RHYNCHOTA. 



197 



mali), which is found also on the pear and sloe tree ; the cherry aphis (A, cerasi), 

 and a host of others named in the same manner. The life-history of plant- 

 lice is very complicated; and although differing somewhat in different species 

 is always characterised by what is known as an alternation of generations. 

 There are several broods or generations of these insects in the course of a year, 

 but it is only in the last autumn brood that true sexual individuals are found. 

 The males are generally provided with wings, but the females are larger 



VINE-PHYLLOXERA (much enlarged). 



1 and 2, The wingless form found on the root, seen from above and below ; 3, The same from the side ; 4, Its 

 piercing organs ; 5, Winged individual ; 6, Rootlets of the vine, with swellings caused by the Phylloxera ; 

 7, An old root stock, with (8) hibernating individuals. 



and wingless ; they lay fertilised eggs, from which, in the following spring, 

 the first brood of the year is produced. The insects of this brood are usually 

 wingless, and give birth to living young, or, as in the genus Phylloxera, lay eggs 

 from which the young subsequently develop. The new brood, thus produced 

 parthenogenetically, resembles the one from which it has sprung, and gives rise to 

 a fresh brood in a similar manner. As many as nine or ten generations may 

 succeed one another in this way during the course of the season, before the appear- 

 ance in the autumn of the last or sexual generation. The brood preceding and 



