Males and females of most "Troides" birdwings are similar and have jet black to brown dorsal forewings, often with the veins bordered in grey to creamy white. At least one of these darkly-coloured species ("T. rhadamantus") possesses thermoreceptors on the anal veins (A2 and A3) of the wings and on the antennal clubs. The antennal receptors of the clubs—which also possess hygroreceptors that measure atmospheric humidity—are known as "sensilla basiconica". The thermoreceptors are sensitive to sudden increases in temperature; they are thought to help the butterfly thermoregulate and avoid overheating while basking.