2S8 THE DESCENT OF MAN. 



colors in any case have been gained as a protection. Mr. Mac- 

 Lachlan, 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 Agrionidae, of which the males 

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

 whilst two other species were attracted by shining white colors. 



It is an interesting fact, first noticed by Schelver, that in sev- 

 eral 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 con- 

 spicuous milky-blue tint, owing to the exudation 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,"' 

 a curious case of dimorphism, some of the females having ordinary 

 wings, whilst 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 net- 

 "ting 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 several species 

 of Agrion, in which some individuals are of an orange color, and 

 these are invariably females. This is probably a case of rever- 

 sion; for in the true Libellulse, when the sexes differ in color, 

 the females are orange or yellow; so that supposing Agrion to be 

 descended from some primordial form which resembled the typi- 

 cal Libellulae in its sexual characters, it would not be surprising 

 that a tendency to vary in this maimer should occur in the fe- 

 males alone. 



Although many dragon-flies are large, powerful, and fierce in- 

 sects, the males have not been observed by Mr. MacLachlan to 

 fight together, excepting, as he believes, in some of the smaller 

 species of Agrion. In another group in this Order, namely, the 

 Termites or white ants, both sexes at the time of swarming may 

 be seen running about, "the male after the female, sometimes 

 "two chasing one female, and contending with great eagerness 

 "who shall win the prize."^* The Atropos pulsatorius is said to 



s2 'Transact. Ent. Soc' vol. i. 1S36, p. Ixxxi. 



5' See abstract in the 'Zoological Record,' for 1867, p. 450. 



" Kirby and Spenoe, 'Introduot. to Entomology,' vol. ii. 1818, p., 35. 



