354 PARASITISM 



Hymenoptera,^ where it exhibits a remarkable sequence of 

 biological processes. In view of the immense number of a species 

 of parasitic Hymenoptera already described, and the very small 

 number whose economy has been adequately studied, it is likely 

 that the incidence of polyembryony will eventually be found to 

 be more frequent. The researches of Martin (1914) in Germany, 

 Silvestri (1906) in Italy, and of Leiby and Hill (1923-24) and of 

 Patterson in America — which followed upon the earlier funda- 

 mental discoveries of Marchal in France — have led to the 

 elucidation of the essential features of the phenomenon. It is 

 known to have arisen independently in three families belonging 

 respectively to the Chalcidoidea, Proctotrypoidea and Vespoidea, 

 and its phases will be briefly considered in each of these groups 

 separately. 



(a) Chalcidoidea. It was in the species Ageniaspis (Encyrtus) 

 fuscicollis that Marchal definitely proved in 1898 the occurrence 

 of polyembryony in insects. It has only so far been observed 

 within the single family Encyrtida?. where it prevails in species 

 belonging to a group of closely related genera. Polyembryony has 

 been recorded in species of Ageniaspis, Encyrtus, Copidosoma, 

 Litomastix, and Berecyntus, whose hosts so far as known are 

 Lepidoptera.2 In all cases the eggs of the parasites are deposited 

 within those of their hosts which duly hatch into larvae, and the 

 developing parasites are contained within their bodies. The 

 main features of the embryonic development are very similar in 

 the individual cases studied, irrespective of whether the eggs arc 

 fertilised or not, vide Fig. 90. Development commences by 

 the oocyte nucleus dividing twice to form two polar bodies, and 

 the first formed of the latter may also undergo division (Fig. 90, A). 



^ In 1923, Noskiewicz and Poluszynski published a very brief preliminary 

 communication on the occurrence of polyembryony in the Stylopid 

 JIalictostylops, which parasitises the bee, Ilalictus simplex. Their much 

 condensed account is difficult to interpret in the absence of any illustrations, 

 but it appears that two types of embryos are formed. In those destined to 

 develop polyembryonically the blastoderm becomes divided into groups of 

 cells which form separate vesicles, and the latter ultimately give rise to the 

 embryos. It is stated that the number of embryos produced may vary from 

 two to more than forty. 



2 These Lepidopterous hosts pertain to the families Gelechiida?, 

 Hyponomeutid.TC, Coleophoridae, Gracilariidae, Tortricidae and Noctuidae. 



