( 20 ) 



CHAPTER II. 



STRUCTURES FOR TROTECTIXG EGGS. — MASON- WASPS ; MASOX- 

 BEES; MINIXG-BEES. 



The provisions which are made by the different species 

 of insects for protecting their eggs, aj^pear in many cases 

 to be admirably proportioned to the kind of danger and 

 destruction to which they may be exposed. The eggs 

 themselves, indeed, are not so liable to depredation and 

 injury as the young brood hatched from them ; for, like 

 the seeds of plants, they are capable of withstanding 

 greater degrees both of heat and cold than the insects 

 which produce them. According to the experiments of 

 Spallanzani, the eggs of frogs that had been exposed to 

 various degrees of artificial heat were scarcely altered in 

 their productive powers by a temperature of 111° of 

 Fahrenheit, but they became corrupted after 133^. He 

 tried the same experiment upon tadpoles and frogs, and 

 found they all died at 111°. Silkworms died at a tem- 

 perature of 108^, Avhile their eggs did not entirely cease 

 to be fertile till 144^ • The larvge of flesh-flies perished, 

 while the eggs of the same species continued fertile, at 

 about the same comparative degrees of heat as in the pre- 

 ceding instances. Intense cold has a still less effect upon 

 eggs than extreme heat. Spallanzani exposed the eggs 

 of silkworms to an artificial cold 23^ below zero, and yet, 

 in the subsequent spring, they all produced caterpillars. 

 Insects almost invariably die at the temperature of 14°, 

 that is, at IS'' below the freezing point.* The care of 

 insects for the protection of their eggs is not entirely 

 directed to their preservation in the most favourable 

 temperature for being hatched, but to secure them against 

 the numerous enemies which would attempt their destruc- 

 * See Spallanzani's Tracts, by Dalycll, vol. i. 



