The labella are situated at the distal end of the prementum, which is the distal part of the labium, which forms the ventral wall of the proboscis. The labella are derived from a pair of united two-segmented labial palpi (ex McAlpine 1981)
The two-segmented condition of the labella is plesiomorphic for Diptera and is most evident in primitive Nematocera, e.g. Blephariceridae and Tanyderidae. The labella usually consist of two membranous, cushionlike lobes. In the Brachycera, and perhaps rarely in the Nematocera (Hennig 1973), the inner surfaces of these lobes are furnished with varying numbers of small, sclerotized, trachea-like grooves called pseudotracheae, which radiate from the terminus of the food canal. these microcanals serve to distribute saliva and to take up dissolved food. In association with the pseudotracheae, many muscoid flies have developed small prestomal teeth, which are used as rasping structures for rupturing surfaces and breaking up tood particles.
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