Muscle Fibres of the Fast and the Slow Contracting Muscles of the Mesothoracic Leg of the American Cockroach (Periplaneta americanaL,)

Abstract

IT is well known that some muscle fibres in the coxa of the mesothoracic leg of the American cockroach contract rapidly while others contract slowly^1,2. Until recently the difference in contraction-speed of insect muscles was attributed mainly to a difference in innervation. Krüger³, however, discovered that in vertebrates the structures of fast and slow muscles did not differ only in degree of innervation. The specific locomotory muscles apparently consist of fibres that show a uniform distribution of the myofibrils in cross-section (Fibrillenstruktur), whereas in posture muscles the fibrils are in small groups, in which the individual muscle fibres are sometimes scarcely discernible. These groups often form angular areas (Felderstruktur). The question now arises whether similar differences occur in insect muscles. In order to investigate this problem some muscles of the coxa have been examined in cross-section. They were fixed in 15 per cent formol, Champy, Susa and Bodian 2, and embedded in the usual way in paraffin (alcohol, methyl benzoate, benzol, benzolparaffin). For frozen sections the muscles were embedded in 15 per cent gelatine.