In certain species of Thomisus the sexes closely resemble each other,
in others they differ much; and analogous cases occur in many other
genera. It is often difficult to say which of the two sexes departs
most from the ordinary coloration of the genus to which the species
belong; but Mr. Blackwall thinks that, as a general rule, it is the
male; and Canestrini (19. This author has recently published a valuable
essay on the Caratteri sessuali secondarii degli Arachnidi, in the
Atti della Soc. Veneto-Trentina di Sc. Nat. Padova, vol. i. Fasc. 3,
1873.) remarks that in certain genera the males can be specifically
distinguished with ease, but the females with great difficulty. I am
informed by Mr. Blackwall that the sexes whilst young usually resemble
each other; and both often undergo great changes in colour during their
successive moults, before arriving at maturity. In other cases the male
alone appears to change colour. Thus the male of the above
bright-coloured Sparassus at first resembles the female, and acquires
his peculiar tints only when nearly adult. Spiders are possessed of
acute senses, and exhibit much intelligence; as is well known, the
females often shew the strongest affection for their eggs, which they
carry about enveloped in a silken web. The males search eagerly for the
females, and have been seen by Canestrini and others to fight for
possession of them. This same author says that the union of the two
sexes has been observed in about twenty species; and he asserts
positively that the female rejects some of the males who court her,
threatens them with open mandibles, and at last after long hesitation
accepts the chosen one. From these several considerations, we may admit
with some confidence that the well-marked differences in colour between
the sexes of certain species are the results of sexual selection;
though we have not here the best kind of evidence, the display by the
male of his ornaments. From the extreme variability of colour in the
male of some species, for instance of Theridion lineatum, it would
appear that these sexual characters of the males have not as yet become
well fixed. Canestrini draws the same conclusion from the fact that the
males of certain species present two forms, differing from each other
in the size and length of their jaws; and this reminds us of the above
cases of dimorphic crustaceans.
The male is generally much smaller than the female, sometimes to an
extraordinary degree (20. Aug. Vinson ( Araneides des Iles de la
Reunion, pl. vi. figs. 1 and 2) gives a good instance of the small
size of the male, in Epeira nigra. In this species, as I may add, the
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