POLY EM BRYONY AND PARTHENOGENESIS 135 



reproduction of an individual by the action of factors upon its mother 

 rather than upon the generation affected. 



Paedogenesis.— This is a form of parthenogenesis in which the eggs 

 develop precociously within the larva or the pupa. Uichanco (1924) 

 has redefined the term "paedogenesis" as ''that method of reproduction 

 whereby during the immature stages of the mother the ovum reaches a 

 condition which enables it to begin a new germ-cell cycle." It has been 

 recorded in the cecidomyiid genera Miastor and OUgarces, in the chi- 

 ronomid genus Tanytarsus, in the beetle genus Micromalthus, in the 

 heteropterous genus Hesperoctenes, and, according to Parker (1922), 

 possibly in the blowfly Calliphora erythrocephala. 



This method of reproduction was first discovered by Wagner in 1861 

 and confirmed by Leuckart (1865) and others. The species concerned 

 was the cecidomyiid Miastor metraloas which was subsequently studied 

 in detail by Kahle (1908). In M. americana, Hegner (1914) found that 

 each of the two ovaries contains 32 oocytes, each oocyte accompanied by 

 a group of nurse cells. As the oocytes increase in size, the nurse cells 

 become smaller. Finally the oocytes with the nurse cells become 

 separated from the ovary and are forced by the movements of the larval 

 mother into some other part of the body. Only a portion of the oocytes 

 produce young; the others degenerate. A polar body is given off at 

 maturation without reduction, the body then dividing mitotically. The 

 development of the eggs resembles that of other nemocerous Diptera 

 but carried on within the body of the mother larva. When the eggs 

 hatch, the young feed for a time on the tissues of the mother in the 

 manner of parasites. When the larvae are well developed, they break 

 through the body wall to become free-living. 



The paedogenetic development of another cecidomyiid, OUgarces sp., 

 has been recorded by Harris (1924). The manner of production of its 

 offspring resembles that of Miastor. Males as well as females arise from 

 mass cultures of larvae that have been produced paedogenetically, but 

 males and females do not arise from the same individual larva under 

 normal conditions, evidence indicating there is a genetic distinction. 

 Ulrich (1936) found that the European 0. paradoxus in nature developed 

 generations of males and females during the summer, whereas during the 

 remainder of the year reproduction occurred paedogeneticall}^ He con- 

 cluded that alternation of generations is due to nutritional changes. 



In 1870 Grimm described the development of a species of Chironomus, 

 the pupa of which, through paired openings at the caudal end of the 

 abdomen, deposited strings of eggs from which the flies hatched. Accord- 

 ing to the observations of both Schneider (1885) and Zavrel (1926), 

 Grimm's statement regarding the presence of paired openings seems to 

 be an error. The genus concerned is what is now known as Tanytarsus, 



