In the genus _Photinus_, certain species of which have parts of a roseate tint, the light-emitting organs are larger in the male than in the female, and vary considerably in position in the different species. In the male they cover the entire ventral segments, from the fourth to fifth inclusive; and on the fifth and sixth segments the little impressions or pores referred to are seen in the females. The light-organs occupy the middle portion of the ventral segments, and resemble a flat elevation upon the fifth segment. There are so many exceptions and differences, that the young naturalist will find it a particularly interesting study. Thus in _P. dimissus_ the male has the usual illuminating apparatus, while it is entirely wanting in the female. In the group _Lampyres_ the lights are bright in the females, but variable in the males. For a long time only the male of 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 daytime; 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 twisting 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 conspicuously 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 “lightning-bugs” in contrast to the steady gleam of the fire-flies or Elaters. Mr. A. E. Eaton has counted the flashes in _Luciola lusitanica_, and found that there were thirty-six in a minute, each flash lasting from one-fourth to one-third of a second.[37]