Jumping Muscle Research Articles

Functioning of the Inhibitory Conditioning Axon Innervating Insect Muscles*

ABSTRACT The jumping muscle of locusts is supplied by three motor axons (Hoyle, 1955 a), one of which gives rise to large twitch contractions when stimulated with a single shock and is termed the fast (F) axon, whilst another gives rise to slow contractions only, when repetitively stimulated, and is termed the slow (S) axon (Hoyle, 1955 b). The third axon was found not to give a contraction when stimulated alone but it did cause a hyperpolarization in some of the muscle fibres Thus, by comparison with peripheral inhibitory action at crustacean neuromusclar junctions (Fatt & Katz, 1953), amphibian heart (Hutter & Trautwein, 1956) and cat spinal cord (Eccles, 1953), its function could be expected to be inhibitory; but Hoyle (1955b) found that stimulation of this axon at the same time as stimulation of either of the two excitatory ones did not lead to mechanical inhibition. Instead, the mechanical responses were enhanced, though the effect was slight so functional significance could not be ascribed to it with confidence. Since this phenomenon could not be fully understood it has remained in the background awaiting further investigation.

The anatomy and physiology of the morabine grasshoppers II. External anatomy and comparisons with Pyrgomorphidae, Acrididae, and Proscopiidae

Grasshoppers of the subfamily Morabinae (Orthoptera: Eumastacidae) bear a protuberance on one of the distal flagellar segments of the antennae, and some species groups also bear a peg-like structure on the fourth flagellar segment. Three ocelli are preserved despite the apterous condition of all morabine species. The eyes bear stripes indicative of the number of moults through which an individual has passed, females generally undergoing one more moult than the males of the same species. The head is prolonged into a marked fastigium which houses the long mandibular apodemes and associated tissues, the mandibles themselves being adapted to the food of the insects and falling into one of two broad categories. Glossae are well developed and subequal, the torma bears a mesa1 hook, and the hypopharynx lacks a marked keel. A basi-occipital slit and two dorsal plates in the cervical membrane are features of the cervical region. In the thorax the anterior pair of spiracles bears a smooth peritremal plate with a protuberance to hold off the pronotal shield; the caudal femora have the ventral margins prolonged into lobes which appear to be longer in some species groups than in others, irrespective of the size of the femur; on the forelegs and midlegs the trochanter articulates slightly with the femur, whereas in acridids it does not. The sheet-like levator muscles of the tibia require less sculptured femoral walls for their insertion than do the bundles of fibres in acridid jumping muscles, and the hind femora are softer and thinner than those of acridids, and are held at an angle to the body when the insect is at rest. Nevertheless, morabines jump well and have a fully developed Brunner's organ, which is nearer the proximal end of the femur than is usual in acridids. The basal external tubercle of the proximal tarsal segment is absent in some species. No auditory tympanum is present on the first abdominal segment, and the abdominal spiracles are carried in the tergosternal membrane. The abdominal sternites have winged processes which replace the apodemes of acridid sternites. The abdominal tergites of morabines, unlike those of acridids, also bear apodemes. In the male, the morabine subgenital plate is unusual in form, being produced into a cultriform section. The cerci are of many forms. The female has heavily sclerotized plates on the dorsal surface of the subgenital plate in several species groups, and these plates receive the cerci of the male. The pattern of the armature formed by the female subgenital plate, its sclerites, and the egg guide are of great diagnostic value in this subfamily. The subgenital plate has a posterior margin which is never ornamented with hairs or other processes as it usually is in acridids. The known resemblances between the Morabinae and the Proscopiidae are tabulated and compared with the condition in Pyrgomorphidae and Acrididae.