THE STRUCTURE, RESPONSE PROPERTIES AND DEVELOPMENT OF A HAIR PLATE ONTHE MESOTHORACIC LEG OF THE LOCUST

A hair plate is present on the proximal anterior face of the pro- and mesothoracic tibiae of the legs of the locust Schistocerca gregaria, b ut not on the metathoracic legs. The hair plate is in a depression of the cuticle and contains about 11 hairs, which are all polarised with their tips pointing towards the dorsal surface of the tibia. The hairs are all of the same trichoid sensilla type and vary in length from 90 to 140 mu m. Associated with the hair plate is a pronounced distal ex tension of the anterior femoral coverplate, the inner face of which is concave, that makes contact with the hairs during flexion and extensi on movements of the tibia. During postembryonic development, no tibial hair plate hairs are present in the first four larval stages. In fift h-instar larvae just three hairs are present, while the full complemen t is attained only after the final moult to adulthood. The distal exte nsion of the posterior coverplate is present through all instar stages , becoming more pronounced after each moult. Sensory neurones innervat ing the hairs of an adult may be divided into two classes on the basis of their responses, The first type responds phasically to imposed def lections and is velocity-sensitive. The second type responds phasotoni cally and is also sensitive to the velocity of the stimulus but has an additional tonic component sensitive to maintained angular deflection s. Both types of afferents are directionally sensitive and respond bes t to deflections against the natural bend of the hair, equivalent to e xtension movements of the tibia. The hairs are deflected by the coverp late only at femoro-tibial angles of less than 90 degrees. During exte nsion movements, they will be deflected in their most sensitive direct ions while during flexion movements they will be deflected in their le ast sensitive directions.