Labellum (plural: labella) is the Latin diminutive of labium, meaning lip. These are anatomical terms used descriptively in biology, for example in entomology and botany.
The labellum is a modified petal and can be distinguished from the other petals and from the sepals by its large size and its often irregular shape. It is not unusual for the other two petals of an orchid flower to look like the sepals, so that the labellum stands out as distinct. 
In entomology the term labellum has been applied variously and in partly contradictory ways. One usage is in referring to a prolongation of the labrum that covers the base of the rostrum in certain Coleoptera and Hemiptera.
In contrast, the commonest current use of the term is in the anatomy of the mouthparts of Diptera, particularly those in which the labium forms the bulk of the proboscis, such as in the housefly family. Typically, the labium is expanded distally into a pair of fleshy labella. In the early twentieth century it was argued that the labella are the modified labial palps, and that point of view still is seen as having merit. In flies such as the mosquitoes, that have long antennae, the labella are two separate organs, attached to the proboscis only at their bases, but in the flies with short antennae, such as the house fly, they are more or less fused to form a single structure. Flies with fused labella have food channels in the surface of the labella. These are called pseudotracheae. They form the "spongy" part of a housefly's "tongue".
Labellum from Phalaenopsis sp
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