Lock-in thermography as a tool in insect thermal biology: Insights from Hoplia argentea

Abstract

The elytron is a hard, intricately patterned exoskeleton of beetles with multiple functions: wing cover, protection from predators and the environment, as well as thermal insulation and heat dissipation. Owing to the complex shape and internal structure spanning millimeters, microns and nanoscales it is difficult to measure and analyze the thermal properties of elytra. Here, we describe a thermal imaging, phase-sensitive (lock-in) technique to assess minute temperature differences and their dependence on elytral anatomy. This is achieved by periodically heating the elytra with a laser while synchronously detecting the emitted thermal radiation with an infrared camera. In this way, the thermal noise was strongly suppressed, revealing small thermal variations along the elytra. Hoplia argentea (Poda, 1761) (Coleoptera: Scarabaeidae: Melolonthinae) was used as a case study, and we were able to relate temperature variation to underlying structures (e.g., wing scales or lacunae). The disclosed technique may be applied to study many thermal processes in insects and other species.