76 
INTRODUCTION TO 
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 pro- 
pagation, happen to he laid in the close of autumn, 
they continue unchanged throughout the winter, and 
disclose their lame in the spring. Notwithstanding 
their generally soft consistency, insects' eggs can 
bear great extremes of temperature without destroy- 
ing their vitality. There has been no deficiency in 
the ordinary number of insects last summer, al- 
though 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 des- 
troyed 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 na- 
tural 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 
