Octopamine and DUM neurons orchestrate the larval firefly aposematic defence

Abstract

Larval fireflies have an aposematic defence. We examined whether the dorsal, unpaired, median (DUM) neurons of the larval CNS, which secrete octopamine (OA), play a key role in coordinating enemy detection, the glow warning signal and the defensive glandular secretion. We experimentally determined whether 1) lateral abdominal setae signal attack, 2) defensive compounds are stored apart from the haemolymph, and 3) DUM neurons affect glandular secretion. Disturbed larval fireflies injected with methylene blue emitted a clear, bitter substance that was untainted by stain, indicating the presence of reservoirs separate from the haemolymph. Setal manipulation always elicited glows. Glow intensity was dependent on intensity of the stimulus. The latency of response to anterior deflection of the setae was significantly shorter than for posterior deflection. DUM neurons released OA in the vicinity of gland dilator muscles. When perfused in saline containing OA, electrical stimuli elicited dilator tetanus, which was not observed with saline alone. Posterio-lateral setae appear to be phasic, polarised mechanoreceptors signalling threats and inducing both glandular secretion and lantern glow. We hypothesise that when an attack escalates, strong irritation of the lateral abdominal setae induces DUM neurons to flood the haemolymph with OA, which serves to orchestrate the aposematic larval defence response. Octopamine plays an analogous role to noradrenalin in the larval firefly. The similarities point to a very early and perhaps homologous origin of adrenergic molecules in defence. However, support is lacking for the hypothesis that the larval firefly aposematic defence is a ‘key innovation.’