Studies on the flight mechanism of insects—II. The innervation and electrical activity of the fibrillar muscles of the bumble bee, Bombus

Abstract

The innervation of the fibrillar flight muscles of the bumble bee, Bombus, is shown to be arranged so that four nerve branches may be separately stimulated, each branch apparently containing only one type of axon. The A1 branch supplies all muscle fibres of both the longitudinal and dorsoventral muscles as the primary innervation. Large fast action potentials result from stimulation of this branch in all fibres. Secondary innervation is limited to the anterior fibres of the dorsoventral muscle. Thirty per cent of the fibres show a relatively large potential with obvious junctional and spike components when the B1 branch is stimulated. Ten per cent of the fibres respond with a small junctional potential to the stimulation of the A2 branch, while only a few per cent respond to stimulation of C2. These last muscle fibres, difficult to find, give a spike characterized by a rapid decay often followed by a small hyperpolarizing potential which may be due to an inhibitory axon in the C2 branch. The mechanical response to the primary innervation is a small contraction summating with multiple stimulation. The secondary innervation probably alters the action of the dorsoventral muscles on the articulation of the wings. Fibrillar muscles appear to play a more complex role in the regulation of the flight mechanism than has been previously considered.