TRACHEATA. 427 



would probably be disadvantageous, there would be a tendency 

 to concentrate into a single moult the transition from the larval 

 to the adult form of mouth parts. At each ordinary moult there 

 is a short period of quiescence, and this period of quiescence 

 would naturally become longer in the important moult at which 

 the change in the mouth parts was effected. In this way a 

 rudimentary pupa stage might be started. The pupa stage, 

 once started, might easily become a more important factor in 

 the metamorphosis. If the larva and imago diverged still more 

 from each other, a continually increasing amount of change 

 would have to be effected at the pupa stage. It would probably 

 be advantageous to the species that the larva should not have 

 rudimentary functionless wings ; and the establishment of the 

 wings as external organs would therefore become deferred to 

 the pupa stage. The same would probably apply to other 

 organs. 



Insects usually pass through the pupa stage in winter in cold 

 climates and during the dry season in the tropics, this stage 

 serving therefore apparently for the protection of the species 

 during the inclement season of the year. These facts are easily 

 explained on the supposition that the pupa stage has become 

 secondarily adapted to play a part in the economy of the 

 species quite different from that to which it owes its origin. 



Heterogamy. The cases of alternations of generations 

 amongst Insects all fall under the heading already defined in 

 the introduction as Heterogamy. Heterogamy amongst Insects 

 has been rendered possible by the existence of parthenogenesis, 

 which, as stated in the introduction, has been taken hold of by 

 natural selection, and has led to the production of generations of 

 parthenogenetic forms, by which a clear economy in reproduction 

 is effected. Parthenogenesis without heterogamy occurs in a 

 large number of forms. In Bees, Wasps, and a Sawfly (Nematus 

 ventricosus) the unfertilized ova give rise to males. In two 

 Lepidopterous genera (Psyche and Solenobia) the unfertilized 

 ova give rise mainly, if not entirely, to females. Heterogamy 

 occurs in none of the above types, but in Psyche and Solenobia 

 males are only occasionally found, so that a series of generations 

 producing female young from unfertilized ova are followed by a 

 generation producing young of both sexes from fertilized ova. It 



