T RAG HE AT A. 355 



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 produce 

 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 parthenogenetically and viviparously. The 

 viviparous females always differ from the females which lay the 

 fertilized eggs. The generative organs are of course differently con- 

 stituted, and the ova of the viviparous females are much smaller than 

 those of the oviparous females, as is generally the case in closely 

 allied viviparous 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 generations 

 of viviparous females may be produced if they are artificially kept warm 

 and supplied with food ; but in the course of nature the viviparous 

 females produce in the autumn males and females which lay eggs 

 with firm shells, and so preserve the species through the winter. The 

 heterogamy of the allied CoccidaB is practically the same as that of the 

 Aphidas. In the case of Chermes and Phylloxera the parthenogenetic 

 generations lay their eggs in the normal way. 



The complete history of Phylloxera quercus has been worked out 

 by Balbiani (No. 401). The apterous females during the summer lay 

 eggs developing parthenogenetically into apterous females, which con- 

 tinue the same mode of reproduction. In the autumn, however, the 

 eggs which are laid give rise in part to winged forms and in part to 

 apterous forms. Both of these forms lay smaller and larger eggs, 

 which develop respectively into very minute males and females with- 

 out digestive organs. The fertilized eggs laid by these forms probably 

 give rise to the parthenogenetic females. 



A remarkable case of heterogamy accompanied by paadogenesis 

 was discovered by Wagner to take place in certain species of 

 Cecydomyia (Miastor), a genus of the Diptera. The female lays a 

 few eggs in the bark of trees, etc. These eggs develop in the winter 

 into larvae, in which ovaries are early formed. The ova become 

 detached into the body cavity, surrounded by their follicles, and grow 

 at the cost of the follicles. They soon commence to undergo a true 

 development, and on becoming hatched they remain for some time in 

 the body cavity of the parent, and are nourished at the expense of its 

 viscera. They finally leave the empty skin of their parent, and sub- 

 sequently reproduce a fresh batch of larvas in the same way. After 

 several generations the Iarva3 undergo in the following summer a 

 metamorphosis, and develop into the sexual form. 



Another case of pa?dogenesis is that of the Iarva3 of Chironomus, 

 which have been shewn by Grimm (No. 41 3) to lay eggs which develop 

 exactly in the same way as fertilized eggs into larvse. 



232 



