140 INVERTEBRATE PHYSIOLOGY 



rise after the completion of the lengthening than if relengthening is de- 

 layed. A record on a large wasp is shown in F, since it gave quite a large 

 fall in tension following shortening. These drops in tension do not appear 

 to result from mechanical factors in the experimental setup but are rather 

 the result of some lengthening of the contractile elements. 



If as in E ( 1 ) the muscle is rapidly lengthened after the initiation of 

 stimulation, but before the tension has increased greatly, one may some- 

 times see the same effect as found when the muscle is lengthened after a 

 rapid shortening. The tension continues to rise after the completion of 

 the stretch. 



A very rapid stretch after a shortening results in a sharp rise in tension 

 followed by a precipitous fall after the completion of the movement, and 

 then a rise to the isometric level, G. In the bee during flight one stroke lasts 

 about 5 msec. In this experiment the muscle has been stretched in 1 msec. 

 This viscous-like behavior of the muscle must set an upper limit to the 

 speed of the system. 



These transient responses of the muscle show that, following a rapid 

 shortening, the contractile elements lengthen slightly and the tension falls. 

 During the subsequent rapid relengthening, only a portion of the tension 

 has returned by the end of the movement, as little as 30% in the best cases. 

 The tension continues to rise very rapidly after the muscle has attained 

 its initial length and may overshoot the isometric tension at which shorten- 

 ing began. The rise is due to the shortening of the contractile elements. 



Many of the characteristics of the flight machinery found in the study 

 of intact insects now find explanation in the physiology of fibrillar muscle. 

 The muscle is basically an elastic system through which chemical energy 

 is furnished by the contractile elements as necessary to overcome the damp- 

 ing forces tending to halt motion. Since the snap action, acting in the same 

 manner as inertia, tends to resist movement, work must be done against 

 it as well as against air damping at the beginning of the stroke. The elastic 

 energy so stored maintains movement toward the end of the stroke when 

 muscle tension is falling. 



If we now place the muscle we have studied back into the insect from 

 which it was isolated, coupling it to the mechanical system described, and 

 to its antagonist, we can give it the proper label and file it away. At some 

 future time, however, as we remove it to study its structure we may per- 

 haps see something more than its external morphology, in fact, one of na- 

 ture's most successful solutions of heavier-than-air flight. 



Summary 

 Insects, making use of the special mechanical properties of an exoskele- 

 ton, have evolved two principal types of flight mechanisms. In the syn- 



