Muscle and Electric Organs 587 



The flight muscles of insects have very brief twitch durations. Contrac- 

 tion times of 15 msec, at room temperature have been recorded, and these 

 are probably longer than the period of contraction in the insect. Wiggles- 

 worth^'*^ discusses insect locomotion in detail. Some of the measurements 

 of wing frequency in beats per second given bv that author-'*- are: honevbee 

 (Apis) 180-250; bumblebee (Bomhus) 130-240; mosquito (Culex) 278-307; 

 fly (Musca) 180-330; butterfly (Pieris) 9, 12; dragonfly (Aeschna) 22, 28; 

 blowfly (Lucilia) 87. In Drosophila repleta the wing beats increase from 

 about 120/sec. at 10° to 200/sec. at 30°.'" In mosquito flight a complete 

 contraction and relaxation cycle must be completed in about 3 msec. It 

 has been suggested''"' that in flight the muscles show an incomplete tetanus 

 and that there is alternation among fibers from beat to beat. Leg muscles 

 of insects are somewhat slower (Table 71). 



Recent evidence^''-" indicates that in flies the wing muscles are stimu- 

 lated by stretch and beat at higher frequencies than are shown by motor im- 

 pulses from the central nervous system. The intrinsic rhythm of the wing 

 muscles is modulated by reflexes initiated in part from sense endings in the 

 halteres as discussed in Chapter 14. 



Metabolism in rapidly beating flight muscles is very high, and many 

 insects require a warming-up before flight. Most insects and humming 

 birds with fast flight muscles feed on sugar. 



Next in speed are the white muscles of vertebrate limbs with contraction 

 times of 25-50 msec, and then the somewhat slower vertebrate "holding 

 muscles," usually red, such as the soleus, with contraction times approaching 

 100 msec. Other red muscles such as the diaphragm are still slower in con- 

 traction. Movements in embryonic muscles, as in a 16-day rat embryo, are 

 slower than they are in adults, thresholds are high, and excitation times 

 are long. Muscle chronaxies of a 5-day chick are reported to be 60 times 

 as long as in the adult fowl.^^^ 



The striated muscles of invertebrates have speeds similar to those of ver- 

 tebrate red muscle. Crustacean claws, squid mantle, and some coelenterate 

 muscles contract in about 100 msec. The adductor muscles of most bivalve 

 molluscs have a striated portion which when isolated contracts in about 

 100 msec, and a smooth portion which contracts more slowly (Table 71). 



It is difficult to get an accurate contraction rate for visceral smooth mus- 

 cles. Spontaneous or induced contractions are often slow rhythmic waves 

 takinp several seconds for contraction. The cat nictitating membrane con- 

 traction time is also several seconds. The contraction rate of single verte- 

 brate smooth fibers is not known, and part of the sluggishness of the whole 

 muscle is due to the local nature of contractions. 



Conduction at Neuromuscular Junctions and in Muscle. Motor nerve 

 impulses activate muscles by way of a neuromuscular junction. The nature 

 of this junction is not well known except in vertebrate skeletal muscle, in 

 which it is a complex end-plate to which b9th nerve and muscle contribute 

 (Fig. 236). In smooth muscles the motor nerve fibers branch extensively, 

 surround, and may penetrate the muscle fibers. 



An impulse in a motor nerve going to vertebrate striated muscle con- 

 sists in part of an electrical wave, the action potential, which travels at high 



