138 THE BIOLOGY OF INSECTS 



the Gipsy Moth {Portheiria dispar), aheady referred to in 

 this chapter, are examples of species as to which perfectly 

 reliable observations have been made. Such exceptional 

 instances of virgin reproduction are of much interest 

 because there can be no doubt that, in the history of animal 

 groups, regularly occurring parthenogenesis is a condition 

 secondarily derived from reproduction through normal 

 sexual union, and that as a starting point for what is now 

 regular parthenogenesis, we must look to an originally 

 exceptional appearance of this mode of development. 



There are insect species of various orders in which 

 parthenogenesis is the usual method of reproduction, males 

 being exceedingly rare in some cases and altogether unknown 

 in others. Of greater interest, however, are those specialised 

 insects in which virgin reproduction alternates definitely in 

 the Hfe-cycle with the usual sexual method. The Aphids 

 (plant- lice or " green-fly ") afford the best known example 

 of this cyclical or seasonal parthenogenesis. Among most 

 aphids there are males and females which pair in autumn, 

 and the females lay fertilised hard-shelled eggs which carry 

 the race over the winter. From these eggs females only 

 are hatched, " stem-mothers " as they are called, whose 

 eggs without fertilisation develop within the oviducts of 

 the female so that active young are born. Successive 

 generations of such ** viviparous " virgin females follow 

 each other through the spring and summer, those of the 

 latest brood giving birth to males as well as to the sexual 

 females of the autumn. Here there are many generations 

 in the course of the yearly life-cycle. The Cynipidae or 

 Gall-flies, a well-known family of the Hymenoptera, have 

 not more than two alternating generations in the year, 

 usually a summer sexual brood, and a winter or spring 

 brood consisting of virgin females only. Among many at 

 least of the social Hymenoptera (ants, wasps, and bees) 

 the mother or " queen " insect may lay either fertilised eggs 

 from which females are normally developed, or unfertilised 

 eggs which as a rule only produce males. In these 

 insects, therefore, the occurrence of parthenogenesis seems 



