PARTHENOGENESIS, PAEDOGEXES1S, IIETEROGENY. 389 



•definite alternation of a generation of parthenogenetic females with 

 a normal generation of males and females, the latter generation 

 reproducing the former hy means of a fertilised egg, these genera- 

 tions being distinguished from each other by certain features in the 

 structure of the body. In the Aphidae, the hibernating fertilised 

 winter-eggs yield in spring a generation that reproduces itself 

 parthenogenetically and viviparously, and which is followed during 

 the spring and summer by a series of generations reproducing 

 themselves in the same way, the individuals of which are often 

 winged, but may also be wingless. This series is closed towards 

 autumn by a generation known as the sexupara, the parthenogenetic 

 and viviparous descendants of which are, as a rule, winged males 

 and wingless females. After copulation has taken place, the female 

 lays the fertilised winter -eggs, from which, in the next spring, the 

 first generation capable of parthenogenetic reproduction hatches. 

 Under certain circumstances it, however, appears that single indi- 

 viduals of the parthenogenetic generations are able to hibernate, 

 and to give origin in the spring to a new parthenogenetic series. 

 In the same way, among other Phytophthires, there are often 

 parallel series of cycles of generations (Dreyfuss, No. 137). 



A further complication in the cycle of development of the Aphidae is brought 

 about in connection with frequent migration from one plant to another. 

 A winged parthenogenetic generation frequently appears, and then may 

 migrate to a different plant, there to reproduce itself, and in a later generation 

 returns to the original host. These wandering generations, the occurrence of 

 which was often pointed out by Lichtenstein, have been distinguished as 

 emigrantes, alienicolae, and remigrantcs by Blochmann (No. 135). In 

 Pemphigus terebinthi, for example, according to Derbes, the fertilised egg 

 gives rise to a wingless parthenogenetic generation (I.), which produces another 

 winged generation (II., emigrantes). This generation leaves the place occupied 

 up to this time and produces a third generation (III., remigrantes = sexupara), 

 which, after hibernating, returns to the original host and produces the small, 

 mouthless, wingless sexual animals without intestine (IV., sexuales). The 

 cycle of generations in Pemphigus terebinthi is interesting because the sexual 

 generation does not here occur, as it usually does, in the autumn, but in the 

 spring, being produced by hibernating parthenogenetic forms. 



Conditions similar to those in the Aphidae are found in the Chermetidae, 

 which have recently been much investigated. The chief distinction between 

 the two is that here the parthenogenetic, like the sexual generation, is also 

 oviparous. In Phylloxera, quercus, according to Lichtenstein, the winter-eggs 

 that are laid on Quercus cocci/era give rise to a mother animal (fundatrix), 

 which produces parthenogenetically a winged generation capable of partheno- 

 genetic reproduction (emigrantes) ; these wander over to the leaves of Quercus 

 pedunculate, and Q. pubescens. Several wingless generations (alienicolae) now 

 follow, which reproduce parthenogenetically, the return to Quercus cocci/era 

 being finally made possible by the production of the winged sexupara. The 



