108 INSECT PHYSIOLOGY 



which control the wings in Diptera and Hymenoptera, or the sound- 

 producing timbals in cicadas, are capable of extremely rapid oscilla- 

 tory contractions. These muscles commonly operate a so-called 

 'click' mechanism such that when they have contracted up to a criti- 

 cal point, the thoracic wall or the timbal cuticle clicks over into a 

 new position and the contracting muscles are suddenly released. This 

 sudden release deactivates the contracting muscle and causes it in- 

 stantly to relax ; at the same moment the sudden stretching of the 

 opposing muscle excites it instantly to contract. 



In addition to their alternate contractions and relaxations the 

 muscles may enter a state of prolonged steady contracture or 'tonus'. 

 In this state they may support the insect in some characteristic atti- 

 tude and may have to bear a considerable weight; but the rate of 

 metabolism is very low. In the cockroach in the standing position 

 there is a steady discharge of impulses passing down the slow fibres 

 to the depressor muscles of the leg and producing the tonic con- 

 traction which raises the animal off the ground. Microscopic exami- 

 nation of a cockroach muscle in this state shows that the contraction 

 is produced by many fibres responding in turn. On this background 

 are superimposed short bursts of impulses in the quick nerve fibres, 

 producing vigorous contractions in the mucles and moving the 

 animal rapidly over the ground. Slow movements are achieved en- 

 tirely by means of the slow fibres. In soft-bodied insects such as 

 caterpillars, the tense form of the body is maintained by the tonic 

 contraction of the so-called 'turgor muscles' of the body-wall. 



Locomotion 



As was shown by Johannes Miiller more than a century ago, the 

 insect rests during walking on a supporting triangle, formed by the 

 anterior and posterior limbs on one side and the middle limb on the 

 other, while it carries forward the other three legs. The fore-leg acts 

 as a tractor, the middle leg serves for support, while the hind-leg acts 

 as a propulsor and also turns the body in the horizontal plane. Thus, 

 as the insect walks the centre of gravity falling within the supporting 

 triangle of limbs is carried forwards and outwards towards the apex 

 of the triangle until it falls outside this base and its support is taken 

 over by the other triangle of legs ; hence the body zig-zags slightly 



