Labrum (arthropod mouthpart)

The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant Euarthropoda. The most conspicuous exceptions are the Pycnogonida, which probably are chelicerate-relatives. In entomology, the labrum amounts to the "upper lip" of an insect mouth, the corresponding "lower lip" being the labium.

The evolutionary origin, embryogenesis and morphological development of the labrum have proved to be by far the most controversial and challenging topic in the study of arthropod head structures.

The labrum is innervated in crustaceans and insects from the , i.e. the back of the brain. However, in development its embryonic primordium often appears at the anterior of the head, and migrates backwards towards its adult position. Furthermore, it often appears as a bilobed structure, with a set of muscles, nerves and gene expression in many ways similar to that of an appendage.

This evidence has been used to suggest that the labrum is in fact a highly reduced appendage. Its tritocerebral from the rear of the brain has suggested to some workers that, if an appendage, it is the appendage of the segment; a point disputed by others who argue that the presence of a well-developed appendage in at least crustaceans in this segment (i.e., the second antenna, corresponding to the intercalary segment of insects) rules this out.

If the labrum is an appendage then it seems possible that its origin is indicated by its developmentally anterior position, i.e., that it is the appendage of a segment anterior to the first antenna. The most obvious choice for this is the segment whose ganglion is the , which in extant Euarthropoda bears no appendage (apart from the eyes). If the labrum really is an anterior appendage that has migrated to the posterior, then it may be homologous to the "antennae" of onychophorans which, as discussed below, seem to be innervated from a very anterior part of the brain, i.e. in front of the eyes. It has even been suggested (e.g., by Roonwal) that the labrum belongs to an even more obscure segment that lies in front of the ocular one. Nevertheless, many workers continue to be highly skeptical about the nature of the labrum, preferring to see it as it appears, i.e. as an outgrowth of the body wall just in front of the mouth.

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