4.2.1. Thermoreception

Insects evidently detect variation in temperature, as seen by their behavior (section 4.2.2), yet the function and location of receptors is poorly known. Most studied insects have antennal sensing of temperature — those with amputated antennae respond differently from insects with intact antennae. Antennal temperature receptors are few in number (presumably ambient temperature is much the same at all points along the antenna), are exposed or concealed in pits, and are associated with humidity receptors in the same sensillum. In the cockroach Periplaneta americana, the arolium and pulvilli of the tarsi bear temperature receptors, and thermoreceptors have been found on the legs of certain other insects. Central temperature sensors must exist to detect internal temperature, but the only experimental evidence is from a large moth in which thoracic neural ganglia were found to have a role in instigating temperature-dependent flight muscle activity.

An extreme form of temperature detection is illustrated in jewel beetles (Buprestidae) belonging to the largely Holarctic genus Melanophila and also Merimna atrata (from Australia). These beetles can detect and orientate towards large-scale forest fires, where they oviposit in still-smoldering pine trunks. Adults of Melanophila eat insects killed by fire, and their larvae develop as pioneering colonists boring into fire-killed trees. Detection and orientation in Melanophila to distant fires is achieved by detection of infrared radiation (in the wavelength range 3.6–4.1 µm) by pit organs next to the coxal cavities of the mesothoracic legs that are exposed when the beetle is in flight. Within the pits some of the 50–100 small sensillae can respond with heat-induced nanometer-scale deformation, converted to mechanoreceptor signal. The receptor organs in Merimna lie on the posterolateral abdomen. These pit organ receptors allow a flying adult buprestid to locate the source of infrared perhaps as far distant as 12 km — a feat of some interest to the US military.

Chapter 4