ANATOMY AND PHYSIOLOGY 



nosity in insects, though sometimes merely an incidental and pathologi- 

 cal effect of bacteria, is usually 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 Pyrophorus (Elateridae) , 

 in which the pronotum bears a pair of luminous spots, and the common 

 fireflies (Lampyridae). In Lampyridae the light is emitted from the 

 ventral side of the posterior abdominal segments, and the structure of 

 the photogenic organ is essentially the same throughout the family. 

 In Photinus this organ (Fig. 169) consists of two layers; a ventral photo- 

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

 consists of polygonal cells containing large quantities of crystals of ur- 

 ates; the former layer is composed 

 of tracheal structures and inter- 

 vening parenchyma cells. The 

 tracheae branch profusely in the 

 photogenic layer, where the larger 

 air- tubes are each surrounded by 

 a more or less cylindrical mass of 

 cells; tracheal branches penetrate 

 between the cells of each cylinder, 

 at the edge of which they pass into 

 tracheoles which penetrate the 

 photogenic tissue and anastomose 



. . 1 ,. ,. , FIG. 169. Transverse section of portion of 



With those Of adjacent Cylinders; photogenic organ of a firefly, Photinus. c, 



in the meshes of the tracheolar 

 network is a granular substance of 



fatty nature ("differentiated fat-body"), the oxidation of which is the 

 source of the luminosity, it is inferred. The photogenic tissues of 

 Photinus, after being dried and kept in sealed tubes, have retained their 

 photogenic power for more than eighteen months, glowing after this 

 interval upon the " application of water in the presence of air or oxygen" 

 (McDermott). Three factors are involved in the production of the 

 light: a substance to be oxidized, oxygen and water. 



Professor W. E. Burge has found that the catalase content of a 

 luminous insect where oxidation is presumably more intense is greater 

 than that of a non-luminous insect where oxidation is less intense. 



The rays emitted by the common fireflies are remarkable in being 

 almost entirely light rays. According to Young and Langley, the radia- 

 tions of an ordinary gas-flame contain less than three per cent, of visible 



