SUB-KINGDOM OF THE ARTHROPODA: CLASS, CRUSTACEA.
In this great class we first meet with undoubted secondary sexual
characters, often developed in a remarkable manner. Unfortunately the
habits of crustaceans are very imperfectly known, and we cannot explain
the uses of many structures peculiar to one sex. With the lower
parasitic species the males are of small size, and they alone are
furnished with perfect swimming-legs, antennae and sense-organs; the
females being destitute of these organs, with their bodies often
consisting of a mere distorted mass. But these extraordinary
differences between the two sexes are no doubt related to their widely
different habits of life, and consequently do not concern us. In
various crustaceans, belonging to distinct families, the anterior
antennae are furnished with peculiar thread-like bodies, which are
believed to act as smelling-organs, and these are much more numerous in
the males than in the females. As the males, without any unusual
development of their olfactory organs, would almost certainly be able
sooner or later to find the females, the increased number of the
smelling-threads has probably been acquired through sexual selection,
by the better provided males having been the more successful in finding
partners and in producing offspring. Fritz Muller has described a
remarkable dimorphic species of Tanais, in which the male is
represented by two distinct forms, which never graduate into each
other. In the one form the male is furnished with more numerous
smelling-threads, and in the other form with more powerful and more
elongated chelae or pincers, which serve to hold the female. Fritz
Muller suggests that these differences between the two male forms of
the same species may have originated in certain individuals having
varied in the number of the smelling-threads, whilst other individuals
varied in the shape and size of their chelae; so that of the former,
those which were best able to find the female, and of the latter, those
which were best able to hold her, have left the greatest number of
progeny to inherit their respective advantages. (8. Facts and
Arguments for Darwin, English translat., 1869, p. 20. See the previous
discussion on the olfactory threads. Sars has described a somewhat
analogous case (as quoted in Nature, 1870, p. 455) in a Norwegian
crustacean, the Pontoporeia affinis.)
[Fig.4. Labidocera Darwinii (from Lubbock). Labelled are: a. Part of
right anterior antenna of male, forming a prehensile organ. b.
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