132 METAMORPHOSES OF MAN 



Bonnet was enabled by a new series of observations, 

 to associate and explain in the most satisfactory 

 manner these apparently contradictory facts. He 

 discovered that the aphides produce larvae without 

 sexual intercourse, during the summer months only, 

 and that when the external temperature diminishes, 

 these insects return to the ordinary process, and 

 propagate by ova, which' can only be produced through 

 the intercommunication of the two sexes. These ova 

 remain attached to the branches of trees during the 

 winter, occupying the position which the colony — 

 now destroyed by cold — formerly held. They are 

 hatched in spring, and viviparous individuals alone 

 are produced; but in the autumn males and females 

 again appear, and with them the oviparous repro- 

 duction presents itself.* 



The facts to which 'we have just called attention 



* De Geer, who may be called the Swedish Keaumur, Lyonnet, 

 the distinguished anatomist of the willow caterpillar, and several 

 others, have confirmed this conclusion. One of them, Kyber, has 

 demonstrated incontestably the influence of temperature upon 

 these phenomena. He placed a twig of carnation covered with 

 aphides, in a chamber, which was maintained at a constant tem- 

 perature all the year round, and observed, that these insects pro- 

 pagated viviparously, and in this manner only, for four successive 

 years. — (Germar's Magazin der Entomologie, 1815.) These strange 

 phenomena have often puzzled naturalists, and among those whose 

 attention they have engaged, we may mention : Duveau (" Nouvelles 

 Becherches sur les Pucerons" — Memoires du Museum, 1825); Morren 

 .(" Memoire sur l'Emigration du Puceron du Pecher et sur les Carac- 

 teres et l'Anatomie de cette Espece," — Annales des Sciences natu- 

 relles, 1836); Siebold . " Ueber die inneren Geschlecteswerkzeuge 

 der viviparen und oviparen Blattlause ; " Froriep's " Neue Noti- 

 zen," 1839 ; and V. Carus " Zur nahern Kenntniss des Generation- 

 wechsels," 1849. 



