Page:Popular Science Monthly Volume 72.djvu/361

 Eighth.—A faculty similar to the preceding which the parasite may possess.

the eggs of the Hyponomeutas are never parasitized, and the adult generation of the Encyrtus will already have disappeared while the Hyponomeutas still continue to lay eggs, which will thus escape the parasite.

Second Example.—With the Hessian fly, a small dipterous insect whose larva is extremely injurious to wheat, the generations succeed one another the whole year, and under particularly favorable conditions there can be five or six generations in a single year. However, the time necessary for an individual to perfect its development is extremely variable, according to conditions in which the pupæ find themselves, and especially in regard to their position on the plant—whether on the green part or near the earth or even dry stubble. Some can complete their development in two weeks, while others, finding the conditions of dryness exceptionally long, wait even as long as two years before issuing. The hymenopterous insects, living at the expense of the insect, which may appear in innumerable quantities, have, on the contrary, only two generations each year at the maximum, and appear only at a definite time, and during a lapse of time of usually short duration. Now, since the parasites never attack more than one of the developmental stages of the insect—egg or larva, according to parasitic species—it results, from what precedes, that there will always be, at the period of egg-laying, existing individuals of the species which will escape them because they are in a developmental condition in which they are not pierced; and when the generation of parasites has passed, these individuals will remain unharmed and constitute the indispensable reserve for the perpetuation of the species. It is not necessary that this reserve should exist throughout the range of the plant-feeding species. On the contrary, they may be annihilated in certain localities by a combination of climatic conditions or factors of some other nature having an unfavorable influence, and it is in this way that the local disappearance of certain species, of which this one is an example, is to be explained.

Third Example.—In the preceding case the average conditions, and in particular the relative dryness, play a part of the first rank in the determination of retarded development, and the adaptation of the plant-feeding species enables it to react in a more or less energetic manner to external influences. In other cases the plant-feeding species has acquired a great variability in the time necessary for the development of individuals. This variability, which appears to be independent of average conditions, consists in reality in the fact that different individuals present a variable power of reaction to identical external influences.

It is thus that, according to Boisduval ("Essay on Horticultural Entomology," Paris, 1867, p. 15), the chrysalids of Bombyx everia and lanestris, which cause great damage in Germany, issue in a very irregular manner. "One sees," says he, "moths of these Bombycids issue in September after three months of metamorphosis, others in the spring of the following year; but what is more astonishing is, that from the same egg laying, from the same brood of caterpillars, reared under the same conditions, the moths have issued during seven years in April and September—a wise foresight on the part of nature, which does not wish to expose the species to sudden destruction."