MOUTH-PARTS, WINGS, AND LEGS 53 



move is not vertical, but, on the contrary, very nearly 

 horizontal. . This plane directs its upper surface somewhat 

 forward, and therefore the main-rib of the wing corre- 

 sponds with this surface. Consequently, it is from below 

 upwards and a little forward that the propulsion of the 

 insect is effected. The greater part of the force exerted 

 by the wing will be employed in supporting the insect 

 against the action of its weight ; the rest of this impulse 

 will carry it forward. By changing the inclination of the 

 plane of oscillation of its wings, which can be done by 

 moving the abdomen so as to displace the centre of 

 gravity, the insect can, according to its wishes, increase 

 the rapidity of its forward flight, lessen the speed acquired, 

 retrograde, or dart toward the side. It is easily to be seen 

 that, when a Hymenopterous insect (e.g. a bee) flying 

 at full speed stops upon a flower, this insect directs the 

 plane of the oscillation of its wings backward with con- 

 siderable force. Nothing is more variable, in fact, than 

 the inclination of the plane in which the wings of different 

 species of insects oscillate. The Diptera (two-winged 

 flies) appear to have this plane of oscillation very nearly 

 horizontal ; in the Hymenoptera (bees, wasps, &c), the 

 wings move in a plane of nearly 45°, but the Lepidoptera 

 (butterflies and moths) flap their wings almost vertically, 

 after the manner of birds." 



We have seen that insects' legs do not work in pairs, 

 but in two sets of three. With the fore-leg a tractile 

 effort is produced, the hind-leg pushes the body forward, 

 and the mid -leg acts in the main as a support, though it 

 also does some of the pushing work. Thus, we may say 

 that while insects creep with their fore-legs, they walk 

 with their hind-legs. Typically, each leg is made up of 

 five principal parts. First there is the haunch, or coxa, 



