MUSCLES AND MOVEMENTS 107 



the maximum load it can raise per square centimetre of cross-section. 

 When expressed in this way there is no great difference between the 

 muscles of insects and of vertebrates. Thus the value for man is 

 6-10 kg. per sq. cm., for the hind-legs of the long-horned grass- 

 hopper Tettigonia A-l kg. 



When the isolated muscles of insects are studied by the methods 

 of classical physiology they show much the same properties as those 

 of vertebrates: they have a similar 'latent period' between stimulus 

 and response, similar 'summation' when a second stimulus follows 

 closely upon the first, and a similar frequency of stimulation is neces- 

 sary to induce a steady state of contraction or 'tetanus'. But the 

 results do depend upon whether the 'slow' nerve fibre or the 'fast' 

 fibre has been excited. For example, in the leg muscles of the cock- 

 roach, the mechanical fusion of twitches begins at a frequency of 

 30 stimuli per second when the slow fibre is excited; whereas if the 

 fast fibre is excited it always produces a brief and powerful tetanus, 

 apparently not susceptible to graduated control. 



When an insect muscle is stimulated by means of the nerve the 

 conduction of the impulses is carried out solely by the nerve fibres : 

 there is no conduction of impulses along the muscle fibres such as 

 occurs in vertebrates; and if the nerve is allowed to degenerate the 

 muscle can no longer be excited by any form of electrical stimulation. 

 In the wing muscles of Periplaneta the contractions begin to fuse at 

 rates of stimulus above 40-50 per second. During flight the wing 

 movements in this same insect occur at a frequency of only 25-30 per 

 second and there are changes in electrical potential occurring at the 

 same rate. It has therefore been concluded that in this insect, in 

 dragon-flies, Lepidoptera, and many other insects there is a nerve 

 stimulus of the conventional type inducing each contraction that 

 leads to a wing beat. 



But in some of the higher insects, notably in Diptera and in 

 Hymenoptera, the muscular contractions occur at very high rates of 

 100-200 or more per second. In these cases the muscle contractions 

 are taking place at a far higher rate than the changes in electrical 

 potential. It has been concluded that the relatively infrequent im- 

 pulses in the nerves serve merely to bring the fibrillar flight muscles 

 of these insects into a reactive state. In this state the fibrillar muscles 



