Correlated structural and contractile properties in specialized fibers of a woodlouse Armadillidium vulgare (Latr.)

In order to define the ultrastructural differences between slow‐contracting (tonic) muscles with differing abilities to develop tension, observaticns were made on Armadillidium vulgare, comparing muscle structure and function. The latero‐ventral muscles, by means of which the animal holds itself rolled up in a tight ball, were compared with the dorsal intersegmental muscles. which pull it back to a normal stance. The morphological differences found are easily interpreted in the light of the different functions of the two types of muscle. Where the muscles are more powerful, and have to hold considerable tension, the contractile system is stronger (myosin filaments are 7 μm long and 200 Å thick; the ratio of thin to thick filaments is 6 or 7: 1; there is a thick filament density of 610 filaments per μm2). In the dorsal intersegmental muscles, which have less resistance, the system is simpler: myosin filaments are 4 μm long and 150 Å thick; the ratio of thin to thick filaments is 4: 1; and the thick filament density is lower — 510 filaments per μm2.