The labrum is a flap-like structure that lies immediately in front of the mouth in almost all extant

Euarthropoda Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, oft ...

. The most conspicuous exceptions are the

Pycnogonida Sea spiders are marine arthropods of the order Pantopoda ( ‘all feet’), belonging to the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). They are cosmopolitan, foun ...

, 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

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

ary 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.

Embryonic nature and origin of the labrum

The labrum is innervated in crustaceans and insects from the

tritocerebrum The supraesophageal ganglion (also "supraoesophageal ganglion", "arthropod brain" or "microbrain") is the first part of the arthropod, especially insect, central nervous system. It receives and processes information from the first, second, and th ...

(the back of the brain). However, in development, its embryonic

primordium A primordium (; plural: primordia; synonym: anlage) in embryology, is an organ or tissue in its earliest recognizable stage of development. Cells of the primordium are called primordial cells. A primordium is the simplest set of cells capable of ...

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 An appendage (or outgrowth) is an external body part, or natural prolongation, that protrudes from an organism's body. In arthropods, an appendage refers to any of the homologous body parts that may extend from a body segment, including anten ...

. This evidence has been used to suggest that the labrum is in fact a highly reduced

. Its tritocerebral

innervation A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system. A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the e ...

from the rear of the brain has suggested to some workers that, if an appendage, it is the appendage of the segment anterior to the first antenna, but this is disputed by others who argue that the presence of a well-developed appendage in at least

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean group ...

s in this segment (the second antenna, corresponding to the

intercalary Intercalation may refer to: *Intercalation (chemistry), insertion of a molecule (or ion) into layered solids such as graphite * Intercalation (timekeeping), insertion of a leap day, week or month into some calendar years to make the calendar foll ...

segment of insects) rules this out. The most obvious choice for this is the segment whose ganglion is the

protocerebrum The protocerebrum is the first segment of the panarthropod brain. Recent studies suggest that it comprises two regions. Region associated with the expression of ''six3'' ''six3'' is a transcription factor that marks the anteriormost part of ...

, which in extant

bears no appendage (apart from the eyes). If the labrum really is a fused pair of anterior appendages known as the

great appendage Great appendages are claw-like appendages which attach to the heads of the "great appendage arthropods", a name usually refers to Megacheira, a class of extinct arthropod characterized by a pair of "short-great appendages" bearing in front of the ...

s that has migrated to the posterior, then it may be homologous to the antennae of

onychophora Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

ns which seem to be innervated from a very anterior part of the brain, in front of the eyes.A Reduced Labrum in a Cambrian Great-Appendage Euarthropod
/ref> It has even been suggested that the labrum belongs to an even more obscure segment that lies in front of the ocular one.Roonwal Nevertheless, many workers continue to be highly skeptical about the

appendiculate This glossary of botanical terms is a list of definitions of terms and concepts relevant to botany and plants in general. Terms of plant morphology are included here as well as at the more specific Glossary of plant morphology and Glossary o ...

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. Evidence of varied origin and nature, though not qualitatively uniform in all

Arthropod Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and Arth ...

a, shows a trend in favour of such interpretations. 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. There do remain difficulties however; the labrum develops in median tissues rather than the lateral sources expected of paired appendages and is part of an anterior nonsegmental tissue. It accordingly has been suggested that the now common genetic network evolved in either the labrum structure or in the trunk appendages and in either case, subsequently was redeployed elsewhere to form the other structure. Another line of evidence arose from a study of the external morphology during the embryogenesis of members of the

Mecoptera Mecoptera (from the Greek: ''mecos'' = "long", ''ptera'' = "wings") is an order of insects in the superorder Endopterygota with about six hundred species in nine families worldwide. Mecopterans are sometimes called scorpionflies after their lar ...

, an order regarded as resembling the common ancestors of the Endopterygota. It too suggests that the labrum derives from paired appendages.

References

{{reflist Mouth Arthropod anatomy Insect anatomy

Embryonic nature and origin of the labrum

See also

References