Labrum (arthropod mouthpart)

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

[3] Nevertheless, many workers continue to be highly skeptical about the appendiculate 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.

Most subsequent work in the first decade of the 21st century has tended to increase support for the impression that the labrum is indeed appendiculate in origin.

For example, there has been progress on lines that increasingly suggest similarities between the networks of regulatory genes in the embryogenesis of the labrum and of segmental appendages of the trunk.

[4] Another line of evidence arose from a study of the external morphology during the embryogenesis of members of the Mecoptera, an order regarded as resembling the common ancestors of the Endopterygota.