TRACHEATA. 429 



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 

 Coccidae is practically the same as that of the Aphidae. Irr-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 continue 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 without digestive organs. 

 The fertilized eggs laid by these forms probably give rise to the 

 parthenogenetic females. 



A remarkable case of heterogamy accompanied by paedo- 

 genesis 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 subsequently reproduce a fresh batch of larvae in the same 

 way. After several generations the larvae undergo in the 

 following summer a metamorphosis, and develop into the sexual 

 form. 



Another case of paedogenesis is that of the larvae of Chiro- 

 nomus, which have been shewn by Grimm (No. 413) to lay eggs 

 which develop exactly in the same way as fertilized eggs into 

 larvae. 



BIBLIOGRAPHY. 



(401) M. Balbiani. " Observations s. la reproduction d. Phylloxera du Chene." 

 An. Sc. Nat. Ser. v. Vol. xix. 1874. 



