HEMIPTERA. 121 



generation began on the ist of August ; it was on this day that the 

 plant-louse I had shut up on the 22nd of July gave birth to this genera- 

 tion. On the 4th of August, about one o'clock in the afternoon, I 

 put into solitary confinement a plant-louse of the third generation. 

 On the ninth of the same month, at six in the evening, a fourth 

 generation, due to this last one, had already seen the light : it had 

 given birth to four little ones. On the same day, towards midnight, 

 all intercourse with its own species was forbiden to the plant-louse of 

 the fourth generation bom at that hour. On the i8th, between six 

 and seven o'clock in the morning, I found this last in the company 

 of four little ones to which it had given birth." * 



In this case, the want of food caused the death of the isolated 

 individual of the fifth generation, and the experiment was brought to 

 a close. 



Bonnet then tried experiments on the plantain aphis, following 

 them up during five consecutive generations, which succeeded each 

 other without interruption, in the space of three months. 



After having stated the extraordinary facts, which he relates with 

 the most perfect simplicity, Charles Bonnet, examining at the end of 

 the fine season specimens of the winged oak-tree aphis, was able to 

 be present at their nuptials. He preserved the females with great 

 care, and saw, not without profound astonishment, that they gave 

 birth, not to small living insects, as was the case in the first ex- 

 periments, but to eggs of a reddish colour, which were stuck fast to 

 each other, on the stem or stalk of the plant. 



A short time afterwards, this illustrious observer was able to 

 convince himself that the oak-tree plant-lice, whose nuptials he had 

 witnessed in the autumn, present the same phenomena of solitary and 

 viviparous propagation, already so often mentioned by him. 



At last some new observations permitted him to establish beyond 

 all doubt the connection of these facts, in appearance so contra- 

 dictory. He discovered that, during the whole of the fine season, 

 the plant-lice are solitary and viviparous, but that towards the autumn 

 these creatures return to the ordinary course of things, and are 

 propagated by eggs, whose development requires the co-operation 

 of a male and female individual. These eggs are hatched in Spring, 

 and produce only viviparous plant-lice. In the autumn the males 

 and females show themselves, and from that moment ovipositing 

 recommences. These curious facts, seen and published more than 

 a century ago, have been verified many times since. 



* "Traite d'Insectologie," &c., pp. 67 — 69. 



