DEGENERATE SEXUAL REPRODUCTION. 1 73 



normal sexual reproductions, but in many insects besides 

 Aphides there are several. It ought to be noted, that the parth- 

 enogenetic Aphides are hardly at the same structural level as the 

 females which are fertihsed ; but as the differences mainly lie in 

 the absence of certain accessory genital organs, there is no 

 reason for regarding the parthenogenetic forms, as some have 

 done, as larval. 



(/) Juvenile Parthenoge?iesis. — Cases do occur, however, 

 where larval forms become precociously reproductive (as some- 

 times happens among higher organisms), and produce offspring 

 parthenogenetically. Such precocious production of partheno- 

 genetic ova must be distinguished from the entirely asexual 

 reproduction exhibited by many larvse. No very firm line can 

 indeed be drawn, but in the last cases no cells which can be 

 called ova are present. In 1865 Professor N. Wagner observed 

 what has been much studied since, that in the larv^ of some two- 

 winged or dipterous midges {e.g.^ Miasfor), the cells of the repro- 

 ductive rudiment develop into larvae within the mother-larva's 

 body. The mother falls victim to her precocity, for the brood 

 of seven to ten larvae literally feed upon her to the death. They 

 finally leave the corpse and begin life for themselves, only how- 

 ever to fall themselves victims to a similar fate. The process 

 may thus go on for several generations, during which the ova, 

 or pseudova as some would insist upon calling them, become 

 smaller and smaller. Eventually the larvae become too constitu- 

 tionally poor to be precociously parthenogenetic, and develop 

 into adult midges — male and female, the latter producing how- 

 ever only a few eggs. 



In another dipterous insect known as Chironomus^ the ova 

 begin to be produced at a very early stage, are laid just at the 

 time when the larval life ends, and develop parthenogenetically. 

 According to Jaworowski, by the rupture of the ovarian mem- 

 brane the ova fall into the body-cavity, where the abundant 

 nutritive stimulus takes the place of fertilisation. Juvenile 

 parthenogenesis is also said by Von Siebold to occur among 

 the Strepsiptera, little insects which mfest bees. 



{g) Total Parthenogenesis. — Lastly, in some of the minute 

 aquatic crustaceans and in many rotifers no males have ever 

 been found. There is every probability that the parthenogenesis 

 is thus total ; and as the numbers are abundant, it has apparently 

 been established without detriment to, at least, the continuance 

 of the species. 



