1858.] the Phenometia of Gemmation. 536 



It was between the years 1740 and 1750, in fact, that Bon- 

 net, acting upon the suggestions of the illustrious Reaumur, isolated 

 an Aphis immediately after its birth, and prove^d to demonstration, 

 that not only was it capable of spontaneously bringing forth 

 numerous living young, but that these and their descendants, to the 

 ninth generation, preserved a similar faculty. 



Observations so remarkable were not likely to pass unheeded ; 

 but notwithstanding the careful sifting which they have received, 

 Bonnet's results have never been questioned. On the contrary, not 

 only have Lyonet, Degeer, Kyber, Duvau, and others, borne ample 

 testimony to their accuracy, but it has been shown that, under 

 favourable conditions of temperature and food, there is practically 

 no limit to this power of asexual multiplication, or as it has been 

 conveniently termed, " Agamogenesis." 



Thus Kyber bred tlie viviparous Aphis Dianthi and Aphis 

 Itos(B for three years in uninterrupted succession ; and the males 

 and true oviparous females of the A. diajithi have never yet been 

 met with. The current notion that there is a fixed number of 

 broods, " nine or eleven," is based on a mistake. 



As, under moderately favourable conditions, an Aphis comes to 

 maturity in about a fortnight ; and as each Aphis is known to be 

 capable of producing a hundred young, the number of the progeny 

 which may eventually result even from a single Aphis during the six 

 or seven warm months of the year is easily calculated. M. Tou- 

 gard's estimate adopted, (and acknowledged) by Morren, and copied 

 from him by others, gives the number of the tenth brood as one 

 quintillioK. Supposing the weight of each Aphis to be no more 

 than Yo'uoth of a grain, the mass of living matter in this brood 

 would exceed that in the most thickly populated countries in the 

 world. 



The agamogenetic broods are either winged or wingless. The 

 winged forms at times rise into the air, and are .carried away by the 

 wind in clouds ; and these migrating hordes har^^e been supposed to 

 be males and females, swarming like the ants and bees ! During 

 the summer months it is unusual to meet other than viviparous 

 Aphides, whether winged or wingless ; but ordinarily, on the ap- 

 proach of cold weather, or even during warm weather, if the sup- 

 plies of food fall short, the viviparous Aphides produce forms 

 which are no longer viviparous, but are males and oviparous females. 

 The former are sometimes winged, sometimes wingless. The latter, 

 with a single doubtful exception, are always wingless. 



The oviparous females lay their eggs, and then, like the males, die. 

 It commonly happens also that the viviparous Aphides die, and then 

 the eggs are left as the sole representatives of the species ; but in 

 mild winters many of the viviparous Aphides merely fall into a state 

 of stupor and hybernate, to re-awake with the returning warmth of 

 spring. At the same time the eggs are hatched and give rise to 

 viviparous Aphides, which run through the same course as before. 



