104 INVERTEBRATE PHYSIOLOGY 



frequency is increased, and the frequency of impulses actually reaching the 

 muscle is always sufficiently low to produce discrete contractions. Only 

 with stimulus intensities well above threshold could the nerve-impulse fre- 

 quency be raised to the level which produces a smooth tetanus in the muscle. 

 There is thus no evidence in the locust tergosternal muscle of any inherent 

 muscular rhythmicity. 



It is probably useful to try to start right from the beginning in a con- 

 sideration of the evolution of insect flight. There is fossil evidence that the 

 first winged insects were large creatures with well-developed locomotor 

 habits and that their wings first arose as lateral expansions of the terga of 

 the thoracic segments. Wings thus first appeared in insects as new struc- 

 tures and not as a modification in function of an existing appendage as in 

 the birds. At first the lateral thoracic expansions of insects must have pro- 

 vided lift as fixed aerofoils, and a control of their incidence is therefore the 

 first requirement in a system of control. Chadwick ( 1953) has argued and 

 Weis-Fogh (1956, Pt. IV) has now demonstrated in the locust that the 

 main control of lift in flight is by a reflex system adjusting wing incidence in 

 certain portions of the stroke. This then may be thought to be the primitive 

 control mechanism, deriving from the time when the wings were fixed 

 aerofoils and preserved in the make-up of insects ever since. Weis-Fogh 

 has pointed out to me that a wing-folding mechanism may have been the 

 second feature to appear, since large lateral expansions must have been 

 difficult to manage under all conditions ; this also is preserved in modern 

 flapping forms. Only later, perhaps, did the flapping machinery arise. 

 Birds, by contrast, probably flapped their wings from the start, using the 

 locomotor muscles of the fore limbs of their running reptilian ancestors. 



The next problem concerns the nature of the rhythmic mechanism. The 

 more primitive insects have flight muscles which give a normal single 

 twitch for a single motor nerve impulse (Roeder's 1 :1 muscle), and if 

 the early insects were large there is initially no requirement for a very 

 high frequency of wing beat and for a very short muscle twitch. A number 

 of workers have studied the efl^ect on wing-beat frequency of alterations in 

 the loading and inertia of the wings ; in 1:1 systems these also provide evi- 

 dence about the nature of the neurogenic rhythmic mechanism, since the 

 frequency of motor nerve impulses corresponds to that of the wing beat. 

 The problem is whether the rhythm-generating mechanism is innate in the 

 ganglion or involves reflexes from wing sense organs. Roeder (1951) re- 

 ports that in Periplaneta and in A gratis (Lepidoptera) amputation of the 

 wings produces little change or a decrease in frequency. Sotavalta ( 1954) 

 confirms that there is no change in Periplaneta, but records a slight in- 

 crease (up to 20%) in Lepidoptera. Tiegs (1955) also finds an in- 

 crease in various noctuids, Neuroptera, and Isoptera (all 1:1 systems). 



