18 THE FLY-EGG, LARVA, AND CHRYSALIS 



As with the domestic mosquitos, cold will delay the 

 hatching of the egg, a prolongation of the larval stage, 

 and hibernation of some of the adult insects. But in 

 winter, it is only the few which survive, the fittest 

 living through the English winter. This hibernation 

 of insects is most interesting. Some of the imagines 

 hibernate as well as the larvag ; probably the pregnant 

 females do this more commonly. But, as Mr. Jepson 

 has pointed out, unprotected flies disappear with 

 the coming of frost. Those that have found for 

 themselves cosy corners in warm houses survive, and 

 in this way some impregnated females live through to 

 the spring, when they go forth to lay their eggs and 

 to start a new generation. But, like the domestic 

 mosquito, the house-fly will proceed to lay her eggs 

 even during an English winter if she finds herself in 

 a regularly warmed conservatory where there is a 

 sufficiency of food for her young ; and then the life 

 of the larva depends upon the temperature. Mr. Jepson 

 has succeeded in keeping flies alive in captivity for 

 eleven weeks in winter ; so the hibernation of the 

 adult insect is certain and that of the larva and chrysalis 

 probable. 



After the fly-maggot has moulted twice and has 

 grown from two millimetres (-^ inch) in length at birth 

 to six millimetres ( J inch), it passes into the pupa, nymph, 

 cocoon, or chrysalis stage. The larva contracts within 

 its own skin, which forms a sort of mummy-case for it. 

 The maggot curls itself up within this case, which be- 

 comes bean -shaped. The skin is white at first, but as 

 the insect inside contracts and the air gains access in be- 

 tween, its colour changes, becoming darker until the pupa 

 is brown. One sees small groups or clumps of brown 

 bean-like objects lying motionless amongst the straw of 



