134 INVERTEBRATE PHYSIOLOGY 



restretching of the muscle for the muscle to take advantage of the deactiva- 

 tion. The possibility of reactivation on stretch was also suggested. Our re- 

 sults with intact flies (Fig. 5E,F) show that after a fast stop the shortened 

 muscle may remain short for the duration of several cycles without the 

 return of tension. When the inhibition to movement is removed, the short- 

 ened muscle can be as rapidly lengthened by its antagonist as in the normal 

 stroke. Lengthening, therefore, must be as important for the redevelopment 

 of tension as shortening is to the fall of tension. 



Since the fly preparation was technically difficult to handle because of the 

 very small movements possible, the study was continued with large bumble 

 bees. Upon removal of the head and abdomen, the thorax was impaled on 

 two needles pushed into a small mounting board. A third needle, inserted 

 into the cuticle at right angles to the other two, firmly anchored the thorax 

 with the posterior end oriented upward. The phragma to which the longi- 

 tudinal muscle is attached was exposed, cut from its connections with the 

 articulation, and fastened with a double hook to an RCA mechanotrans- 

 ducer for recording muscle tension. The transducer could be raised and 

 lowered precise amounts in order to study tension at different muscle 

 lengths. 



Fibrillar muscle of the bumble bee exhibits marked summation to a series 

 of maximum stimuli, as does that of the fly and of the tymbal muscle. The 

 response of the bee preparation to stimulation at 8 per second is shown in 

 Fig. 11 A. The single isometric twitch is quite small as compared to the 

 complete tetanus attained at about 40 stimuli per second. Relaxation is 

 very slow, taking one-half to one second for the tension to drop to zero 

 after the cessation of stimulation. The slow relaxation explains the stops 

 noted in Fig. 6B,C. 



A typical tension-length diagram of the muscle in maximum tetanus is 

 shown in Fig. 9. The maximum amount the muscle can shorten is only 

 about 12% of its rest length, or 0.9 mm. With the arms of the phragma 

 attached to the articulation, the muscle is held in the thorax at the length 

 at which it develops maximum isometric tension, and shortening is limited 

 to 0.1-0.2 mm. The curve of passive tension is also shown. This limited 

 movement means that only a portion of the tension-length curve is used 

 in flight. Since the isometric tension varies little over this range (Fig. 9), 

 the fall in tension necessary for the work cycle must result from the rapid 

 shortening. In these isometric contractions there was no evidence of 

 oscillatory behavior, the muscle acting as other skeletal muscle in all 

 regards. 



The relation between isometric and isotonic contraction is shown in Fig. 

 10. For these experiments the mounting board with the bee thorax prepara- 

 tion was fastened to a thin piece of metal hinged to a support. To load the 



