Superextension and supercontraction in locust ovipositor muscles

Abstract

A ten times elongation of certain abdominal intersegmental muscles occurs in female locusts during digging prior to oviposition. During and after oviposition the muscles contract, shortening by up to 90% or more, restoring the resting positions of the abdominal segments. Discontinuous Z-discs permit supercontraction at the resting length and then fragment into Z-bodies when the muscle is stretched, so enabling it to superextend without loss of the contractile property. In this superextended state the fibres resemble smooth muscles. After oviposition, the muscle fibres contract but the sarcomeres are not restored completely, some of the Z-bodies being unevenly distributed in the recontracted fibres. Locust ovipositor muscle has the most extreme example of Z-disc disagregation known from the insects and is the insect muscle which approaches most closely the smooth muscle condition. Two types of motor nerve innervate this muscle, one is ordinary and the other, containing granules, resembles an octopaminergic fibre possibly involved in regulating a catch mechanism in the muscle. The physiological requirements for egg-laying with an extensible ovipositor, which is also part of the normally functioning abdomen, are well met by the ultrastructural specializations of locust ovipositor muscles.