VIII APHIDAE 585 



winged individuals becomes large/ The appearance of winged 

 individuals is frequently accompanied by a peculiar change of 

 habit ; the winged individuals migrating to another plant, which 

 in many cases is of a totally different botanical nature from that 

 on which the apterous broods were reared : for instance Aphis 

 ■mali, after producing several apterous generations on apple, gives 

 rise to winged individuals that migrate to the stems of corn or 

 grass, and feeding thereon commence another cycle of generations. 

 The study of this sort of Aphis-migration is chiefly modern, but 

 many very curious facts have already been brought to light ; 

 thus Drepanosiphum platanoides, after producing a certain number 

 of viviparous generations on maple {Acer), quits this food-plant for 

 another, but after two or three months returns again to the 

 maple, and produces sexual young that lay eggs." Histories such 

 as this are rather common. Even more interesting are the cases 

 of those species that, after some weeks of physiological activity 

 on a plant, pass into a state of repose on the same plant, and 

 then after some weeks produce sexual young. On the whole, it 

 would appear that the appearance of winged forms is a con- 

 comitant of decreasing nutrition. It is a very remarkable fact 

 that the sexually perfect females are invariably apterous, and this 

 is frequently also the case with the males. It is also highly 

 remarkable that the sexually perfect individuals are of com- 

 paratively small size. There are at least three kinds of males 

 in Aphidae — 1, winged males ; 2, wingless males with mouth 

 well developed ; 3, wingless small males with mouth absent. 

 As regards some of these points the conditions usual in Insect 

 life are reversed.^ Huxley inclined to treat all these products of 

 a fertilised egg, that are antecedent to another process of gamo- 

 genesis {;i.e. production with fertilisation), as one zoological indi- 

 vidual : in that case the Aphis zoological individual is winged 

 before attaining the mature state, and is wingless and smaller 

 when mature. Some species may have as a rule two, others 

 three, winged generations in a year. 



Parallel series. — In certain cases individuals of one genera- 

 tion assume different habits, and so set up the phenomenon 



^ Seventeenth Rc2J. Insects Illinois, 1891, p. 66. 

 2 Kessler, Acta Ac. German, li. 1887, pp. 152, 153. 



'^ In connection with this the absence of a functional month in the imago state 

 of numerous Lepidoptera, and of Oestrid Diptera, should not be forgotten. 



