ANATOMY AND PHYSIOLOGY 



FIG. 166. 



CEnocytes and accom- 

 panying tracheae, from 

 abdomen of a silkworm. 



fat-tissues. The corpuscles (leucocytes, or phagocytes] \\1iidi 

 in some insects absorb effete larval tissues during meta- 

 morphosis have been by some authors regarded as wandering 

 fat-cells. Cells constituting the pericardial fat-body are at- 

 tached to the lateral muscles (alary muscles) of the dorsal 

 vessel, but almost nothing is known as 

 to their function. Associated with the 

 fat-body proper are the peculiar cells 

 known as oenocytes. These occur in 

 most insects, in segmentally-arranged 

 clusters on each side of the abdomen, 

 and consist of exceptionally large cells, 

 more or less round or oval (Fig. 166), 

 each with a large round, oval or elon- 

 gate nucleus. These peculiar cells are 

 usually separate from one another, but 

 are held in clusters by tracheal branches. 

 Their function is unknown. Finally, the 

 fat-body is the basis of the luminosity, or so-called phospho- 

 rescence, of insects. 



Luminosity. This phenomenon appears sporadically and 

 by various means in protozoans, worms, insects, fishes and 

 other animals. Luminosity in insects, though sometimes 

 merely an incidental and pathological effect of bacteria, is usu- 

 ally produced by special organs in which light is generated 

 probably by the oxidation of a fatty substance. 



There are not many luminous insects. Those best known 

 are the Mexican and West Indian beetles of the genus Py- 

 rophorus (Elateridse), in which the pronotum bears a pair of 

 luminous spots, and the -common fire-flies (Lampyridse). In 

 Lampyridse, the light is emitted from the ventral side of the 

 posterior abdominal segments. In our common Photinus, the 

 seat of the light is a modified portion of the fat-body a 

 photogenic plate, situated immediately under the integument 

 and supplied with a profusion of fine tracheal branches. The 

 cells of the photogenic plate, it is said, secrete a substance which 



