354 IN SECT A. 



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 es- 

 tablishment 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. Hetero- 

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

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

 ducing female young from unfertilized ova are followed by a genera- 

 tion producing young of both sexes from fertilized ova. It 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 parthenogenesis has become confined to 

 special generations, which differ in their character from the genera- 

 tions which reproduce themselves sexually. The partheuogenetic 

 generations generally flourish during the season when food is abun- 

 dant ; 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 (CynipidaB). 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, however, Spathegaster, but is a species considered to 



1 Petites Nouvclles Entomologiques, May, 1878. 



