314 THE PRINCIPLES OF Part II. 



With Siagonium (Staphylinidae), in which the males are furnished 

 with horns, " the females are far more numerous than the opposite 

 " sex." Mr. Janson stated at the Entomological Society that the 

 females of the bark-feeding Tomicus villosus are so common as to 

 be a plague, whilst the males are so rare as to be hardly known. 

 In other Orders, from unknown causes, but apparently in some in- 

 stances owing to parthenogenesis, the males of certain species have 

 never been discovered or are excessively rare, as with several of the 

 Cynipida?. 67 In all the gall-making Cynipida? known to Mr. Walsh, 

 the females are four or five times as numerous as the males ; and so 

 it is, as he informs me, with the gall-making Cecidomyiia; (Diptera). 

 With some common species of Saw-flies (Tenthredime) Mr. ¥. 

 Smith has reared hundreds of specimens from larvae of all sizes, 

 but has never reared a single male : on the other hand Curtis says, 68 

 that with certain species (Athalia), bred by him, the males to the 

 females were as six to one ; whilst exactly the reverse occurred with 

 the mature insects of the same species caught in the fields. With 

 the Neuroptera, Mr. Walsh states that in many, but by no means 

 in all, the species of the Odonatous groups (Ephemerina), there is a 

 great overplus of males : in the genus Heta?rina, also, the males are 

 generally at least four times as numerous as the females. In certain 

 species in the genus Gomphus the males are equally numerous, 

 whilst in two other species, the females are twice or thrice as 

 numerous as the males. In some European species of Psocus thou- 

 sands of females may be collected without a single male, whilst 

 with other species of the same genus both sexes are common. 69 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 here seen. 70 With most 

 of these species (excepting, as I have heard, with the Tenthredina 3 ) 

 there is no reason to suppose that the females are subject to parthe- 

 nogenesis ; and thas we see how ignorant we are on the causes of the 

 apparent discrepancy in the proportional numbers of the two sexes. 

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

 still less information. With Spiders, Mr. Blackwall, who has care- 

 fully attended to this class during many years, writes to me that 

 the males from their more erratic habits are more commonly seen, 



67 Walsh, in 'The American Entomologist,' vol. i. 1869, p. 103. F. Smith, 

 ' Record of Zoological Literature,' 1867, p. 328. 



68 ' Farm Insects,' p. 45-46. 



69 ' Observations on N. American Neuroptera,' bv H. Hagen and B. D. 

 Walsh, 'Proc. Ent. Soc. Philadelphia,' Oct. 1863, p. 168, 223, 239. 



70 ' Proc. Ent. Soc. London,' Feb. 17, 1868. 



