172 THE EVOLUTION OF SEX. 



flight. The sperms thus received are stored up, and used to 

 fertihse the eggs as she lays them in the cells. Not all the eggs, 

 however, but only those which will produce future queens or 

 else workers. Other eggs, to all appearance similiar, are un- 

 fertilised, and these, as Dzierzon first clearly showed, develop 

 solely into drones. We cannot, however, say that the absence or 

 presence of fertilisation is the sole difference, though if fertilis- 

 ation be prevented by the imperfect development of the 

 wings, or by clipping them, the queen only lays drone eggs. 

 The same happens when she is old and her store of male 

 elements exhausted, or when the sperm receptacle has been 

 removed. Von Sie])old carefully examined the eggs from drone- 

 cells, and found that they never contained spermatozoa. Hensen 

 notes an interesting side fact, obviously corroboratory, that 

 " German queen-bees, fertilised by Italian or Cyprian drones, 

 produced hybrid females but pure drones, a proof that on the 

 latter the sperm does not operate." Again, it sometimes happens 

 that what are called " fertile workers " crop up, which in con- 

 sequence of some accident or misdirected intention in the 

 nutrition, become less abortive than the host of semi-females 

 which make up the body of workers. They are fertile enough 

 to lay eggs, but their female organs do not seem to admit of 

 their being impregnated. Certain it is that they only produce 

 drones. What has just been said in regard to bees, is also 

 true of some w^asps and ants. 



(e) Seasonal Parthenogenesis. — In some of the minute aquatic 

 crustaceans (Cladocera), popularly included under the general 

 title of water-fleas, parthenogenesis only occurs for a season, and 

 is periodically interrupted by the birth of males, and the 

 occurrence of the ordinary sexual reproduction. Males generally 

 reappear in the disadvantageous conditions of autumn, but 

 Weismann denies that there is a direct connection between these 

 facts. The common Aphides are parthenogenetic for a succes- 

 sion of generations, sometimes as many as fourteen, throughout 

 the summer, but the cold and hard times of autumn bring back the 

 males and the sexual process. The fertilised egg lives on through 

 the winter, and develops with the warmth of the next spring. 

 By keeping up the temperature and nutritive optimum for three 

 or four years in the artificial summer of a glass case, Reaumur 

 and Kyber succeeded in rearing as many as fifty continuous 

 parthenogenetic generations. In the gall-wasps {Cynipidie) there 

 is usually only one parthenogenetic generation between the 



