88 The Life-History of Insects 



After fertilization the polar-bodies are, in most insects, 

 re-absorbed into the egg-substance (fig. 57> B) (54, 



55). 



Virgin Reproduction. — Without fertilization, no 

 egg will, as a rule, develop into a new individual, 

 but among insects there are a fair number of excep- 

 tions to this rule. Eggs laid by virgin female Moths 

 are known occasionally to develop. In certain Saw- 

 flies reproduction by virgin females {parthenogenesis) 

 is common, and also in many Gallflies. Males of 

 some kinds of the latter insects have never been dis- 

 covered, so that sexual reproduction must be exces- 

 sively rare among them, if it ever takes place at all. 

 The rapid multiplication of Plantlice in summer-time, 

 too well known to gardeners, is due to successive 

 generations brought forth by virgin females. With 

 few exceptions all the eggs which have been studied 

 of insects and other animals capable of virgin repro- 

 duction, cast off but a single polar body (fig. 57, 

 C,p) ; that is, the halving of the nuclear matter of the 

 egg-cell preparatory to the addition of the nuclear 

 matter of the sperm-cell does not take place as a rule 

 in parthenogenetic eggs. It appears therefore that the 

 amount of nuclear matter present in the egg before 

 the extrusion of the second polar body is enough to 

 start the formation of a new individual. But if this 

 amount be halved (as it invariably is in eggs which 

 develop sexually) reproduction becomes impossible 

 unless the loss is made good by fertilization. But 

 several parthenogenetic insect eggs are known in which 

 two polar bodies are produced. Judging from what 

 occurs in the similar case of the crustacean Artemia, 

 it is likely that, in these eggs, the second polar body 

 reunites with the egg-nucleus before segmentation (56). 

 Among Honey-bees the mother insect (queen) is able 

 to lay, at will, eggs either fertilized or unfertilized j 



