Evolution of the arthropod neuromuscular system. 1. Arrangement of muscles and innervation in the walking legs of a scorpion: Vaejovis spinigerus (Wood, 1863) Vaejovidae, Scorpiones, Arachnida.

(1) The musculature of the walking legs is analysed with regard to both morphology and function in the scorpion, Vaejovis spinigerus (Wood, 1863) (Vaejovidae, Scorpiones, Arachnida), and selected other species. Conspicuous features are multipartite muscles, muscles spanning two joints, and partial lack of antagonistic muscles. The muscle arrangement is compared to that in the walking limbs of other Arthropoda and possible phylogenetic implications are discussed. (2). Histochemical characterisation of selected leg muscles indicates that these are composed of layers of slow, intermediate and fast muscle fibres. Anti-GABA immunohistochemistry shows that mainly the intermediate fibres receive innervation from putative inhibitory motoneurons. (3). Intracellular recording from muscle fibres reveals both excitatory and inhibitory muscle innervation. Individual muscle fibres may receive input from more than one inhibitory motoneuron, as indicated by different IPSP amplitudes. (4). The motoneuron supply of the leg muscles is analysed by retrograde fills of motor nerves. The general arrangement of leg motoneurons in the central nervous system and motoneuron anatomy conforms to the situation in pterygote insects and decapod crustaceans. For example, there are an anterior and a posterior group of leg motoneurons in each hemineuromere, and two contralateral somata near the ganglion midline. Between 12 and 20 motoneurons are found to supply each muscle. Most motoneuron cell bodies supplying a given muscle are arranged in a single cluster with a specific location.