52 Observations on Imects affect Imj the Turnip Croj^s. 



maximum;* so that if they all lived, everything on the face of 

 the earth would be covered with them. About the middle of 

 September, the last generation, consisting of males and females, 

 is produced; the former generally becoming winged. When 

 they have attained maturity, the sexes pair, and the females no 

 longer bring forth young, but lay eggs, which are able to resist 

 the severity of winter, and these, hatching in the succeeding 

 spring, again produce viviparous mothers. The autumnal stock, 

 having provided for the continuation of its race in the following 

 year, generally dies off before the approach of winter. 



Towards the end of March, or the beginning of April, we 

 often have a succession of cold drying winds from the north and 

 north-east, at which time the Aphides occasionally make their 

 appearance so suddenly as to be termed a blight : these must be 

 the offspring of the autumnal eggs, or broods which had lived 

 through the winter. Their increase in damp and sultry weather, 

 at a more advanced period, is equally surprising ; and the uni- 

 versal diffusion of such myriads soon after a thunder-storm has led, 

 as with the black caterpillars, to the vulgar error of their having 

 fallen from the clouds. Electricity probably causes the simul- 

 taneous appearance of insects in many instances, for the irrita- 

 bility of the nervous system is excited by the increased action of 

 oxygen, so that insects, both in the egg and pupa states, may be 

 more speedily developed, the dormant eggs may thus be called 

 instantly into life, and the viviparous aphides bring forth their 

 young simultaneously ; which may in some measure account for 

 the vast swarms of plant-lice that so frequently cover, in a few 

 hours, the flowers in our gardens and the crops in our fields. 



I have already stated that there are three, if not four, species 

 of Aphides which live upon the turnips: the first (figs. 1 and 2) 

 I have found under the rough leaves of the English varieties, as 

 well as one which I believed to be distinct (fig. 4) ; another 

 (figs. 5 and 6) appears to be attached to the swedes ; and the last 

 (figs, 7 and 8) is secreted amongst the flower-stalks. During 

 the first few generations they are all wingless, but as the summer 

 advances they appear to arrive at greater perfection, and assume 

 a more complete state of development, so that eventually indi- 

 viduals of both sexes are furnished with wings, when they are 

 capable of doing incredible mischief by the extended field of their 

 operations ; for, flying about, they form colonies in every direc- 

 tion, and they are thus enabled to select a proper nidus for the 

 eggs, which are laid by the last generation in the auiumn, after 



* Reaumur (vol. vi. p. 566) has proved that, in five generations, one 

 aphis may be the progenitor of 5,904,900,000 descendants ; and it is sup- 

 posed that, in one year, there may be twenty generations. — Vide Kirby and 

 Spence's Introd. to Ent, vol. i. p. 174. 



