554 Comparative Animal Physiology 



Perhaps the most striking examples of hght production in arthropods are 

 to be found among the Coleoptera, specifically the lampyrids and elater- 

 ids,-*' "'■ >" In the "fireflies" or "lightning bugs" the photogenic organ is typ- 

 ically located ventrallv in the posterior abdominal region. The organ is 

 composed of two cellular lavers, a ventral layer of light-producing cells and 

 a dorsal or internal layer of "reflecting" cells. The reflecting cells contain 

 minute particles of a purine base, probably urates. This layer is said to serve 

 as a diffusing reflector for the photogenic cells, while simultaneously being 

 a dorsal shield. The photogenic organ is richly proA'ided with tracheal ves- 

 sels which terminate in numerous tracheal end-cells. Nerves pass into the 



organ. 



Flashing of the firefly is controlled by the ner\'ous s\'stem. One remark- 

 able aspect of the activity of these organs, which indicates this, is the syn- 

 chronous flashing of some tropical fireflies.'" The fireflies normally flash at 

 random, but may, when large numbers of individuals are assembled together, 

 flash synchronously, or exhibit some coordination of activity in the assemblage 

 through waves of flashing which proceed out rapidly from some individual 

 of the group. 



The elaterid "Cucujo" beetle, Pyrophorns, of the West Indies has a pair 

 of greenish luminescent organs on the dorsolateral aspects of the prothorax 

 and an orange-vellow organ on the ventral aspect of the first abdominal seg- 

 ment. The South American "railway worm," Phrixothrix, possesses a reddish 

 luminescence on the head and greenish luminous spots segmentally ar- 

 ranged along the body.-^- A North American species of the related genus, 

 Phengodes (Fig. 252), in its photogenic organs resembles somewhat the 

 South American species, except that it lacks the red head organ. The North 

 American species has been reported to glow continuously, unlike most lum- 

 inescent animals. 



Several species of mvriapods secrete a luminous slime in much the same 

 manner as the luminous earthworms. A luminous pycnogonid has been de- 

 scribed. 



The bivalve mollusc, Pholas, has a number of photogenic glands, which 

 secrete into the siphon.''^ Some nudibranchs have distributed over the body 

 luminescent cells which flash when the animal is appropriately stimulated."^ 

 Of the molluscs, however, the cephalopods as a group show the highest 

 development of this capacity. A large number show luminescence. In many, 

 as for example, Loligo, the luminous organs are open structures and con- 

 tain luminous bacteria. In others, probably the majority, light is produced by 

 cells of the animal itself (Fig. 251). In one squid, Hetcroteiithis, there is an 

 unpaired luminous organ which opens into the mantle cavity and expels a 

 luminous cloud through the siphon when the animal is disturbed.^^' *''^ Some 

 species of squids, such as the firefly squid, Watasenia,^''- "- have complex 

 patterns of luminous organs of the intracellular type over the body, reach- 

 ing a high state of differentiation in such deep-sea forms as Lycoteiithis dia- 

 dema. In the latter as many as four different colors are produced by the va- 

 rious luminescent organs present. 



Only the Ophiuroidea among the echinoderms contain luminous species. 

 A number of these have unicellular photogenic organs scattered over the 

 body, and luminesce in response to any disturbance of the animal. 



