132 



INVERTEBRATE PHYSIOLOGY 



Fig. 8. A start during stimulation of the thoracic ganglion of the fly, Sarcophaga 

 bullata. AP is a record of the action potentials obtained with external leads, each 

 potential preceded by a shock artifact. SC is the simultaneous record of movements 

 of the scutellum recorded by an optical lever. 



muscle. After one of these movements, a very slight vibration is noted. The 

 next stimulus increased the oscillatory response, which, with continuing 

 stimulation, gradually built up to normal amplitude. This start contrasts 

 with normal starts (Fig. 6). The wings were held straight out and the vi- 

 brations began from this position. Although some of the direct muscles 

 must have been excited, the articulation was apparently not set. The medial 

 positions of the wing are unstable in an articulation set for flight. The be- 

 ginning of the vibrations may be correlated with the shortening of the 

 direct muscles, principally the pleurosternal, but under conditions where 

 the indirect muscles were already under tension and resisting the shorten- 

 ing. Excitation of the opposing fibrillar muscles without the proper me- 

 chnical situation results in each muscle preventing the shortening of the 

 other. To get out of this impasse, movement must be initiated by an acces- 

 sory agent, in this case the direct muscle. Once one muscle is shortened 

 slightly, and its antagonist necessarily lengthened, vibrations begin and 

 with proper setting of the articulation grow to full amplitude. The smallest 

 vibrations in the record of Fig. 8 are about one-twentieth normal amplitude 

 but at very nearly normal frequency. As the shortening of these muscles 

 during flight is 0.02-0.04 mm., the shortening in the vibration is less than 

 2 /x or 0.047o of muscle length. In wasps, similar vibrations have been 

 noted. In one case the ring stand to which the wasp was fastened could be 

 felt to vibrate while the animal appeared to be perfectly still. 



In these very small vibrations one cannot believe that any snap action 

 occurs. All that is necessary is for the opposing excited fibrillar muscles to 

 be moved in opposite phase, and this movement need not be very fast. The 

 system having a negative resistance, once started, will then tend to build 

 up. For the attainment of full amplitude with a damped load, such as the 

 wings moving through the air, the snap action may be necessary. 



Although much useful information can be obtained by the study of in- 

 tact insects, the correctness of our conclusions and the details of the 



