The orthopteran descending movement detector (DMD) neurones: a characterisation and review

Summary1.The thoracic connectives of the Acridoidea, Grylloidea and Tettigonioidea contain 2 units which appear functionally identical except that they are activated by ipsilateral and contralateral eyes respectively. They are purely visual, responding to dimming of small areas of the retina or to moving objects subtending small angles, but not to whole field stimulation (Fig. 8). The axons run from the optic lobe to the metathoracic ganglion; the contralateral unit crosses the brain in the tritocerebrum. Their relative sizes differ in the different taxa, but they appear functionally homologous.2.The common identity of the units previously described by Palka and by Rowell and Horn is established, and areas of conflict are reexamined. The units do not respond to whole field stimulation caused by movement of the animal in a structured visual field. Palka found that the response to small area stimulation was totally inhibited during such movement, whereas Rowell and Horn did not. It is shown that the inhibition is graded, and is complete only at high angular velocities; also that it wanes if movement is sustained. Both descriptions of the units are correct, but were made at different angular velocities. At medium angular velocities of the whole field the unit responds adequately to normal stimuli, but at a somewhat diminished level (Fig. 3–5).3.The units play no part in the optokinetic memory previously described in the locust (Fig. 6).4.Activity of the visual units has no direct effect on any of the major thoracic motor output systems, nor upon the behaviour of chronically implanted freely moving animals (Fig. 7).5.The occurrence, anatomy, response characteristics, afferent and efferent connections and function of the DMD units is reviewed. Their apparent specialisation for a “warning” function, plus the absence of direct motor output, leads to the speculation that they may switch the thoracic neuropile from thoracic to cerebral inputs.