NERVOUS CONTROL OF INSECT MUSCLES 75 



vation of Decticus thoracic and leg muscles and Dytiscus wing muscles ; 

 Montalenti (1928) described a triplotomic branching in Hydro philus leg. 

 Pringle (1939), using a physiological method, showed that the extensor 

 tibiae muscle of the metathoracic leg of Periplaneta receives two motor 

 axons. The locust metathoracic extensor tibiae muscle receives three axons 

 (Hoyle, 1955a). The homologous muscles of the pro- and mesothoracic 

 legs only receive two (Hoyle, unpublished) ; many other locust muscles, 

 e.g., the retractor unguis, also receive two. The flexor tibiae of several 

 species, e.g., Romalea microptera (Ripley, 1954), Acanthacris riificornis 

 and Zonocerus sp. (Ewer, 1954), do however appear to receive four or 

 even more axons, as indicated by a consideration of the number of steps 

 which can be obtained in the tension developed by the muscles when a very 

 carefully graded stimulus is applied to the motor nerve.^ The flexors of the 

 tibiae of all the legs of the locusts Locusta migratoria and Schistocerca 

 gregaria also seem to receive several axons. I have obtained graded steps 

 from these muscles during stimulation of the motor nerve and at the same 

 time have recorded intracellular action potentials from various muscle 

 fibers in different parts of the muscles. As the stimulus strength is raised, 

 groups of fibers in different regions come into twitch activity. The action 

 potentials in all the fibers are nearly identical electrical responses of the 

 "fast" type (see below). Evidently the orthopteran flexor tibiae muscles 

 are composed of four to six motor units, each of which has a separate nerve 

 supply. From the evidence in the literature we may tentatively regard the 

 Pm/?/aM^/a flexor trochanteris (Pringle, 1939) and the Z^yfwcM.? extensor 

 trochanteris as being similarly constructed. Graded contraction in these 

 muscles could be effected not only by the special arthropod methods to be 

 described, but also by the vertebrate method of varying the number of 

 motor units in action at a time. The "fast-fiber motor unit, i.e., the comple- 

 ment of muscle fibers supplied by a single "fast" axon, of the flexor tibiae 

 muscles consist of a single, coherent bundle of muscle fibers, in contrast 

 to the vertebrate motor unit which is probably composed of fibers scattered 

 throughout the whole muscle (data in Tiegs, 1953). The smallest insect 

 muscles are formed of single bundles of fibers and the larger ones are con- 

 structed of several such bundles (termed "muscle units" in Hoyle, 1955a). 

 The locust and cockroach extensor and flexor tibiae are all composed of 

 several muscle units ; those of the extensors all receive branches from the 

 same two (or three) axons, whereas those of the flexors may receive inde- 

 pendent nerve supplies. Perhaps this complexity of innervation imparts a 

 greater degree of controllability to the flexors. In the pro- and mesothoracic 

 legs of locusts and in all the legs of the cockroach, the flexor tibiae play a 



1 The same is also true of some coxal muscles of the cockroach (Becht and Dres- 

 den, 1956). 



