Among insects far better known than the rare luminescent May-flies, are the glow-worms, a family of beetles of which several species are known besides our own familiar one, called Lampyris noctiluca. The fire-flies of Southern Europe--Luciola italica--are small beetles allied to the glow-worm, but both sexes fly and both are luminous, whilst in the common glow-worm the female is wingless, and the flying male, who is guided to the female by her light (which she can vary in intensity), gives but a feeble light. The swarms of Italian fire-flies consist of as many as a hundred males to one female, and the males are far more brilliant than the females. My fellow-student Moseley showed some in oxygen gas at the Royal Society's soirée many years ago. The gas greatly increased their brilliancy. Many valuable experiments in search of an explanation of the brilliance of the male Luciolæ and their excess in number could be carried out in North Italy. A peculiar grub-like female glow-worm, three inches long, is found in South America, which produces a red light at each end of the body and numerous points of green light on each side of it. It is called the "railway-beetle" in Paraguay.
Another family of beetles besides the Lampyrids, or glow-worms, is celebrated for the brilliant luminescence of some of its species. These are the click-beetles, or Elaterids (see Fig. 22, C). In South America there are upwards of a hundred species of this group, showing various degrees of luminosity. The "Cucujos" (Pyrophorus noctilucus) of tropical America is one of the most abundant and largest. It is as much as an inch and a half long, and has three "lamps," or luminous organs, one on each side of the body and one below the tail. The light given off is extremely beautiful, and the live insects are used by the women for ornament and by the country-folk as lamps on nocturnal excursions. Erroneously the term "fire-fly" is applied to these beetles; it should be reserved for the little Italian Luciola, which swarms, as countless thousands of dancing lights, in the nights of early summer over the marsh lands of North Italy. I have seen it at the end of June as far north as Bonn, on the Rhine. In Australia a small true "fly"[7]--that is to say, a two-winged fly or Dipteron like our gnats, midges, and house-flies--is known, the maggot of which is luminous. And in New Zealand there is another of which both the maggot and the perfect insect are luminous. The grub is called the New Zealand glow-worm.[8]