Built to break: the antenna of a primitive insect, Petrobius brevistylis (Archaeognatha)

Jumping bristletails (Archaeognatha) have flagellate antennae with frequent breakpoints, structures destined to break when undue mechanical stress is applied. In natural populations of Petrobius brevistylis at least 67% of animals had broken antennae. In twenty-four trials, all antennae broke at a breakpoint under the animal's weight. Breakpoints consist of two cylinders of laminated cuticle, one stuck inside the other. For half the length of the breakpoint the cylinders are separated by a non-laminated layer of cuticle; thus a tripartite cuticle is formed. During a breakage, the two cylinders slide apart along that non-laminated middle layer that seems to act like a lubricant. While the cylinders pull apart, a thin cuticle unfolds that closes off the new tip of the antenna formed by the outer breakpoint cylinder. Later on, the cylinder is not replaced by a new terminal antennomere. Instead, the antennomere directly proximal to a breakpoint, i.e. the one that after the break forms the last antennomere, already possess the same complement of sensilla as the original terminal antennomere. This includes sensilla basiconica Type A and B found exclusively on these antennomeres and the original terminal antennomere. Breakpoints are common to Archaeognatha and Thysanura, insects that moult throughout their lifetime.