84 INVERTEBRATE PHYSIOLOGY 



tivity of a "fast" axon. Pringle (1939) found a similar type of nonfacili- 

 tating response when stimulating the nerve to the cockroach metathoracic 

 extensor tibiae muscle. The "fast" responses of several muscles of Locusta 

 migratoria have been studied in a series of papers (del Castillo, Hoyle, and 

 Machne, 1953; Hoyle, 1955b,c). Similar techniques have recently been 

 used in studies on Calliphora vomitoriu, Dytiscus marginalis, and Schisto- 

 cerca gregaria (Hoyle, unpublished). The "fast" responses have also been 

 studied in cockroach leg muscles (Wilson, 1954; Hoyle, 1955c), the wing 

 muscles of Locusta migratoria danica, Gampsocleis hurgeri, and Mecopoda 

 elongata and the sound muscles of the cicadas Graptopsaltria nigrojuscata 

 and Platy pleura kaenipferi (Hagiwara, 1953; Hagiwara and Watanabe, 

 1954). 



In the extensor tibiae muscles of all the legs of the locusts Locusta 

 migratoria migratorioides and Schistocerca gregaria a sufficient number 

 of intracellular insertions has been made to make it possible to state con- 

 fidently that in them the single "fast" fibers innervate every muscle fiber. 

 This is probably true of all the other "fast" fiber systems in the locusts and 

 also applies to each unit of the locust flexor tibiae muscles and no doubt 

 also to the extensor trochanteris of Dytiscus (Kraemer, 1932) and others. 



The responses of each muscle fiber are substantially similar. In some in- 

 stances a microelectrode has been used to record the responses in different 

 parts of a single muscle fiber, and these investigations showed that there 

 is very little difference between the responses recorded at the different 

 points. The fibers on which these studies have been made are all ones which 

 receive multiple nerve endings at intervals of less than 0.1 mm. along their 

 entire length. 



The intracellularly recorded resting potentials average 60 mV (50-65 

 mV) in all Locusta and Schistocerca leg muscles in good condition when 

 bathed in a saline containing 10 mM K per liter, which is the lower limit 

 of their haemolymph potassium content. In zero-5 mM K per liter saline 

 the resting potentials approach 70 mV. In Gampsocleis wing muscles and 

 Platy pleura sound muscles the average resting potential is 60 mV in 3 mM 

 K per liter saline, whilst in Mecopoda and Graptopsaltria the recorded 

 values were only 42 mV (Hagiwara and Watanabe, 1954). In Calliphora 

 and Dytiscus values of 60 mV are common in 5 mM K per liter saline. 

 In Periplaneta flexor tibiae Wilson ( 1954) obtained resting potentials in 

 2.7 mM K per liter saline averaging 45 mV (30-70 mV ) . There are reasons 

 for being skeptical about the lower means of 42 and 45 niV, since these 

 result from several very low values of resting potential which were re- 

 corded and included in the analysis. In most insect haemolymph the 

 potassium concentration is rather high ; in the omnivorous cockroach it 

 may reach 30 mM and in the grass-feeding locust nymph 40 mM. Values 



