CHAPTER X 



POLYEMBRYONY AND PARTHENOGENESIS 



POLYEMBRYONY 



The term " polyembryony " is applied by zoologists to cases in which 

 two or more embryos develop from a single egg, although its use in cases 

 of twinning is not strictly correct. The phenomenon occurs sporadically 

 among many animals, both vertebrates and invertebrates, and habitually 

 among certain bryozoans, annelids, parasitic Hymenoptera, and the 

 mammals (armadillos). 



Polyembryony in insects was discovered by Marchal in 1898, but not 

 until 1904 did he publish a full account. It is known to occur in the 

 groups Chalcidoidea, Serphoidea (Proctotrupidae), and Braconidae of the 

 parasitic Hymenoptera. Recently Noskiewicz and Poluszynski (1935) 

 have recorded its occurrence also in the Strepsiptera, a group of parasitic 

 insects related to the beetles. Nearly a decade before Marchal's work, 

 Bugion (1891) in a paper on Ageniaspis fuscicollis called attention to the 

 novel fact that embryos were placed in a fiexuous tube which floated in 

 the haemolymph of the host caterpillar (Hyponomeuta) at the side of the 

 intestine, but he did not suspect that all the embryos originated from a 

 single egg. The principal conclusion of Marchal's researches were that 

 the egg of A. fuscicollis is laid in the egg of its host but continues its 

 development into the larva and that from the single egg of the parasite 

 many individual parasites develop. Some points in the development left 

 untouched by Marchal were treated subsequently by Silvestri (1906) in 

 his study of Litomastix truncatellus wherein he described the formation of 

 the polar bodies, fertilization, cleavage, and the development of asexual 

 larvae. In a series of later papers he dealt with the embryology of several 

 species of parasitic Hymenoptera, both monembryonic and polyembry- 

 onic. Martin (1914) was the next investigator dealing with this subject, 

 placing especial emphasis on the development of the oocyte in the ovarium 

 of A. fuscicollis. In 1915 Patterson pubHshed an account of the late 

 stages of development of Copidosoma gelechiae and since then a series of 

 papers covering the development of Paracopidosornopsis floridana. The 

 latter species, which is a common parasite of the cabbage looper {Auto- 

 grapha hrassicae), is apparently identical with the insect known under the 

 name of Litomastix or C. truncatellum. Following Patterson's account of 

 C. gelechiae was a more extended one by Leiby (1922) on the same species. 

 This was followed by papers by Leiby and Hill on Platygaster hiemalis and 



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