GREEN-FLIES AND PLANT-LICE 327 



a plentiful nourishment for the lady-birds which prey on 

 them. But the most curious thing is this, that these 

 abundant and rapidly reproducing broods of aphis are 

 all females, and that they do not lay eggs, but extrude their 

 young in a more or less complete state of development, 

 that is to say, they are viviparous. They are all females ! 

 It is only late in the season that males are produced ! 



In fact, the summer broods of the " green-fly " and other 

 aphides which do so much damage to rose bushes, hops, 

 and other cultivated plants, are produced by females 

 alone, without the intervention of a male. These minute 

 insects present true instances of that very remarkable and 

 interesting occurrence which is called " parthenogenesis," 

 or virginal propagation. It is further a noteworthy thing 

 that the virginal aphis mothers do not lay or deposit eggs, 

 but that the young grow from the eggs inside their mothers 

 (Fig. 61), and are only extruded when they are complete 

 little six-legged insects, capable of walking, and ready to 

 feed themselves by stabbing the soft leaves of the plant 

 on which they find themselves, and sucking up its juices. 

 The summer aphides are spoken of as being both " vivi- 

 parous " and " parthenogenetic." The words are really 

 useful, and we cannot get on without them. 



No case is known to medical men or to naturalists 

 of the birth of young from an unimpregnated or virgin 

 mother among what are called the higher animals those 

 which are classed as vertebrates, and include mankind, 

 mammals, birds, reptiles, batrachians, and fishes. But 

 though uncommon, this virginal reproduction (or "par- 

 thenogenesis ") does occur constantly in a very few kinds 

 of small insects and in some small shrimp-like creatures. 

 It has excited the greatest interest amongst naturalists 

 from the early days when it was first observed until the 

 present, and it has been very carefully studied in the past 

 thirty years. 



