often it extends to a month, and occasionally to several months. When the eggs of such species as have several broods in the course of the season, and are speedily hatched to admit of such frequent propagation, happen to he laid in the close of autumn, they continue unchanged throughout the winter, and disclose their larvæ in the spring. Notwithstanding their generally soft consistency, insects' eggs can bear great extremes of temperature without destroying their vitality. There has been no deficiency in the ordinary number of insects last summer, although many of them would be exposed, in the egg state, to the unusual rigours of the preceding winter. Spallanzani placed some with impunity in an artificial mixture, which reduced the thermometer to 22° below zero; and experiments of the same nature have frequently been made since, with a similar result. The same skilful observer found, that a temperature of 90° did not materially injure the eggs of the silk-worm, but a higher degree destroyed the fertility of many, and none could resist a heat of 144°. The situation selected by the parent insect, is often exposed to the highest degree of natural heat that can be obtained. The Melasomas of America lay their eggs in sandy places where the thermometer has been found to rise above 70° (R.) during the heat of the day, and those of Nyctelia have been observed in similar places, where it was impossible to hold the hand for a few seconds on account of the heat.
Want of air is more speedily fatal to them than
