388 INSECTA. 



any such abrupt changes. We can, however, in the main, distinguish, 

 according to the vital functions belonging to them, the forms of the- 

 larval, the pupal, and the imaginal stages. 



III. Parthenogenesis, Paedogenesis, Heterogeny. 



A capacity for developing unfertilised eggs in a parthenogenetic 

 manner has repeatedly been observed in the Insecta. Partheno- 

 genesis may here be either occasional (e.g., many Lepidoptera, 

 Bombyx, Liparis) or may be of normal occurrence, often recurring 

 at fixed intervals in the ontogenetic cycle.* The males of the social 

 "Wasps and Bees, for instance, are produced from eggs that develop 

 parthenogenetically. This is also the case in the Ants, and in 

 Nematus and other Tenthredinidae, while, in the Cynipidae, only 

 females are produced from the parthenogenetic eggs. In the 

 Lepidoptera it seems to be the rule that females come from the- 

 parthenogenetic eggs. In Psyche and Solenobia, for example, a 

 succession of many parthenogenetic generations was observed, while 

 males were only seldom met with. The same is the case in Apatania 

 among the Trichoptera (Klapalek). In certain Cynipidae there is- 

 a cyclic alternation of parthenogenetic females and male and female 

 sexual forms of a different shape (true heterogeny). There thus 

 develops, in the galls produced by a form known as Spathegaster 

 baccarum, a gall-wasp of different shape called Neuroterus ventricidaris r 

 of which only parthenogenetic females are known. The unfertilised 

 eggs laid by Neuroterus, which develop in peculiarly shaped galls, 

 give rise again to the sexual generation of Spathegaster. 



With the possibility of attaining reproduction by means of un- 

 fertilised eggs is connected the shifting back of this process to an 

 early stage of development (paedogenesis). Thus, according to 

 Grimm, in one species of Chironomus, the pupa lays eggs, while 

 other Diptera (Cecidomyia), even as larvae, are capable of reproducing 

 themselves parthenogenetically and viviparously. The partheno 

 genetic reproduction of the Ap>hidae must also to some extent be 

 regarded as paedogenesis ; in these Insects it may happen that the 

 embryo produced parthenogenetically may in its turn reproduce 

 itself. 



The heterogeny of the Phytophthires seems to be founded on the 



* [In this connection Nussbaum (No. XXXV. ) has recently made a series of care- 

 ful experiments on certain Lepidoptera, A'iz. , Bombyx, Porlhesia, and Liparis. He 

 only succeeded in demonstrating the parthenogenetic condition in Bombyx, in 

 which form two per cent, of the unfertilised eggs (1100) showed segmentation, 

 hut never hatched. — Ed.] 



