300 THE COURTSHIP OF ANIMALS 



duties and the manner of the evolution of these types, 

 are themes foreign to these pages : enough has been 

 said already to indicate the nature of the problems 

 they present when discussing the Hfe-history of the 

 Bees. 



The subject of parthenogenesis need be pursued no 

 further in this volume than is sufficient to bring out its 

 retrograde character. It is a form of reproduction which 

 may be limited to a small number of generations, as with 

 the Aphides, or to a single generation alternating with 

 normal sexual generations, as in many Cynipidse or Gall- 

 flies, or it may be the only mode of reproduction, as in 

 some other Gall-flies, some Saw-flies and some Crustacea, 

 wherein no males have ever been seen. In some species 

 this form of reproduction gives rise to females only — 

 the Thelyotokous parthenogenesis of scientific text-books 

 — as in the Saw-flies and Gall-flies, and the parasitic 

 Tomognathous. In some other Saw-flies, unfertilized 

 queens and workers of Ants, Bees, and Wasps, which 

 occasionally produce offspring, the progeny is always 

 male, and this is known as Arrhenotokous partheno- 

 genesis. In one or two species of Saw-fly, e.g., Nematus 

 curtispina, both males and females may be produced, 

 when the species is said to be Deuterotokous. 



In the case of the Aphides, winged males normally 

 appear in large numbers at the end of the summer, and 

 these fertilize the females ; but if kept in a warm green- 

 house, parthenogenetic reproduction may be sustained 

 for as long as four years. Under quite normal circum- 

 stances these tiny insects show a singular range of 

 variability, for egg-laying and viviparous individuals are 

 met with; while winged and wingless generations appear 



