Many insects besides butterflies possess a periodicity which responds to the alternation of day and night. Most of the Hymenoptera are diurnal, among others the bees and the wasps. The predatory insects are also almost all diurnal, such as the robber flies (Asilidce) among the Diptera, the Cicindelidce (tiger-beetles) among the Coleoptera, the Neuroptera of the genus Panorpa, (scorpion-flies) the libellulids or dragon-flies, which belong to the Pseudo-Neuroptera. Nocturnal insects are also very numerous. Among the commonest of these I will mention the June-beetle, the stag-beetle, the mosquitos, the bedbug, the cockroaches, and the white ants. It is during the night that the processionary caterpillars of the oak leave their silken nest to travel in processions over the branches until they reach the leaves on which they feed. It is during the night also that a great numbers of weevils shake off their daily torpor to climb on plants and to puncture the leaves or buds. Some very closely related species may have a different periodicity. ,The European click-beetles ordinary fly in full daylight, as is the case with the commonest form, Lacon murinus, but Perris observes that several of them are nocturnal, among others Atlious rufus, Elater sanguineus, and E. crocatus.

Has periodicity with all these insects the rank of a definite acquirement, as with the group of nocturnal moths ? We cannot tell, for the lack of experiments. But it is not to he doubted that many possess periodicity in this way. Perris (1853) noted, as a matter of fact, that the nocturnal click-beetles of our regions remain asleep under the bark when the sun is bright, but "that when enclosed alive in boxes, they remain motionless during the day and begin to move when night comes." Dixippus morosus is an orthopteron often found in laboratories to which it has been brought from India, which is its original country. This curious phasmid (walking-stick) is remarkable not only on account of its form, but also on account of its mimetic attitude, in which, drawing its legs and antennae together, it resembles a dry branch. But, according to Pieron (1910), "the mimetic attitude is assumed by the Dixippus only during a limited time. This time lasts exactly from the beginning of day until night. As soon as the darkness arrives the period of immobility ceases; the animal walks around on its six legs, its antennae spread, sipping and exploring, and it feeds itself with ivy leaves which it chews with its mandibles." But here the acquirement of the rhythm does not as yet seem to be perfect. Pieron observes, in fact, that one "can keep Dixippus in continuous obscurity without their taking on the period of immobility with the mimetic attitude. Their activity is evidently not absolutely uniform," and everybody who has reared these insects knows that they are frequently active in the middle of the day.

Much more strongly fixed is the rhythm which produces the light with phosphorescent insects. In an interesting study on these insects, Professor Bugnion (1916) observes that their phosphorescence is always a nocturnal phenomenon, while with certain of them, like the Pyrophoras, it can be provoked in the daylight by mechanical excitations. But the specific differences are very great; with the Luciolas, or fireflies, which abound in tropical countries and in the South of Europe, the male is more luminous than the female, and "its phosphorescence is extinguished more or less completely" when it stops flying. With the Lam-pyrids on the contrary, the male is very slightly luminous even when flying, while the female (the glowworm), which is sedentary and wingless sends out a great phosphorescence. The luminosity of the latter, observes Bugnion, "is not intermittent like that of the firefly, but it is continuous or persistent during a considerable length of time. Every one knows that one can take the adult Lampyris in the hand, turn it over, and put it in a tube, without fearing that it will extinguish its torch. The functioning of the luminous organs is, on the other hand, regularly periodical; it stops almost completely during the day and when the moment is favorable (the period of sexual maturity), begins each evening with its customary brilliance." It is possible that the luminosity of certain myriapods with long slender bodies (Orya barbarica and some other geophilids) has the same rhythm.

The phenomena of phosphorescence and their rhythm have been patiently studied by Raphael Dubois (1896) with a great American elaterid, namely, Pyrophorus noctilucus, known in the West Indies by the popular name of cucujo. As with the nocturnal moths, this curious beetle is positively phototropic to the lights which burn during the night, but this phenomenon is independent of its own luminosity. This latter is in close connection with muscular activity, so that it may be provoked at any time by exciting the insect. However, it has a closer relation to the nocturnal periodicity, for under normal conditions it shows itself only during the night. When the Pyrophorus is enclosed "for several days in a black cabinet" the temperature of which is nearly constant, it becomes "active every evening at the same hour," and soon gives out a bright light even when it has been fatigued during the day by experiments. This rhythm was doubtless acquired like that of the nocturnal moths, although it is of another kind. The Auchmeromyias and the Choeromyias have shown us how acquirements of this kind can be made. We may note, in passing, that the photogenic periodicity is not necessarily due to the existence of differentiating visual organs; according to Massart, as a matter, of fact, the protozoa of the genus Noctilucus become luminous at night when kept in a closed closet.