That this is really so-that is to say, that the diurnal periodicity of butterflies has been acquired in the course of their history-is what the pretty experiments of Eoubaud1 have shown concerning the actual acquisition of a similar periodicity by the larvae of an African fly, Auchmeromyia luteola, which resembles our bluebottle, but is very different in habits. The larvae of this fly are called "house-worms,'y because they are found exclusively in houses, and only in those in which people lie on the ground with or without mats. During the day the larvae are to be found at a certain depth below the surface of the ground, which serves as the flooring of these primitive habitations. At night, while the sleepers are lying down, the larvae crawl up toward them, wound them with their mandibles, and gorge themselves with their blood through the wound thus made. After this, they go back into the earthen floor to digest there in peace, and to await the following night. Here are temporary parasites whose periodicity is like that of nocturnal moths.
This periodicity is the result of their thermic sensibility. Experiments establish the fact that their optimum temperature varies between 25°
1 E. Roubaud, Les Producteurs de myiasis et agents similiares chez l'Homme et les animaux, 1916, and 30°, which they find at a certain depth in the soil of these houses, and that they are endowed with a positive thermotropism which leads them to the sleepers, whose temperature is 37°, and that, once they are filled, their thermotropism becomes negative, which leads them back underground. A temperature of 35° is manifestly unfavorable to them when it is prolonged for some hours. Thus the nocturnal periodicity of these larvae is produced by thermic irritability in conditions of determined hunger.
This rhythm not only is acquired in the course of the larval existence, but is changeable at the will of the experimenter. On their birth, says Roubaud, these larvae, having taken no nourishment "are awake during the day as well as during the night." Fed from the egg stage regularly each day at the laboratory in the morning or afternoon, they "remain completely motionless during the night"; and when, after having been nourished periodically for several days, they are left to fast, "they awake irregularly," some during the day and others during the night. "Thus/' says Roubaud, "the nocturnal awakening is above all a case of adaptation of the organism of the larva to a certain rhythm of nutrition," regulated under normal conditions by the habits of the sleepers, who return to the houses and sleep when evening comes. It is a rhythm acquired in the course of the larval existence, experimentally modified with the young larvae, and moderately tenacious with those in which it is well established, for it only slightly resists a long deprivation from food.
The work of Roubaud upon the larvae of Chcero-myia chcerophaga completely justifies the foregoing conclusions. These blood-sucking larvae in nature are less completely nocturnal than the preceding, for they have as their hosts certain tropical pigs which leave their holes only in the cool hours of the morning, and in the afternoon when the great heat has passed. During the warm hours of the day they rest in their obscure lairs, where, just as during the night, they are bitten by their parasites. But the physiological plasticity of the larvae of the Choeromyias is just as great as that of the house-worms: "gorged regularly with blood and only during the day, after their hatching they manifest only a slight diurnal activity during the meal hours, and when filled, pass the night in a condition of complete immobility."
The rhythm of the Auchmeromyias and the Choeromyias is acquired, but it is not yet innate, because it is not passed on to the progeny. That of the nocturnal moths is innate, for we observe it in moths issuing from chrysalids, but it was certainly acquired like the preceding, and there were ancestors who transmitted it to their descendants.