Contribution of the Sarcolemma to the Force Exerted by Resting Muscle of Insects

Conclusions and Summary. 1. Many insect muscles are composed of parallel, non‐tapering fibres with little connective or tendinous tissue to influence the forces measured during rest and activity. This applies to the three muscles used here: (a) the dorsal longitudinal depressor of the hindwings of the desert locust ( Schistocerca gregaria Forskål); (b) the dorsal longitudinal abdominal muscle of the same insect; and (c) the dorsal longitudinal wing depressor of the water beetle Hydrophilus piceus L. (= Hydrous p. ). The relevant morphological data are given in Table I and figures 3 to 6. 2. Both types of flight muscles are much less extensible (passive stretch) than other striated muscles but, in locusts, the static modulus of longitudinal elasticity is of the same order of magnitude (about 8 kg . cm ‐2 ) as in the abdominal muscle and in frog muscle. 3. The sarcolemma is present as a tough membrane in locust muscle; slight squeezing of the stretched fibres produces empty sarcolemma tubes. In the flight muscle of Hydrophilus the sarcolemma, if present, has no mechanical strength compared with that of the fibrils. 4. In contrast to frog muscle, the breaking force of the empty sarcolemma tube is several times less than that of the intact locust fibre. In spite of its reduced sarcolemma, Hydrophilus flight muscle exerts a considerable resistance to passive stretch. In insect muscle, therefore, the content of the fibres is responsible for the major part of the resistance to passive stretch. 5. The resistance to stretch must either be due to passive‐elastic elements inside the fibre and parallel to the non‐elastic contractile elements, or the contractile system must be elastic even in the non‐active state.