Crataerina pallida, a ‘Flat’ or ‘Louse’ Fly, is a highly modified blood-sucking ectoparasite on swifts (Apus apus).
As the common names suggest, the body is flat, allowing it to burrow to the base of the host’s feathers to reach the skin. The focus-stacked image showing the claws at the end of the legs, and the short hairs covering the body and legs, suggests how the fly is attached to the bird.
A blood meal is necessary for successful mating. Eggs are fertilised and larvae develop within the female’s body. The larvae emerge, pupate, and the new adults appear in the spring to coincide with the return of swifts from overwintering in Africa. Blood is taken from both nestlings and adult birds.
Being closely associated with their prey’s nest, the wings of C.pallida have atrophied and so this insect is not capable of sustained flight.
