428 INSECTA. 



would be interesting to know if the unimpregnated female would 

 not after a certain number of generations give rise to both males 

 and females ; such an occurrence might be anticipated on 

 grounds of analogy. In the cases of true heterogamy partheno- 

 genesis has become confined to special generations, which differ 

 in their character from the generations which reproduce them- 

 selves sexually. The parthenogenetic generations generally 

 flourish during the season when food is abundant; while the 

 sexual generations occur at intervals which are often secondarily 

 regulated by the season, supply of food, etc. 



A very simple case of this kind occurs, if we may trust the 

 recent researches of Lichtenstein 1 , in certain Gall Insects 

 (Cynipidae). He finds that the female of a form known as 

 Spathegaster baccarum, of which both males and females are 

 plentiful, pricks a characteristic gall in certain leaves, in which 

 she deposits the fertilized eggs. The eggs from these galls give 

 rise to a winged and apparently adult form, which is not, how- 

 ever, Spathegaster, but is a species considered to belong to a 

 distinct genus known as Neuroterus ventricularis. Only females 

 of Neuroterus are found, and they lay unfertilized ova in peculiar 

 galls which develop into Spathegaster baccarum. Here we have 

 a true case of heterogamy, the females which produce partheno- 

 genetically having become differentiated from those which pro- 

 duce sexually. Another interesting type of heterogamy is that 

 which has been long known in the Aphides. In the autumn 

 impregnated eggs are deposited by females, which give rise in 

 the course of the spring to females which produce partheno- 

 genetically and viviparously. The viviparous females always 

 differ from the females which lay the fertilized eggs. The gene- 

 rative organs are of course differently constituted, and the ova of 

 the viviparous females are much smaller than those of the ovi- 

 parous females, as is generally the case in closely allied vivi- 

 parous and oviparous forms; but in addition the former are 

 usually without wings, while the latter are winged. The reverse 

 is however occasionally the case. An indefinite number of gene- 

 rations of viviparous females may be produced if they are arti- 

 ficially kept warm and supplied with food ; but in the course of 



1 Petites Nouvelles Entomolog iyues, May, 1878. 



