Homonomies within the ventral muscle system and the associated motoneurons in the locust, Schistocerca gregaria (Insecta, Caelifera)

A new kind of auxiliary heart in insects: functional morphology and neuronal control of the accessory pulsatile organs of the cricket ovipositor

Introduction

In insects, the pumping of the dorsal heart causes circulation of hemolymph throughout the central body cavity, but not within the interior of long body appendages. Hemolymph exchange in these dead-end structures is accomplished by special flow-guiding structures and/or autonomous pulsatile organs (“auxiliary hearts”). In this paper accessory pulsatile organs for an insect ovipositor are described for the first time. We studied these organs in females of the cricket Acheta domesticus by analyzing their functional morphology, neuroanatomy and physiological control.

Results

The lumen of the four long ovipositor valves is subdivided by longitudinal septa of connective tissue into efferent and afferent hemolymph sinuses which are confluent distally. The countercurrent flow in these sinuses is effected by pulsatile organs which are located at the bases of the ovipositor valves. Each of the four organs consists of a pumping chamber which is compressed by rhythmically contracting muscles. The morphology of the paired organs is laterally mirrored, and there are differences in some details between the dorsal and ventral organs. The compression of the pumping chambers of each valve pair occurs with a left-right alternating rhythm with a frequency of 0.2 to 0.5 Hz and is synchronized between the dorsal and ventral organs. The more anteriorly located genital chamber shows rhythmical lateral movements simultaneous to those of the ovipositor pulsatile organs and probably supports the hemolymph exchange in the abdominal apex region. The left-right alternating rhythm is produced by a central pattern generator located in the terminal ganglion. It requires no sensory feedback for its output since it persists in the completely isolated ganglion. Rhythm-modulating and rhythm-resetting interneurons are identified in the terminal ganglion.

Conclusion

The circulatory organs of the cricket ovipositor have a unique functional morphology. The pumping apparatus at the base of each ovipositor valve operates like a bellow. It forces hemolymph via sinuses delimited by thin septa of connective tissue in a countercurrent flow through the valve lumen. The pumping activity is based on neurogenic control by a central pattern generator in the terminal ganglion.

Common and specific inhibitory motor neurons innervate the intersegmental muscles in the locust thorax

The inhibitory innervation of the intersegmental (body wall) muscles between the first and the second thoracic segment of the migratory locust, Locusta migratoria, was investigated using neuroanatomical, immunocytochemical and electrophysiological techniques. Three neurons located in the prothoracic ganglion show GABA-like immunoreactivity and project into the intersegmental nerve. Two are common inhibitors. One of those innervates the oblique intersegmental muscle M59 and two dorsal longitudinal muscles (M81 and M82). The second common inhibitor also innervates M59 and the ventral longitudinal muscle M60. The third neuron innervates M60 exclusively and, for that reason, has to be regarded as the first specific inhibitor ever observed in insect neuromuscular assemblies. According to their innervation pattern, we term these neurons CI(59/60), CI(59/81/82), and SI(60). CI(59/81/82) and CI(59/60) appear to be segmentally homologous to CI(a) and CI(b) neurons, respectively, in the other body segments.

Inhibitory motor neurones supply body wall muscles in the locust abdomen

Inhibitory motor neurones in the abdominal ganglia of the locust Locusta migratoria were identified by combining extra- and intracellular electrophysiology, labelling of motor neurones by peripheral nerve backfills, and immunocytochemistry directed against the inhibitory transmitter gamma-aminobutyric acid. The fifth and sixth abdominal ganglia were studied in particular detail, although general findings were verified in all other abdominal segments. In each abdominal ganglion half, there are two inhibitory motor neurones, CIa and CIb, which supply dorsal (CIa) and ventral (CIb) longitudinal muscles. Their cell bodies are located in the next anterior ganglion to where the axons leave the ventral nerve cord via nerve 1. Both inhibitors have contralateral somata in the posterior ventral soma cortex, looping primary neurites and bilateral dorsal arborisations. There are homonomous (segmentally homologous) motor neurones in the fused abdominal neuromeres, the thoracic ganglia, and at least the third subesophageal neuromere. These body wall inhibitors are distinctly different from the limb muscle inhibitors, CI(1-3), described previously. This is signified, for example, by the fact that both types of inhibitory motor neurones coexist in the prothoracic segment and innervate leg and body wall muscles, respectively.