THE INTERNAL ANA TOMY OF INSECTS 165 



The most commonly observed examples of light-producing insects are 

 certain members of the Lampyridas, the fireflies and the glow-worms, 

 and a member of the Elateridas, the "cucujo" of the tropics. With 

 these insects the production of light is a normal function of highly 

 specialized organs, the light-organs. 



Examples of insects in which the production of light is occasionally 

 observed are larvee of mosquitoes, and certain lepidopterous larvae. 

 In these cases the production of light is abnormal, being due either to 

 the presence in the body of light-producing bacteria or to the ingestion 

 of luminescent food. 



The position of the specialized light-organs of insects varies 

 greatly; in the fireflies, they are situated on the ventral side of the 

 abdomen; in the glow-worms, along the sides of the abdomen; and in 

 the cucujo, the principal organs are in a pair of tubercles on the dorsal 

 side of the prothorax and in a patch in the ventral region of the 

 metathorax. 



The structure of the light-organs of insects varies in different 

 insects, as is shown by the investigations of several authors. A good 

 example of highly specialized light-organs are those of Photinus 

 marginellus, one of our common fireflies. An excellent account of 

 these is that of Miss Townsend ('04), to which the reader is referred. 

 A detailed account of the origin and development of the light-organs 

 of Photurus pennsylvanica is given by Hess ('22). 



