202 



OUR HOUSEHOLD INSECTS 



insect is shown just after the lid had been forced off, with 

 its body still enclosed within the lower part of the pupa 

 case, and the distension of the frontal sac 

 is still at its height. The fly soon wriggles 

 out of the case, and, while its body is still 

 soft and pale, runs about with its wings in 

 a crumpled and baggy condition ; as it thus 

 runs along, the membranous sac on the 

 head alternately dilates and contracts, 

 being connected with the breathing-tubes. 

 But as the skin gradually hardens and the 

 wings become fully expanded, the move- 

 ments of this sac become less perceptible, 

 and finally, when the hardening process is 

 completed, and the fly is ready for flight, 

 all power of dilatation is lost. 



It is impossible to speak definitely with 

 regard to the duration of the life of the 

 perfect fly ; but it would appear to be longer than that of 

 the larva and pupa, except when these exist through the 

 winter. Bluebottles are often caught with very ragged 

 edges to their wings, and with their proper proportion 

 of hairs much reduced, both of which mutilations seem 

 to imply a good deal of knocking about in the world. In 

 addition to this, bluebottles, at least, hybernate, and are 

 thus able to exist from one year into the next, though 

 probably only a very small proportion of the specimens 

 of a given season survive in this way. During the 

 winter they stow themselves away in the most perfect 

 concealment in unsuspected corners and crevices, from 

 which, however, an unexpectedly warm and sunny day 

 in spring will at once entice them. Most people have 

 probably noticed the sudden appearance of flies on 

 such occasions, and their equally sudden and complete 



FIG. 65. Fly 

 inPuparium, 

 lid removed. 

 a, Frontal 

 sac. 



