The structure and physiology of the tarso-pretarsal chordotonal organ in the larva of Anax imperator Leach (Anisoptera: Aeshnidae)

The tarso-pretarsal (TP) chordotonal organ is described. It is unusual in that only a small number (ca 8) of bipolar sensory cell bodies are present in the cellular strand. Each scolopidium encloses a single centriolar derivative. The cilium arises from a cross-striated ciliary root and has a 9 + 0 axoneme configuration running from base to apex of the centriolar derivative. A scolopale cell surrounds each cilium to form the intracellular scolopale, extracellular scolopale space and distal cap. No attachment cell is seen distally, instead each scolopidium appears in close association with the surrounding strand cells. Unidirectional sensory units were recorded from the TP organ. Most of these were sensitive to flexion. New names are proposed for some insect joint chordotonal organs to conform with crustacean terminology.