Chap. X. 
INSECTS. 
345 
less atrophied, with the tibiae and tarsi reduced to mere 
rudimentary knobs. The wings, also, in the two sexes 
often differ in neuration , 10 and sometimes considerably 
in outline, as in the Arkoris 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 excres- 
cences on the discs of the posterior pair . 11 In several 
British butterflies, the males alone, as shewn by Mr. 
Wo n for, are in parts clothed with peculiar scales. 
The purpose of the luminosity in the female glow- 
worm is likewise not understood ; tor it is very doubtful 
whether the primary use ot the light is to guide the 
teale to the female. It is no serious objection to this 
latter belief that the males emit a feeble light; for 
secondary sexual 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 Muller 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. T\ ilh insects 
of all kinds the males are commonly smaller than the 
females ; 12 and this difference cau often be detected 
even in the larval state. So considerable is the difference 
10 E. Doubleday, 1 Annals and Map:. ofNat. Hist.’ vol. i. 1848, p. 379. 
I may add that the winsjs in certain Hymenoptera (see Slmckard, 
'Eossorial Hymenop.’ 1S37, p. 39-43) differ in neuration according to 
sex. 
11 H. W. Bates, in ‘ Journal of Proc. Linn. Soc.’ vol. yi. 1862, p. 74. 
Fr. Wonfor's observations are quoted in ‘Popular Science Review, 
l8 68, p. 343. 
12 Kirby and Spence, ‘ Introduction to Entomology,’ vol. iii. p. 299. 
