What influence is exerted upon the organism by external -stimuli which make themselves felt at regular periods? That is what we are going to study in the present chapter.

The Paradox Of Reaumur; Diurnal Variations In Excitability

"It is singular," says Eéaumur, "that the Lepidoptera which avoid the light of day are precisely those which fly into lighted rooms and are attracted to light when it is carried into the garden. "

This fact would really seem paradoxical, and it is readily understood that biologists have tried to explain it. Eomanes contents himself with a word, attributing to nocturnal moths a curiosity regarding strange objects, but Loeb has studied the paradox more seriously and has made many experiments.

In a room which received the daylight through a window, he placed in a glass box two nocturnal moths, the Euphorbia sphinx (Celerio euphorbiœ) and the willow Bombyx (Eriogaster lanestris).

These moths flew about, then all gathered on the side toward the window, in which direction they collected when the box was turned around. Moths issuing from pupae acted in exactly the same way. Allowed to fly in the room with a kerosene lamp placed far from the window, they went toward the latter when they were at equal distances between the two luminous spots (the window and the lamp), but they went to the lamp when they were at least a yard nearer to it. Thus, nocturnal Lep-idoptera do not flee from the light of day, as Reaumur believed, and they are positively phototropic, just as are the diurnal species, but, adds Loeb, we find with insects as with many plants, "periodical variations of irritability, and these variations correspond to the change of day to night. Just as certain flowers open their corollas only during the night and others only during the day, certain Leopidoptera fly only by day and certain others only by night".

This periodicity is independent of actual luminous stimulations. There are, in fact, with these insects, "periodic variations of irritability,'9 but these variations can manifest themselves in the absence of all actual luminous stimulation.

Reaumur has already noted this phenomenon, which is not less curious than suggestive:

Most of the night-flying moths which are at liberty in the country, fly only at night or on the approach of night. Some, moreover, of the same class, when kept enclosed in boxes or cages, show the time when their inclination leads them to flight. During the day they are quiet in their prisons, passing hours and often days without moving, in the same place. But when night has come and sometimes even before the sun is ready to set, they move their wings and fly as much as the box will permit.

Loeb has tried this experiment with the Euphorbia sphinx, and has noted these periodic nocturnal movements during two or three days. He does not know if this periodicity will ultimately disappear after long confinement in the dark box, but he has noted that the sphinx in the clear day does not take its flight when a quick diminution of the intensity of the light occurs. This reaction is produced only in the morning a little after dawn, and toward evening when 'twilight begins,-that is to say, at hours close to those when the free insect ordinarily takes flight.

Thus, the periodicity appears to us to be manifest and independent of luminous variations.