52 A BOOK OF INSECTS 



ship, propelled by powerful machinery, and possessing 

 ample " steering- way " ; while the latter resembles a sail- 

 ing ship " riding light " with all sails set, driven merci- 

 lessly before the prevailing wind. 



Insects' bodies contain air-sacs, which are in com- 

 munication with the tracheae ; and these are especially large 

 in swift-flying forms, such as dragon-flies, moths, bees, 

 and flies. Before flight, they are charged by a deliberate 

 act of inspiration, the specific gravity of the insect 

 being thereby slightly reduced. We have seen, however, 

 that too much stress must not be laid upon mere lightness 

 of body ; and there can be little doubt that the air-sacs 

 are chiefly important because they act as reservoirs, and 

 assist respiration during the severe muscular exertion 

 coincident to rapid flight. 



While up and down movements of the wings suffice 

 for the simplest kind of insect flight, the process becomes 

 more elaborate as efficiency increases, and the muscles are 

 proportionately more numerous and complicated. Thus, 

 in the case of dragon- flies, the two pairs of wings are 

 capable of independent action, although they can also be 

 worked in unison ; while in addition to the elevator and 

 depressor muscles there are others by means of which the 

 wings may be rotated and otherwise adjusted. In many 

 four- winged insects, however, the two wings of each side 

 are united when in use by a series of hooks, or some other 

 simple mechanism, and strike the air as one. As the 

 precise method of attachment has an important bearing 

 on insect affinity, we shall refer to it again in the next 

 chapter. 



With respect to the way in which an insect regulates 

 its flight, Professor Marey has the following passage : " We 

 need only observe the flight of certain insects, the common 

 fly, for instance, to see that the plane in which the wings 



