50 Observations on hisects affecting the Turnip Crops. 



those left for seed : one is exceedingly like those which attack 

 the cabbages,* and I believe they are identical : if such be the 

 case, it may be admitted as an exception to a general rule, but it 

 must be remembered that it is tlie swedes which this species 

 infests. 



We have in a former report alluded to the remarkable economy 

 of the Aphides, which during the summer bring forth young with- 

 out sexual intercourse, and in the autumn lay eggs ; the fecunda- 

 tion of the first female being sufficient to render twenty successive 

 generations fertile. f No writer that I am aware of has paid par- 

 ticular attention to the turnip- aphides, although the leaves are 

 often infested to a great extent towards the end of summer, as 

 well as in the autumn, and by their piercing the nerves the leaves 

 become curled and distorted, at the same time affording the in- 

 sects a habitation, protected both from heat, wet, and cold : the 

 green tops thus become less serviceable as food for cattle, and 

 the growth of the roots is unquestionably retarded by the ex- 

 haustion of the leaves, and the outer ones, turning yellow, fall 

 off. They are sometimes attacked at a much earlier period ; for 

 Mr. Marshall observed the Aphides extremely numerous on 

 seedling leaves, from whence he concludes that they may be the 

 cause of the very slow progress which is sometimes made by the 

 young turnip-plants to push into rough-leaf. 



I wished to watch the economy of these insects, and having 

 sown some turnip-seed in a garden in the spring, I found the 

 leaves infested with the Aphides in the middle of July, when the 

 wingless females (fig. 2) were surrounded by their young broods, 

 unmolested by the heavy rains which had incessantly fallen during 

 the previous fortnight ; and two winged ones were sheltered in 

 the same situation (fig. 1). On the underside of one small leaf 

 I counted 168 Aphides; they comprised about a dozen large 

 green ones, like fig. 2, one of which was just giving birth to a 

 young Aphis (fig. 3) ; apparently, to me, it was enclosed in a thin 

 membrane, and came forth backwards, which would render its 

 inclusion in a sac the more necessary ; it was exactly like a pupa, 

 with two little black eyes shining through, and the antennae were 

 folded backward, as well as the legs, so that the limbs were not free, 

 but the instant it was perfectly excluded it began to move iis horns, 

 and immediately afterwards used its legs. J The females are green 



* Mr. F. Walker says, " Two distinct species infest the turnip ; the one 

 is bright pale-red in colour, the other o;reen, covered with white down, which 

 is also abundant on tlie cabbage." — Vide ' The Entomologist,' p. 173. 



t M. Bonnet, who first tried the experiment, thinks that Aphides would 

 produce in this way to the thirtieth generation, t^ 



% Mr. W. Curtis says that the young of A. salicis were able to use their 

 legs before they were perfectly disengaged from their mother, and thus 

 assisted in liberating themselves. — Linn. Trans., vol. vi. p. 79. 



