part of develops from labial disc adult labial segment labellum labial palpus FBbt:00004531 There is significant sexual dimorphism on the size of the labellum, with the male labellum being bigger than the female, in the anterior-posterior axis (Shanbag et al., 2001). labium fly_anatomy.ontology labella The broadened, cushion-like, most distal part of the proboscis. It consists of two regions; its lateral surface has a hemispherical, on which there are approximately 33-34 gustatory (chemosensory) and two mechanosensory bristles. The gustatory bristles are arranged into three ill-defined rows of 10-11 bristles: two medial and one located peripherally. The mechanosensory bristles are located in the most dorsolateral area of the labella surface and are shorter than the gustatory sensilla. The inner (medial) surface bears a series of open collecting channels, the pseudotrachea, that converge medially and conduct food to the tip of the labrum. Taste pegs are present in between the pseudotrachea, with 30-40 on each side. distiproboscis integumentary specialization ventral ectoderm derivative