PHYSIOLOGY AND BEHAVIOR OF THE ARTHROPODA 



335 



originates within the muscles. Such myogenic rhythms, as opposed to 

 the neurogenic rhythms in other insects, produce wing beat frequencies 

 that may reach 300 or 400 per second. 



A critical feature of such myogenic rhythms is the tension in the 

 system. If one set of flight muscles is cut, the other will not develop its 

 rhythm. The two must act together, each alternately stretching the 

 other. The frequency of contraction depends upon the tension in the 

 flight muscles and this tension not only is caused by the opposing flight 

 muscles, but can be increased by other smaller muscles in the thorax. If 

 these smaller muscles contract steadily, they increase the tension and 

 raise the frequency of wing beat. The hum of a mosquito, fly or bee 

 can be used as an accurate indicator of this frequency, since changes 

 in tone indicate changes in frequency. 



Pringle found further that a single set of muscles could produce 

 its rhythm if opposed by powerful springs as a substitute for the op- 

 posing set of muscles. As one might expect, the frequency of contraction 

 was a function of the stretch of the springs. 



In the evolution of these small, fast-flying higher insects the co- 

 ordination of the flight muscles has been taken from the nervous system 

 and built into the muscles themselves. The ability to contract repeatedly 

 under tension is apparently a result of adjustment in the internal physi- 

 ology of the muscle cells. 



In many insects with slow neurogenic rhythms the path traced by 

 the wing tip as it moves up and down shows that the wing moves evenly 

 from the up or down position to the horizontal, and then "clicks" sud- 

 denly the rest of the way down or up. Until very recently the "click" 

 mechanism was not understood. It has turned out to be a marvel of sim- 

 plicity: Some of the small muscles of the thorax attach to the inner 

 upper edge of the "box," just belo^v' the point Avhere the wings articu- 

 late (Fig. 17.10). The steady contraction of these muscles tends to pull 

 the sides of the box together. A study of the figure shows that the dis- 

 tance between the upper edges of the box is least when the wings are 



= — Notutn 



Pleurumn. 

 PI e-ural muscle 



Figure 17.10. The "click" mechanism. A comparison of the three figures, using the 

 dotted hne as a reference Une, shows that the sides of the thorax are pushed out when 

 the wings are horizontal, and closer together when the wings are up or down. Oblique 

 muscles to the sides of the thorax produce tension to enhance the "click," which is a rapid 

 conclusion of the wing beat upward or downward. 



