ALTERNATION OF GENBRATIONH. 053 



of which hiy eggs, while the redia or p7-oscoIex of the same 

 worm produces cercarice by internal budding. Here also 

 may be cited the cases of strobilation of the Aurelia, the 

 tape-worm, the Nais, Syllis, and Autolycus, among Anne- 

 lids. Thus among Ccelenterates and worms, as well as some 

 Crustacea, a large number of individuals are produced, 

 not from eggs, but by budding. 



Similar occurrences take place among insects, as the 

 Aphis or plant-louse, in which a virgin Apliis may bring 

 forth in one season nine or ten generations of Aphides, so 

 that one Aphis may become the j)arent of millions of 

 young. These young directly develop from eggs or buds 

 which are never fertilized, hence the term parthenogenesis, 

 or virgin -reproduction, sometimes called agamogenesls (or 

 birth without marriage). The bark-lice as well as the 

 Aphides develop in this manner during the warm wea- 

 ther ; but at the approach of cold both male and female 

 Aphides and Coccidse appear, the females laying fertilized 

 eggs, the first spring brood thus being i^roduced in the 

 normal, usi^al manner. 



Still more like the production of young in the redia of 

 the Trematode worms is the case of the larva of a small gall- 

 gnat [Miastor), which during the colder part of the year from 

 autumn to spring produces a series of successive generations 

 of larvae like itself, until in June the last brood develops 

 into sexually mature flies, which lay fertilized eggs. 



While the larval Miaster produces young like itself, the 

 pupa of another fly, Chironomus, also lays unfertilized eggs. 



A number of moths, including the silk-worm moth, are 

 known to lay unfertilized eggs which produce caterpillars. 

 Among the Hymenoptera, the currant saw-fly, certain gall- 

 flies, several si:)ecies of ants, wasps [Polistes), and the honey- 

 bee, are tnown to produce fertile young from unfertilized 

 eggs ; in the case of the ants and bees, the workers lay eggs 

 which result in the production of males, while the fertilized 

 eggs laid by the female ant or queen bee produce females 

 or workers. 



Taking all these cases together, parthenogenesis is seen to 

 be due to budding, or cell-division, or multiplication. Now, 



