INSECTS. 329 



forms throughout the animal kingdom similar cases of the 

 sexes differing greatly, or very little, or not at all, are of 

 frequent occurrence. Although there is so wide a differ- 

 ence in color between the sexes of many Libellulidae, it is 

 often difficult to say which is the more brilliant; and the 

 ordinary coloration of the two sexes is reversed, as we have 

 just seen, in one species of Agrion. It is not probable that 

 their colors in any case have been gained as a protection. 

 Mr. MacLachlan, who has closely attended to this family, 

 writes to me that dragon-flies the tyrants of the insect- 

 world are the least liable of any insect to be attacked by 

 birds or other enemies, and he believes that their bright 

 colors serve as a sexual attraction. Certain dragon-flies 

 apparently are attracted by particular colors; Mr. Patterson 

 observed* that the Agrionidse, of which the males are blue, 

 settled in numbers on the blue float of a fishing line; while 

 two other species were attracted by shining white colors. 



It is an interesting fact, first noticed by Schelver, that, 

 in several genera belonging to two sub-families, the males 

 on first emergence from the pupal state, are colored exactly 

 like the females ; but that their bodies in a short time 

 assume a conspicuous milky-blue tint, owing to the exuda- 

 tion of a kind of oil, soluble in ether and alcohol. Mr. 

 MacLachlan believes that in the male of Libellula depressa 

 this change of color does not occur until nearly a fortnight 

 after the metamorphosis, when the sexes are ready to pair. 



Certain species of Neurothemis present, according to 

 Brauer,f a curious case of dimorphism, some of the 

 females having ordinary wings, while others have them 

 " very richly netted, as in the males of the same species." 

 Brauer "explains the phenomenon on Darwinian principles 

 by the supposition that the close netting of the veins is a 

 secondary sexual character in the males, which has been 

 abruptly transferred to some of the females, instead of, as 

 generally occurs, to all of them." Mr. MacLachlan 

 informs me of another instance of dimorphism in sev- 

 eral species of Agrion, in which some individuals are 

 of an orange color, and these are invariably females. 

 This is probably a case of reversion ; for in the 

 true Libellulae, when the sexes differ in color, the females 



*" Transact. Ent. Soc.," vol. i, 1836, p. 81. 



f See abstract in the "Zoological Record " for 1867, p. 450. 



