Anatomy of Motoneurones Innervating Mesothoracic Indirect Flight Muscles in the Silkmoth, Bombyx Mori

Abstract

ABSTRACT The anatomy and innervation of mesothoracic indirect flight muscles, dorsal longitudinal (DLM) and dorsoventral muscles (DVM) are described. The major DLMs depress the forewing, the major DYMs elevate it. The morphology of the motoneurones and unpaired median neurones (UM-neurone) innervating these muscles was investigated by back-filling with cobalt. The DLMs are innervated by eight paired motoneurones and one UM-neurone: one of these motoneurones has its cell body in the contralateral side of the mesothoracic ganglion, and the others have theirs in the ipsilateral side of the prothoracic ganglion. The DVMs are innervated by nine paired motoneurones and one UM-neurone, with their cell bodies in the ipsilateral and the median portion of the mesothoracic ganglion, respectively. All of the paired DLM and DVM motoneurones have many dendritic processes, mainly in the dorso-ipsilateral neuropile of the ganglia, but the DLM motoneurones also have processes in the contralateral neuropile. Each UM-neurone has its cell body in the postero-medial cortex of the mesothoracic ganglion, extending a single median neurite anteriorly to the dorsal neuropile where it bifurcates to form a right and left axon. The arrangement of the mesothoracic indirect flight motoneurones in different insect orders is discussed in terms of the neural organization of the flight system.