ANATOMY AND PHYSIOLOGY 



105 



FIG. 166. (Eno- 

 cytes and accom- 

 panying tracheae, 

 from abdomen of a 

 silkworm. 



but gradually increase in size (Wheeler) , and their size is in a general way 

 proportional to that of the fat-body. 



Their function has been problematical until recently. Many ob- 

 servers have regarded them as ductless glands, having seen "microscop- 

 ical exudations around the periphery of the cytoplasm, 

 especially at times when the nucleus is greatly ramified, 

 and therefore manifesting its great activity" (Glaser). 

 R. W. Glaser has recently thrown light upon the 

 nature of the cenocytic fluid. By using three-year-old 

 caterpillars of Zeuzera pyrina, which have a great 

 amount of fatty tissue and correspondingly large 

 cenocytes, he was able to extract enough of the fluid 

 for chemical experiments. He found by carefully con- 

 ducted tests that the fluid had the power of oxidizing 

 fats, by means of enzymes known as oxidases (though 

 no fat-splitting enzyme, or lipase, was present), and 

 concluded that the secretion of the cenocytes is used 

 to oxidize the reserve food stored up ' by the larva in the form of fat. 

 Photogeny. This phenomenon appears sporadically and by various 

 means in protozoans, worms, insects, fishes and other animals. Lumi- 

 nosity in insects, though sometimes merely an incidental and pathological 



effect of bacteria, is usually pro- 

 duced 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 Pyrophorus 

 (Elateridae), in which the pro- 

 notum bears a pair of luminous 

 spots, and the common fire-flies 

 (Lampy ridae) . In Lampyridae 

 the light is emitted from the 

 ventral side of the posterior ab- 

 dominal segments, and the struc- 

 ture of the photogenic organ is essentially the same throughout the family. 

 In Photinus this organ (Fig. 167) consists of two layers : a ventral photogenic 

 layer and a dorsal reflecting layer. The latter, white and opaque, consists 

 of polygonal cells containing large quantities of crystals of urates; the 



FIG. 167. Transverse section of portion of 

 photogenic organ of Photinus. c, cylinder; p, 

 photogenic layer; r, reflecting layer; /, trachea. 

 After TOWNSEND. 



