These sensitive creatures are unable to resist the attractions of artificial lights. Réaumur noticed this fact many years ago, and since the introduction of the electric light, notes may frequently be seen in journals recording that myriads of these Insects have been lured by it to destruction. Their dances may frequently be observed to take place in peculiar states of light and shade, in twilight, or where the sinking sun has its light rendered broken by bushes or trees; possibly the broken lights are enhanced in effect by the ocular structures of the Insects. It has recently been ascertained that a species of Teleganodes is itself luminous. Mr. Lewis,[[369]] who observed this Insect in Ceylon, states that in life the whole of the abdomen was luminous, not brightly so, but sufficient to serve as a guide for capturing the Insect on a dark night. It has also been recorded that the male of Caenis dimidiata gives a faint blue light at night.
Nearly 300 species of Ephemeridae are known, but this may be only a fragment of what actually exist, very little being known of may-flies of other parts of the world than Europe and North America. One of the more curious forms of the family is Oniscigaster wakefieldi; the body of the imago is unusually rotund and furnished with lateral processes. In Britain we have about forty species of may-fly. The family is treated as a distinct Order by Brauer and Packard, and is called Plectoptera by the latter.
Fig. 284.—Oniscigaster wakefieldi. New Zealand. (After M‘Lachlan.)
That Insects so fragile, so highly organised, with a host of powerful enemies, but themselves destitute of means of attack or defence, should contrive to exist at all is remarkable; and it appears still more unlikely that such delicate Insects as Ephemeridae should leave implanted in the rocks their traces in such a manner that they can be recognised; nevertheless, such is the case,—indeed, the may-fly palaeontological record is both rich and remarkable. Several forms are preserved in amber. In the Tertiary bed of the old lake at Florissant, Scudder has been able to distinguish the remains of no less than six species; while in the Jurassic layers of the Secondary epoch, in more than one locality, the remains of several other species have been detected and described. Still more remarkable is the fact that in the Devonian and Carboniferous layers of the Palaeozoic period, remains are found that appear to be akin to our existing Ephemeridae. Palingenia feistmantelii from the Carboniferous of Bohemia is actually referred to a still existing genus; it is said to have been of gigantic size for a may-fly.
The families Megasecopterides, Platypterides, and Stenodictyopterides of the Carboniferous epoch (see p. 343) are all more or less closely allied to the Ephemeridae, and in addition to these Brongniart has established the family Protephemerides for some Insects that he considers to have been the precursors in the Carboniferous epoch of our existing may-flies. These ancient Insects differed in having the wings of another form from those of existing Ephemeridae, and in having the hind wings equal in size to the front pair. Besides this, these Insects had, as shown in Fig. 285, prothoracic dorsal appendages; some had also projections from the abdominal segments, considered by Brongniart to be of the nature of gills. Some doubt must exist as to this point, for we find in the imago of one of our existing Ephemeridae, Oniscigaster wakefieldi, Fig. 284, abdominal processes that are not gills.
Fig. 285.—Homaloneura bonnieri; Carboniferous of Commentry. (After Brongniart.)
It is remarkable that may-flies, which now form a comparatively unimportant part of the Insect tribe, should in far distant times have been represented by so great a variety of allied forms. Our fragile, short-lived may-flies appear to be, as Scudder says, the lingering fragments of an expiring group.