S66 THE DESCENT OF MAN 



destruction. He further believes that the luminous species 

 profit by being at once recognized as 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 larvse 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 larvse 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 (JBomhyx mori), thai in 

 France they are separated by a particular mode of weigh- 

 ing." 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. Wal- 

 lace has suggested a much more probable explanation. He 

 finds, after carefully attending to the development of the 

 caterpillars of Bomhyx cynihia and yamamii, and especially 

 to that of some dwarfed caterpillars reared from a second 

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

 vidual moth is finer, so is the time required for its meta- 

 morphosis longer; and for this reason the female, which is 

 the larger and heavier insect, from having to carry her nu- 

 merous 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 impregnated 

 as soon as possible. This end would be gained by the males 

 being first matured in large numbers ready for the advent 



" Eobinet, "VersiSoie," 1848, p. 207. 



" "Tranaact. Ent. Soc," 3d series, vol. t. p. 486. 



