76 INTRODUCTION TO 



often it extends to a month, and occasionally to 

 several months. When the eggs of such species ; 

 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 larvae 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 



