356 PARASITISM 



from a single egg varies in different species ; in Ageniaspis 

 testaceipes 13 are produced (Marchal) ; in Copidosoma gelechice 

 from about 163 to 191 are recorded (Patterson, Leiby), while in 

 Litomastix truncatellus an average of about 1,500 embryos results 

 (Silvestri). It appears that, as the number of embryos derived 

 from a single egg increases, the polyembryonic process becomes 

 more complicated. The first-formed morula may each divide 

 into two, as in G. gelechice, while in L. truncatellus secondary 

 morulas divide to form tertiary bodies which finally develop into 

 larvae. AVhen fully formed the individual larvae break through 

 the investing membranes and become free within the body of 

 the host. 



[h) Proctotrypoidea. Polyembryony occurs in this grouj) in 

 the genera Polygnotus and Platygaster, belonging to the family 

 Platygasteridse. Their hosts in all cases are Cecidomyidse, which 

 are parasitised either as eggs or as young larvae. Polygnotus 

 minutus utilises the Hessian fly (Mayetiola destructor) and the oat 

 midge (M. avence) as its hosts and deposits its egg in the mid- 

 intestine of the young larva, where it ultimately produces usually 

 10 to 12 embryos. Since the egg is bathed in the chyle of the gut 

 no adventitious sheath is formed around the embryonic mass. 

 Platygaster hiemalis, according to Lieby and Hill, also parasitises 

 the Hessian fly, attacking the eggs or, more occasionally, the 

 young larvae. It is of particular interest because the egg may 

 develop either monoembryonically or polyembryonically. From 

 4 to 8 eggs are laid in an individual host ; some develop into 

 single embryos, others give rise to two embryos, and the remainder 

 degenerate. The embryos become dispersed in the body-cavity 

 of the host embryo or larva, as the case may be, and become 

 invested by certain of the tissues which form an adventitious 

 sheath. The embryonic nucleus divides to form four nuclei 

 and subsequently the whole embryonic mass divides, and the 

 two daughter masses become separated by an extension of the 

 trophamnion between them. The cells of each embryo thus 

 formed then undergo further division until a hollow 16-celled 

 blastula is formed (Fig. 91). Without following the development 

 further, it may be added that finally the resulting larvae feed upon 



