174 INJURIOUS AND USEFUL INSECTS 



sometimes resting long in this position. When the feet are 

 sufficiently hard to hold firmly, the fly erects itself again, 

 grasps its support, and bends the abdomen vigorously until it 

 becomes free. During the casting of the larval skin the linings 

 of the great air-tubes are withdrawn from the thoracic spiracles, 

 and left adhering to the empty integument as white threads. 

 When the fly is disengaged, it is flabby and incapable of active 

 movement. The abdomen has not yet gained its full length, 

 while the wings are still soft and crumpled. They slowly 

 unfold and increase in firmness. During this expansion it can 

 be seen with a lens that a clear fluid, no doubt the blood, is 



Fig. 92.— Adult dragon-fly (Libellula). After Charpentier. 



injected into the veins of the wing. In a quarter of an hour 

 the wings may be fully expanded, but they are not yet firm 

 enough to support the body in the air. Two or three hours 

 often elapse before the fly can trust its weight to them. 

 During the expansion and hardening of the wings the colours 

 of the body become more deep and vivid. The rapidity with 

 which the tints develop, the rapidity with which they are lost 

 after death, and the fact that little pigment can be extracted 

 from a dragon-fly by alcohol and other solvents, suggest that 

 the coloration is largely due to surface markings which modify 

 the reflection of light. In insects, as in many birds and a very 

 few mammals, bright and varied patterns may be got by inter- 

 ference colours with little or no use of pigment. 



The fly is magnificently equipped for flight. Each of the 



