54 LIVING LIGHTS. 



the genus Phengodes was known, the female being described 

 as another insect. The mistake was made owing to the fact 

 that the female never attains a development beyond the 

 larval condition, and is the only instance among beetles 

 where the larval female produces fertile eggs. The female is 

 about two inches in length, of a creamy-white hue in the day- 

 time ; but at night it presents a truly magnificent appearance, 

 emitting from the sides or margins of the segments a rich 

 green phosphorescent light. 



Another light-giver rarely seen is the larva of Mastinocerus, 

 a slender* cylindrical form of a pale color. It lives upon 

 snails, and is feebly luminous. Mrs. King thus writes to Dr. 

 le Conte concerning it : " June 4, saw running rapidly over 

 the table, near a lighted lamp, a small Coleopter ; it was twist- 

 ing its abdomen up over its wings, and evidently trying to 

 straighten them out, as they seemed moist and twisted at 

 their ends. The general appearance suggested Mastinocerus ; 

 and, acting on this thought, I captured it, and sat up till 

 a late hour to be assured of the truth. The insect was in a 

 small phial, and moved quickly. It gave out light conspicu- 

 ously from the head, feebly from the anal end, and still more 

 so from about the base of the abdomen. The light seen in 

 the head, though visible in the dark as a round spot, yet, 

 when taken into a room obscurely lighted, was invisible from 

 above ; but, when the insect was suddenly thrown upon its 

 back, a light no larger than a pin-point was seen just about 

 the junction of the head and prothorax." 



The method of illumination in this group is intermittent, 

 the light appearing as repeated flashes : hence the term " light- 

 ning-bugs " in contrast to the steady gleam of the fire-flies or 

 Elaters. Mr. A. E. Eaton has counted the flashes in Luciola 



