MUSCLES AND MOVEMENTS 109 



from right to left as it advances. Actually, when the walking insect 

 is studied by cinematographic methods, there are found to be many 

 variants of this basic plan: three, four or five legs may be on the 

 ground at the same time, and the course followed in walking may be 

 practically straight. 



In the walking of caterpillars there is a co-ordination between the 

 'turgor muscles' which are maintaining the general form of the body 

 and the true 'locomotor muscles' which move the limbs. For the 

 extension of any part of the body is brought about by the relaxation 

 of its own muscles while the general internal tension is maintained by 

 the turgor muscles elsewhere. In other larvae, such as maggots of 

 flies, legs are entirely absent and progression is effected by peristaltic 

 movements or lateral twisting movements of the body-wall combined 

 with friction against the surface due to backwardly directed spines. 

 In walking upon rough surfaces, insects make use of their tarsal 

 claws to maintain their hold; but many can walk or climb on smooth 

 leaves or on glass. For this purpose they use 'adhesive organs' which 

 commonly take the form of expansions of the tarsal surface or of 

 special 'pul villi'. These may perhaps function in more than one way, 

 but the usual mechanism seems to be the provision of 'tenent hairs', 

 moistened with some greasy secretion, which have soft delicate 

 extremities that can be brought into such intimate contact with the 

 surface that seizure or adhesion takes place and the insect is held by 

 the surface molecular forces. 



Flight 



The most characterstic form of locomotion among insects is flight. 

 This is effected chiefly by 'indirect muscles' : vertical and longitudinal 

 columns which deform the thoracic capsule by their contraction and 

 so move the wings which are articulated to it. The vertical muscles 

 elevate the wing, while the longitudinal muscles depress it. The com- 

 plex articulation of the wing gives a wide amplitude to the small but 

 powerful movement imparted to the wing base: in the wasp, for 

 example, the wings vibrate through a sector of 150°. In addition, 

 there are direct muscles inserted into the wing itself. Some of these 

 serve for the flexion and extension of the wings; others will rotate 

 the wing around its long axis or make other adjustments in the wing 



