SPIDERS. 307 



be much more numerous than the females; they differ also 

 in the larger size of their chelae. In some species of the 

 genus, probably in all, the sexes pair and inhabit the same 

 burrow. They are also, as we have seen, highly intelligent 

 animals. From these various considerations it seems proba- 

 ble that the male in this species has become gayly orna- 

 mented in order to attract or excite the female. 



It has just been stated that the male Gelasimus does not 

 acquire his conspicuous colors until mature and nearly 

 ready to breed. This seems a general rule in the whole 

 class in respect to the many remarkable structural differ- 

 ences between the sexes. We shall hereafter find the same 

 law prevailing throughout the great sub-kingdom of the 

 vertebrata; and in all cases it is eminently distinctive of 

 characters which have been acquired through sexual selec- 

 tion. Fritz Muller * gives some striking instances of this 

 law; thus the male sand-hopper (Orchestia) does not, until 

 nearly full grown, acquire his large claspers, which are 

 very differently constructed from those of the female; while 

 young his claspers resemble those of the female. 



Arachnida (Spiders). The sexes do not generally 

 differ much in color, but the males are often darker than 

 the females, as may be seen in Mr. Black wall's magnificent 

 work.f In some species, however, the difference is con- 

 spicuous; thus the female of Sparas&us smaragditlus is 

 dullish green, while the adult male has the abdomen of a fine 

 yellow, with three longitudinal stripes of rich red. In 

 certain species of Thonrisns the sexes closely resemble each 

 other, in others they differ much; and analogous cases occur 

 in many other genera. It is often difficult to spy which of 

 the two sexes departs most from the ordinary coloration of 

 the grenns to which the species belong; but Mr. Blackwall 

 thinks that, as a general rule, it is the male; and Oanctstrini} 

 remarks that in certain genera the males can be specifically 

 distinguished with ease, but the females with great difficulty. 



* " Facts and Arguments," etc., p, 79. 



f" A History of the Spiders of Great Britain," 1861-64. For tlie 

 following facts, see pp. 77. 88, 102. 



J This author lias recently published a valuable essay on the 

 Sf Oarat.teri sessuali secoiidarii degli Arachnioi," in the " Atti della 

 Soc. Veueto-Treiitina di So. Nat. Padova," vol. i, Fasc. 3, 1873. 



