Chap. X-l INSECTS. . 335 



in outline, as in the Aricoris epitus, which was shown to 

 me in the British Museum by Mr. A. Butler. The males 

 of certain South American butterflies have tufts of hair on 

 the margins of the wings, and horny excrescences on the 

 disks of the posterior pair. 11 In several British butterflies, 

 the males alone, as shown by Mr. TVonfor, are in parts 

 clothed with peculiar scales. 



The purpose of the luminosity in the female glow- 

 worm is likewise not understood ; for it is very doubtful 

 whether the primary use of the light is to guide the male 

 to the female. It is no serious objection to this latter be- 

 lief that the males emit a feeble light ; for secondary sexu- 

 al characters proper to one sex are often developed in a 

 slight degree in the other sex. It is a more valid objection 

 that the larvae shine, and in some species brilliantly : Fritz 

 Miiller informs me that the most luminous insect which he 

 ever beheld in Brazil was the larva of some beetle. Both 

 sexes of certain luminous species of Elater emit light. 

 Kirby and Spence suspect that the phosphorescence serves 

 to frighten and drive away enemies. 



- 



Difference in Size between the Sexes. — "With insects of 

 all kinds the males are commonly smaller than the fe- 

 males ; 12 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 (JBombyx 

 mori), that in France they are separated by a particular 

 mode of weighing:. 13 In the lower classes of the animal 



I may add that the wings in certain Hymenoptera (see Shuckard, ' Fosso 

 rial Hymenop.' 1837, pp. 39-43) differ in neuration according to sex. 



11 H. W. Bates, in 'Journal of Proc. Linn. Soc' vol. vi. 1862, p. 74. 

 Mr. Wonfor's observations are quoted in ' Popular Science Review,' 1868, 

 p. 343. 



11 Kirby and Spence, ' Introduction to Entomology,' vol. iii. p. 299. 



' 3 Robinet, 'Vers a Sole,' 1848, p. 207. 



