3 i2 ANIMAL MOVEMENT 



and downward movements, and hence the body is propelled 

 onwards as well as upwards, on the same principle as in Birds 

 (see p. 303). The comparative rigidity of the front part of the 

 wing conduces to obliquity. There appear to be several ways by 

 which the direction of flight is regulated, and turning, stopping, 

 and even reversing (in some cases) brought about. For example, 

 the wings of opposite sides are capable of independent action, and 

 the abdomen can be moved so as to throw the weight of the body 

 to this or that side. 



It is interesting to note that, as in Birds, air-sacs are present 

 within the body, and these are especially large in swiftly-flying 

 forms, such as bees, dragon-flies, moths, and flies (see vol. ii, 

 p. 439). This undoubtedly reduces the weight of the body in 

 proportion to its bulk, but, according to the most recent view, 

 mere lightness is not the chief advantage thus gained. The large 

 air-sacs are now thought to be important mainly because they 

 render vigorous breathing possible, and this is a matter of great 

 moment when muscular exertion is severe and prolonged. 



Modification of the Wings in Different Insects (fig. 841). In 

 Dragon- Flies and some other forms the four wings are of equal or 

 nearly equal size, and capable of acting independently. But in 

 such insects as Bees and Moths there is an arrangement which to 

 a great extent deprives the hind-wings of their independence, and 

 makes them of secondary importance. The arrangement in ques- 

 tion consists of an interlocking device, the nature of which varies 

 in insects of different kind, whereby the hind-wings are attached 

 during flight to the fore- wings. In a Moth, for instance, there are 

 usually one or more bristles {frenuluni) projecting from the front 

 edge of the hind-wing, near its attachment, and capable of being 

 made to catch in a small flap or tuft of scales (retinaculum) situated 

 on the under surface of the fore-wing. A more elaborate arrange- 

 ment fulfils the same purpose in Bees and similar insects, consisting 

 of a series of hooks on the front of the hind-wing, which can be 

 slipped over a ridge on the hinder edge of the fore-wing. The 

 hooks are arranged " in a slightly-twisted or spiral direction along 

 the margin of the wing, so as to resemble a screw, and when the 

 wings are expanded attach themselves to a little fold on the pos- 

 terior margin of the anterior wing, along which they play very 

 freely when the wings are in motion, slipping to and fro like the 

 rings on the rod of a window-curtain " (Newport). 



