CHAPTER 4. SEXUAL PECULIARITIES AND DIMORPHISM.
Our Tanais, which in nearly all the particulars of its structure is an
extremely remarkable animal, furnished me with a second fact worthy of
notice in connection with the theory of the origin of species by natural
selection.
When hand-like or cheliform structures occur in the Crustacea, these are
usually more strongly developed in the males than in the females, often
becoming enlarged in the former to quite a disproportionate size, as we
have already seen to be the case in Melita. A better known example of
such gigantic chelae is presented by the males of the Calling Crabs
(Gelasimus), which are said in running to carry these claws "elevated,
as if beckoning with them"--a statement which, however, is not true of
all the species, as a small and particularly large-clawed one, which I
have seen running about by thousands in the cassava-fields at the mouth
of the Cambriu, always holds them closely pressed against its body.
A second peculiarity of the male Crustacea consists not unfrequently in
a more abundant development on the flagellum of the anterior antennae of
delicate filaments which Spence Bate calls "auditory cilia," and which I
have considered to be olfactory organs, as did Leydig before me,
although I was not aware of it. Thus they form long dense tufts in the
males of many Diastylidae, as Van Beneden also states with regard to
Bodotria, whilst the females only possess them more sparingly.
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