2/8 The Descent of Man. Part ±i. 



Elaters, both sexes of which are highly luminous. It is not 

 known why the wings of the female glow-worm have not been 

 developed ; but in her present state she closely resembles a 

 larva, and as larvae are so largely preyed on by many animals, 

 we can understand why she has been rendered so much more 

 luminous and conspicuous than the male ; and why the larvae 

 themselves are likewise luminous. 



Difference in Size behveen the Sexes. — With insects of all kinds 

 the males are commonly smaller than the females; and this 

 difference can often be detected even in the larval state. So 

 considerable is the difference between the male and female 

 cocoons of the silk-moth (Bombyx mori), that in France they are 

 separated by a particular mode of weighing. 13 In the lower 

 classes of the animal kingdom, the greater size of the females 

 seems generally to depend on their developing an enormous 

 number of ova ; and this may to a certain extent hold good with 

 insects. But Dr. Wallace has suggested a much more probable 

 explanation. He finds, after carefully attending to the develop- 

 ment of the caterpillars of Bombyx cynthia and yamamai, and 

 especially to that of some dwarfed caterpillars reared from a 

 second brood on unnatural food, " that in proportion as the in- 

 •' dividual rnoth is finer, so is the time required for its metamor- 

 " phosis longer ; and for this reason the female, which is the 

 " larger and heavier insect, from having to carry her numerous 

 " eggs, will be preceded by the male, which is smaller and has 

 " less to mature." 14 Now as most insects are short-lived, and as 

 they are exposed to many dangers, it would manifestly be ad- 

 vantageous to the female to be impregnated as soon as possible. 

 This end would be gained by the males being first matured in 

 large numbers ready for the advent of the females ; and this 

 again would naturally follow, as Mr. A. E. Wallace has re- 

 marked, 15 through natural selection; for the smaller males 

 would be first matured, and thus w r ould procreate a large 

 number of offsj^ring which would inherit the reduced size of 

 their male parents, whilst the larger males from being matured 

 later would leave fewer offspring. 



There are, 'however, exceptions to the rule of male insects 

 being smaller than the females : and some of these exceptions are 

 intelligible. Size and strength would be an advantage to the 

 mules, which fight for the possession of the females ; and in 

 those cases, as with the stag-beetle (Lucanus), the males are 

 larger than the females. There are* however, other beetles 



13 Robinet, « Ver.' a Soie, 1348, vol. v. p, 486. 



p. 207. >5 'Journal of Proc. Ent. Soc. 



14 *T- -naact. Eut. Soc.' 3rd series, Feb. 4th, 1867, d. lxxi. 



