The Descent of Man

The tarsi of the front-legs are dilated in many
male beetles, or are furnished with broad cushions
of hairs; and in many genera of waterbeetles
they are armed with a round flat sucker,
so that the male may adhere to the slippery
body of the female. It is a much more unusual
circumstance that the females of some waterbeetles
(Dytiscus) have their elytra deeply
grooved, and in Acilius sulcatus thickly set
with hairs, as an aid to the male. The females of
some other water-beetles (Hydroporus) have
their elytra punctured for the same purpose. (6.
We have here a curious and inexplicable case of
dimorphism, for some of the females of four
European species of Dytiscus, and of certain
species of Hydroporus, have their elytra
smooth; and no intermediate gradations between
the sulcated or punctured, and the quite
smooth elytra have been observed. See Dr. H.
Schaum, as quoted in the 'Zoologist,' vols. v.-vi.
1847-48, p. 1896. Also Kirby and Spence, 'Introduction
to Entomology,' vol. iii. 1826, p. 305.) In