358 PARASITISM 



whether it undergoes division in the morula stage or not. Since 

 a number of the embryos abort during development, the brood 

 finally results on an average of only eight parasites being produced 

 from one Qgg. 



{c) Ichneumonoidea. Among the ichneumonoid flies poly- 

 embryony occurs in the Braconid genus Macrocentrus, which 

 parasitises larvse of the Lepidoptera. According to Parker (1931) 

 the species M. gifuensis, which has been introduced into North 

 America in connection with the control of the European corn 

 borer, produces eggs each of which may yield up to eight or ten 

 larvae. In this species there appears to be no proliferation of the 

 host tissue to form an adventitious sheath. Polyembryony is also 

 stated to occur in M. ancylivorus, which parasitises the oriental 

 fruit moth larva among other hosts. According to Daniel (1932) 

 only a single polyembryonically produced individual survives in 

 each host. 



(d) Vespoidea. The only example of polyembryony in solitary 

 parasitic wasps is afforded by Aphelopus thelice, a member of 

 the family Dryinidae. According to Kornhauser (1919) its host 

 is Thelia bimaculata, one of the largest and commonest of the 

 Membracidae found in north-eastern America. The parasite 

 usually lays a single egg in each host, selecting individuals which 

 are in a nymphal instar. The full details of the polyembryonic 

 process are not given, but it is evident, from Kornhauser's account, 

 that a trophamnion is present surrounding the developing poly- 

 germinal mass. The latter becomes converted into branching 

 chains of embryos which ultimately result in the production of 

 about 50 parasites. When the larvai become fully grown, the 

 host is either in its last nymphal instar, or has become an adult, 

 and is killed as the result of the larvae boring their way out to 

 pupate beneath the soil. 



Certain features of the polyembryonic process require further 

 mention, viz., the origin of polyembryony, the function of the 

 embryonic membranes, and the significance of mixed broods of 

 parasites. 



Origin of Polyembryony. Among parasitic Hymenoptera 

 polyembryony occurs where the eggs are immersed in a fluid 



