INSECTS. 315 



unpalatable. It is probable that the same explanation may 

 be extended to the 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 con- 

 spicuous than the male; and why the larvae themselves are 

 likewise luminous. 



Difference in Size Between 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 (Eombyx mori), that 

 in France they are separated by a particular mode of 

 weighing.* 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 development of 

 the caterpillars of Bombyx cyntlila and yamamai, and espe- 

 cially to that of some dwarfed caterpillars reared from a 

 second brood on unnatural food, "that in proportion as 

 the individual moth is finer, so is the time required for its 

 metamorphosis 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. "\ Now as most insects are 

 short-lived, and as they are exposed to many dangers, it 

 would manifestly be advantageous to the female to be im- 

 pregnated 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 remarked, J through nat- 

 ural selection; for the smaller males would be first matured, 

 and thus would procreate a large number of offspring which 

 would inherit the reduced size of their male parents, while 



*Robinet, " Vers a Sole," 1848, p. 207. 



f " Transact. Ent. Soc.," 3d series, vol. v, p. 486. 



\ " Journal of Proc. Ent. Soc.," Feb. 4, 1867, p, 71. 



