptera or Dytiscide. 
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 roeselu, 
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) isred 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 praemorsus, No. 916). In all these cases it is the males 
that have the ventral sezments 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 Dytiscidze 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 Dytiscide 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 Scunrrurn of the surface we find that the Dytiscidze show less variety 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 punctures 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- 
