PROPORTION OF THE SEXES. 251 



excess, whilst in two other species, the females are twice or 

 thrice as numerous as the males. In some European species of 

 Psocus thousands of females may be collected without a single 

 male, whilst with other species of the same genus both sexes are 

 common.'"' In England, Mr. MacLachlan has captured hundreds 

 of the female Apatania muliebris, but has never seen the male; 

 and of Boreus hyemalis only four or five males have been seen 

 here.™ With most of these species (excepting the Tenthredinse) 

 there is at present no evidence that the females are subject to 

 parthenogenesis; and thus we see how ignorant we are of the 

 causes of the apparent discrepancy in the proportion of the two 

 sexes. 



In the other Classes of the Articulata I have been able to collect 

 still less information. With Spiders, Mr. Black wall, who has 

 carefully attended to this class during many years, writes to me 

 that the males from their more erratic habits are more commonly 

 seen, and therefore appear more numerous. This is actually the 

 case with a few species; but he mentions several species in six 

 genera, in which the females appear to be much more numerous 

 than the males."' The small size of the males in comparison with 

 the females (a peculiarity which is sometimes carried to an ex- 

 treme degree), and their widely different appearance, may ac- 

 coimt in some instances for their rarity in collections." 



Some of the lower Crustaceans are able to propagate their 

 kind asexually, and this will account for the extreme rarity of the 

 males: thus Von Siebold" carefully examined no less than 13,000 

 specimens of Apus from twenty-one localities, and amongst these 

 he found only 319 males. With some other forms (as Tanais 

 and Cypris), as Fritz Miiller informs me, there is reason to be- 

 lieve that the males are much shorter-lived than the females; 

 and this would explain their scarcity, supposing the two sexes to 

 be at first equal in number. On the other hand, Miiller has in- 

 variably taken far more males than females of the Diastylidae 

 and of Cypridina on the shores of Brazil; thus with a species in 

 the latter genus, 63 specimens caught the same day included 57 

 males; but he suggests that this preponderance may be due to 

 some unknewn difference in the habits of the two sexes. With 

 one of the higher Brazilian crabs, namely a Gelasimus, Fritz 



*» 'Observations on N. American Neuroptera,' by H. Hagen and B. 

 D. Walsh, 'Proc. Bnt. Soo. Philadelphia,' Oct. 1863, pp. 168, 223, 239. 



M 'Proc. Bnt. Soc. Ixindon, Feb. 17, 1868. 



"Apother irreat authority with respect to this class, Prof. Thorell 

 of TJpsala ('On European Spiders,' 1869-70, part i. p. 205) speaks as if 

 female spiders were generally commoner than the males. 



"2 See, on this subject, Mr. O. P. Cambridge, as quoted in 'Quarterly 

 Journal of Science,' 1868, p. 429. 



63 'Beitrage zur Parthenogenesis,' p. 174. 



