Hawk Moths hold their antennae under their wings at rest and deploy them forward when in flight. How do the moths know where their antennae are positioned?
The joints in insect cuticle frequently have arrays of sensory hairs known as hair plates, a type of proprioceptor (gives positional information). As the joint is bent, hairs in the array contact the surface of the neighboring segment. The contact mechanically stimulates a nerve within the hair and causes a nerve to respond. The patterns of nerve responses generated by the hairs that respond and the hairs that do not respond are processed by the nervous system and indicate where the appendage is located. The image shows a hair plate in the joint between the pedicel (left) and scape (right) of a hawk moth antenna. When the joint is straight, few hairs in the plate on the pedicel contact the scape. As the joint is bent, increasing numbers of hairs contact the scape.
Hair plates are commonly associated with many different types of joints. Hair plates on the antenna are called Böhmʼs bristles after L.K. Böhm who first described them* in 1911. Krishnan and colleagues** removed the Böhmʼs bristles from the antenna of a hawk moth and found that the moths could not properly position their antennae for flight. The antennae often collided with the beating wings. Clearly taking the hairs off makes the antenna position more than a hair off.
*Böhm L.K., 1911. Die antennale sinnesorgane der Lepidopteren. Arbeiten aus dem
Zoologischen Instituten der Universita ̈t Wien und der Zoologischen Station in Triest
**Anand Krishnan, Sunil Prabhakar, Subashini Sudarsan and Sanjay P. Sane. 2012. The neural mechanisms of antennal positioning in flying moths. The Journal of Experimental Biology 215, 3096-3105.