The Corethrellidae (“frog-biting-midges”) is a small group among the lower Diptera. The family contains approximately 110 species. Their predaceous larvae feed mainly on other insect larvae. They capture prey with their antennae; a mechanism which also can be found in the closely related Chaoboridae. This unusual feature and the lack of morphological data of internal structures induced us to investigate the cephalic structures of the larvae of Corethrella appendiculata (Grabham 1906). The closely adjacent antennae are one segmented and insert on the anterior margin in a horizontal position in specific pockets of the head capsule. A strongly developed antennal muscle causes the abduction of the antennae leading to catch prey. In larvae of Chaoboridae the antennae are also located on the anterior margin of the head; but in a vertical position. In contrast to the Corethrellidae the abduction is caused indirectly by increasing the hemolymph-pressure. The adduction is caused by the antennal muscle. Because of this unique mechanism a sistergroup relationship of Corethrellidae and Chaoboridae is confirmed.