MESODERMAL DERIVATIVES 121 



one view they developed from the ectoderm ; according to the other, they 

 are related to the fat body and therefore mesodermal. The results of the 

 work of Williams (1916) on both Photuris pennsylvanicus and Photinus 

 consanguineus and of Hess (1922) on Photuris pennsylvanicus have demon- 

 strated their origin from the mesoderm, Hess states that in the embryo 

 of Photuris the first indication of the formation of the light organs is 

 noticeable just as the embryo revolves from its backward-turned position 

 and starts to coil up. At this time groups of fat cells migrate ventrally 

 in the eighth segment and come to lie in the region of the future light 

 organs. Soon after these cells become localized, they separate from 

 the other fat cells and appear to have fewer and smaller fat globules. 

 Three or four days later there is a differentiation into photogenic and 

 reflector areas, the cells of the former being denser, less vacuolate, and 

 with denser granulation than the latter. Light is produced before emer- 

 gence of the larva. Williams' account deals mainly with the development 

 of the organs of the adult. 



THE SUBESOPHAGEAL BODY 



The structure that Wheeler (1893a) designated as the "subesophageal 

 body" (Fig. 310, suboesh) consists of a complex of unusually large cells 

 which in the embryo is found in the tritocerebral segment. Although 

 primarily an embryonic organ, it occurs in the nymphs of Gryllus and the 

 Blattidae and in the adults of termites. The structure is now generally 

 regarded as a derivative of the tritocerebral (premandibular) mesoderm, 

 although Nusbaum and Fulinski (1906, 1909), Hirschler (1907), Schwang- 

 ert (1904), and Wray (1937) considered it as developing from the anterior 

 entodermal mass. It occurs in the embryos of the Orthoptera, Plecoptera 

 (Miller, 1939), Isoptera (Strindberg, 1913&), Mallophaga (Strindberg, 

 1916a), Coleoptera (Wray, 1937; Tiegs and Murray, 1938), and Lepidop- 

 tera. In Stenohothrus (Graber, 1891) and Locusta (Roonwal, 1937) it 

 arises from the mandibular segment. In Calandra oryzae according to 

 Tiegs and Murray it is derived from the mesoderm but becomes second- 

 arily part of the mid-gut wall and has the appearance of developing from 

 the latter. This may account for the reports of an entodermal origin. 

 The cells of the organ are characterized by their large size and their larger, 

 paler nuclei. In most insects the organ degenerates before emergence 

 of the larva. In the more generalized Orthoptera and Plecoptera it 

 originates as a paired structure, but in the Lepidoptera it is either single 

 or vaguely bilobed as in Diacrisia. 



HEAD GLANDS IN THE APTERYGOTA 

 Head glands in the Apterygota as in the Pterygota are for the most 

 part of ectodermic origin; only the paired tubular glands of Campodea, 



