The jump of the sea-shore bristletails, Petrobius brevistylis and P. maritimus, has been investigated. A brief study of jumping distance and orientation showed that jumping behaviour was partly random, particularly with regard to the direction of a jump. A high speed cine camera was used to analyse both forward and backward jumps. The jumping action consists of a tail beat which reflects the strong depression of the posterior abdomen. This swings up the anterior part of the body, including the centre of gravity. The model suggested to explain this action proposes two movements spreading from either end of the bristletail: 1. A straightening of the abdomen starting posteriorly, 2. An increasing ventral curvature of the thorax starting anteriorly. Movement 1 will account for the raising of the centre of gravity, and the rapidity of movement 2 will determine the final direction of the jump.

The muscular basis of these movements has been discussed using the data of Manton (1972), Barlet (1967) and Bitsch (1973). A catapult system was proposed which depended on the dynamic balance between two abdominal muscular systems. It was suggested that the dorsal longitudinal muscles were antagonised by the deep oblique muscles. A short study of the energetics of jumping was made in order to estimate the times needed for sufficient energy to be stored for particular jumps. These times were then compared with known times spent by Petrobius in the cocked (energy storing) pre-jump position.