Motor supply of the dorsal longitudinal muscles II: Comparison of motoneurone sets in Tracheata

The set of motoneurones (MNs) supplying a specific ensemble of dorsal longitudinal muscles was studied in several segments of a primarily apterygote insect, Lepisma saccharina (Zygentoma), and a centipede, Lithobius forficatus (Chilopoda). In all preparations, a distribution of the MNs in two adjacent ganglia is observed. The cell number and their morphology in the thorax of L. saccharina are similar to the equivalent neural system of two Caelifera species, whereas in the L. saccharina abdomen there is some reduction in cell number. It appears that for both dicondylean insect groups homologous MNs exist. For L. forficatus, 12 MNs are present in the anterior ganglion and 4 in the posterior one. The morphology of these neurones is different compared to the insect MNs supplying an apparently equivalent set of muscles. Additionally, the neural supply of the intersegmental dorsoventral muscle was studied in Schistocerca gregaria, L. saccharina and L. forficatus. Both insect species show a pronounced similarity of the MN set. Again, the neural set in L. forficatus is different from that of the Dicondylia. Our results support the idea that the structure of MNs in the largest present taxon of Insecta, the Dicondylia, is conserved irrespective of crucial changes in the periphery (e.g. primarily apterygote vs oterygote). The muscles and their neurones are probably part of a basic neuromuscular ground pattern. The pronounced differences in a centipede are discussed in phylogenetic terms.Abbreviations DLM dorsal longitudinal muscle - DUM dorsal unpaired median - ISM intersegmental dorsoventral muscle - MN motoneurone - M muscle (number) - N nerve (number)