HEXAPODA : HEMIPTERA PLANT-LICE, ETC. 329 



this multiplication they admirably illustrate what has been 

 called Parthenogenesis. 



It is well understood among physiologists that it is the contact 

 of the male sperm-cell with the yolk which fertilizes the egg, and 

 that from the moment of this contact the life of the embryo, which is 

 to be the future animal, begins. This is the general rule among 

 animals in which the sexes are distinct. But among some kinds of 

 the lower animals as some kinds of Jelly-fishes, Worms, Crustaceans, 

 and Insects there are exceptions, so that in some species an embryo 

 begins its life without the interposition of the male ; and this mode 

 of reproduction has been called, by Owen, " Parthenogenesis," and 

 by Steenstrup, '' alternation of generations," and by Huxley and 

 others, "agamic reproduction."* 



For example, the young aphides are hatched in the spring from 

 impregnated eggs laid the previous autumn, and soon they come to 

 maturity, and the whole brood consists of wingless females. These 

 females bring forth living young, each female producing in some 

 cases twenty in a day. These young are also wingless females, and 

 soon they bring forth living young, which are also wingless females, 

 and in their turn bring forth living young. And in this way brood 

 after brood is brought forth, even to the fourteenth generation, in a 

 single season, in some cases ; and this, too, without the appearance of 

 a single male. But the latest .brood in autumn is composed of both 

 males and females, which have wings ; these pair, stock the plants 

 with eggs, and then perish. 



"We get some idea of the rapidity of the multiplication 

 of these animals when we remember that Reaumur has 

 proved that a single Plant-louse may become, in five gen- 

 erations, the progenitor of 6,000,000,000 descendants ! 



* In the case of one of the highest of the Insects the Hive Bee there is 

 something quite different from the ordinary modes of reproduction, and is 

 probably one phase of parthenogenesis. It has been shown by Siebold that 

 the fertilized eggs of the queen bee produce either queens or workers, accord- 

 ing to the conditions to which they are subjected, and the nature of the 

 food given the larvae ; and that the unfertilized eggs produce drones, that is 

 males. 



Parthenogenesis has been observed even in the larva of one insect a 

 Cecidomyian fly. 



