Observations on Insects affecting the Turnip Crops. 51 



and finely shagreened ; most of them were surrounded by eight or 

 ten young ones, of a yellower colour, and there was a considerable 

 number of larger young ones, of a somewhat rosy tint. I likewise 

 observed eight dark specimens (fig. 4), each of which was accom- 

 panied by five or six young ones, of a similar colour, with two 

 dark oval spots upon the head ; and these, I am inclined to be- 

 lieve, belonged to a different species. 



It may readily be conceived that, under favourable conditions 

 of the atmosphere, the multiplication of the Aphides must be 

 beyond calculation; in 1827 and 1836 they are stated to have 

 committed very extensive ravages on the turnip-crops.* In the 

 absence, however, of any positive data immediately connected 

 with the natural history of the turnip -aphides, I shall avail myself 

 of information relating to kindred species, which will equally 

 well illustrate the habits and economy of this family, and supply 

 us with the means of studying any of its members.-}" Most of the 

 plant-lice, I believe, come forth in the spring, as soon as the 

 foliage is apparent, but some are seen even before the leaves are 

 expanded: at this time they are all wingless females, which have 

 hatched from eggs that had been laid the foregoing autumn : 

 these soon disperse, and forcing their rostrums up to the base 

 in the tender stalks and leaves of plants, begin to produce young 

 in ten or twelve days, which likewise seem to be wingless females; 

 and as no males make their appearance until the autumn, when 

 pairing takes place, it is evident that the females must be preg- 

 nant at their birth, without sexual intercourse ; and this occurs 

 for several successive generations. The females produce about 

 two young ones daily during fifteen or twenty days ; \ and Kollar 

 says that, in ten days, the third generation from the eggs is able 

 to bring forth young, comprising winged as well as wingless 

 females, the former flying away as soon as they have arrived at 

 maturity, and the latter remaining on the plant where they were 

 produced. Both the winged and wingless females of the third 

 generation are able to produce young in eight, and even in four 

 days, which are capable of the same power as their parent at the 

 expiration of a similar space of time ; and proceeding thus until 

 the middle of September, the generations often amount to sixteen 

 or twenty — thus, from one e^^ only, 729,000,000 of Aphides 

 would be produced in seven generations, taking thirty as the 

 average of each brood — twenty JDeing the minimum, and forty the 



* In 1793 the Aphides were the chief, and in 1798 the sole cause of the 

 failure of the crops of hops.— W. Curtis, Linn. Trans., vol. vi. p. 79. 



t Schmidberger, in Koliar's Naturgeschichte der schaedlichen In- 

 secten : vide Aphis pyri, malt, pnini, and persiccu, p. 291. 



% On opening the body of a willow-aphis, Mr. W. Curtis counted 61 

 young ones, large and small, and in another 46. 



E 2 



