MYOGENIC RHYTHMS 105 



Weis-Fogh (unpublished) tells me that in Schist ocerca the usual result of 

 amputation is a pronounced irregularity in the wing beat, but that some 

 rhythmic activity can still occur when the wings are reduced to mere 

 stumps. One difficulty in intepreting results of this sort on otherwise intact 

 insects is that we now know^ that the incidence-control reflex must have 

 been operating ; and, since loading as well as inertia must aflfect the magni- 

 tude of the forces at the wing base to which sense organs must respond, it 

 is not easy to predict the nature of the change produced by amputation on 

 the form and intensity of sensory excitation. It appears likely, however, 

 that there is, at least in some of the 1 :1 groups, a reflex effect from the 

 wing sense organs on the rhythmic mechanism, but that something in the 

 nature of a pacemaker is present in the ganglion capable of discharging 

 rhythmic bursts of motor nerve impulses even in the absence of sensory 

 feedback. It is relevant to note here that in Diptera (with a myogenic 

 rhythm in their indirect flight muscles) amputation produces a fall in the 

 frequency of motor nerve impulses (Roeder, 1951), although it leads to 

 a large rise in wing-beat frequency. In all 1:1 insects it is necessary to 

 suppose that there is a considerable measure of inherent organization in the 

 flight motor centers in the thoracic ganglion producing coordination of 

 activity in the various muscles which move the wings; Tiegs (1955) has 

 emphasized that the evolution of the flight mechanism has resulted in a 

 reduction rather than an increase in the number of muscles involved. 



The first rhythmic mechanism in the insect wing system was thus 

 probably of a nature similar to that found in the rhythms of swimming and 

 locomotion, where there is an inherent pattern of coordination in the central 

 nervous system, but where reflex feedback is always very important and 

 may sometimes be essential (toad; Gray and Lissmann, 1946). As some 

 insects got smaller, the frequency of action demanded from this system 

 increased to the point where it became difficult for the flight muscles to 

 perform one complete contraction and relaxation during a single beat. This 

 seems to be the present condition in some Orthoptera, Lepidoptera, and 

 Odonata. Tiegs (1955) has recorded a normal wing-beat frequency of 

 57/sec. in the hawkmoth Hippotion. By direct observation he finds that 

 electrical stimulation at 30/sec. produces a partial tetanus, although com- 

 plete fusion of twitches is found only at 70/sec. Heidermans (1931) re- 

 ported a similar condition of partial tetanus in Odonata. Weis-Fogh (un- 

 published) tells me that in Schistoccrca stimulation experiments suggest 

 a similar result, but that this is misleading, since there is a rise of tempera- 

 ture of about 8° C in the thorax of a flying locust, and that at the real tem- 

 perature of the muscles during flight there is time for a complete contrac- 

 tion and relaxation ; twitch duration is known to have a high temperature 

 coefficient in frog muscle (Hill, 1951). 



