Behind each middle leg sits a pit organ containing dozens of small spheres, each a cuticular dome enclosing fluid and connected to a mechanoreceptor. Infrared radiation from a fire warms the fluid, it expands, and the resulting pressure deforms the receptor. The beetle is therefore not sensing heat chemically but converting radiation into a mechanical signal, an arrangement not known in other insects and different from the pit organs of snakes.
The payoff is a monopoly on a resource. A conifer killed by fire has lost the resin defences that would otherwise drown boring larvae, and for a short window it is nearly undefended. Beetles arriving first get the wood.
Estimates of detection range have varied widely, from a few kilometres to figures of 50 kilometres or more that circulate in popular accounts. Laboratory sensitivity measurements support detection at substantial distances, but the largest field figures are inferences from beetle arrivals rather than direct measurement, and should be treated cautiously.