21 



CHAPTER II. 



STRUCTURES FOR PROTECTING EGGS. MASON-WASPS; MASON- 

 BEES J MINING-BEES. 



THE provisions which are made by the different species of 

 insects for protecting their eggs, appear in many cases 

 to be admirably proportioned to the kind of danger and 

 destruction to which they may be exposed. The eggs them- 

 selves, 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 

 temperature of 108, while their eggs did not entirely cease 

 to be fertile till 144. The larvae of flesh-flies perished, 

 while the eggs of the same species continued fertile, at about 

 the same comparative degrees of heat as in the preceding 

 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 in- 

 variably die at the temperature of 14, that is, at 18 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 destruction ; and, above all, to protect the grubs, 



* See Spallanznni's Tracts, by Dalyell, vol. i. 



