Faecal firing in a skipper caterpillar is pressure-driven

Many leaf-rolling caterpillars have a rigid anal comb attached to the lower surface of the anal plate (or shield) situated above the anus. This comb is widely assumed to be a lever used to 'flick' away frass pellets. An alternative mechanism to explain pellet discharge is proposed on the basis of observations on the caterpillar of the skipper Calpodes ethlius. The model proposes that the underside of the anal plate serves as a blood-pressure-driven surface for the ejection of faecal pellets. Rather than acting as a lever, the anal comb serves as a latch to prevent the premature distortion of the lower wall of the anal plate until the anal haemocoel compartment is fully pressurized. The anal comb is swung into position during pellet extrusion by retractor muscles attached at its base and held in place by a catch formed by a blood-swollen torus of everted rectal wall. As the caterpillar raises the blood pressure in its anal compartment by contracting its anal prolegs, the comb eventually slips over the toral catch. This causes the underside of the anal plate to move rapidly backwards as the blood pressure is released, projecting the pellet resting against it through the air. Simulation suggests that a local blood pressure of at least 10 kPa (75 mmHg) would be required to accelerate the lower surface of the anal plate outwards at a rate fast enough to discharge a 10 mg pellet at an observed mean velocity of 1.3 m s-1.