When the usual swarms of may-flies emerge from the water, fishers make straw fires on their boats, and the insects singe their wings. The innumerable bodies incapable of flight fall into the water, and provide a coveted food for the fish.[44] This disharmonious and fatal instinct is displayed chiefly by nocturnal insects that rest during the day and do not leave their retreats till after sunset. In the cornfields Coleoptera of the genera _Anisoplia_ and _Rhizotrogus_, resembling each other in form and general appearance, are to be found. When a fire is lighted in the darkness of the night it is only the _Rhizotrogus_ that approaches it at the risk of its life. The _Anisoplia_ remains quiet in the midst of the corn. The latter kind of beetle pairs during the day, while the _Rhizotrogi_ satisfy their sexual desires during the night. Moreover, it is the males only of this species that fly about in the darkness and approach the fire, whilst the females rest at home in the plants.[45] It is probable, therefore, that light induces a sort of sexual excitement in these male beetles. The males, searching for the female, believe her to be in the midst of the flames, towards which they fly without being conscious of the danger they incur. Such an interpretation of this disharmonious and suicidal instinct is confirmed by the fact that the moths attracted by fire are also almost exclusively males. Moreover, entomologists have advised against the lighting of fires by agriculturists in the belief that they destroy the noxious _Botys_, as they maintain that the females are not attracted. These latter therefore live on, and, being capable of laying eggs, produce a generation of voracious caterpillars. Of the _Ephemera_ attracted by fire in such great quantities males are by far the more numerous. It is therefore really very probable that the mad excitement which leads to the destruction of so many male insects, represents a sort of sexual aberration. In this connection it is to be remembered that, amongst Coleoptera, species exist of which the females, hidden in the grass, produce intense light which attracts the males. In the common glow-worm, the female, which is devoid of wings, alone shines with the familiar greenish glitter. Even in species of which the two sexes are luminous, the female shines more vividly. It is true that there are some beetles with luminous larvæ, a fact that led Darwin[46] to remark that the production of light by insects may serve to frighten enemies. This is possible, and it is also possible that certain insects make use of their luminosity to light their way in the darkness.[47] But, notwithstanding this, the sexual character of the luminous organ is so manifest in certain species that it is impossible to doubt its function as a means of attracting the male.