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

Euarthropoda Arthropods ( ) are invertebrates in the phylum Arthropoda. They possess an exoskeleton with a cuticle made of chitin, often mineralised with calcium carbonate, a body with differentiated ( metameric) segments, and paired jointed appendages. ...

. The most conspicuous exceptions are the

Pycnogonida Sea spiders are marine arthropods of the class Pycnogonida, hence they are also called pycnogonids (; named after ''Pycnogonum'', the type genus; with the suffix '). The class includes the only now-living order Pantopoda ( ‘all feet’), alongs ...

, which are probably chelicerates. In entomology, the labrum amounts to the "upper lip" of an insect mouth, the corresponding "lower lip" being the labium. The

evolution Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

ary origin,

embryogenesis An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male ...

, 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.

Embryonic nature and origin of the labrum

The labrum is innervated in crustaceans and insects from the tritocerebrum (the back of the brain). However, in development, its embryonic

primordium A primordium (; : primordia; synonym: anlage), in embryology, is an Organ (anatomy), organ or tissue in its earliest recognizable stage of development. Cell (biology), Cells of the primordium are called primordial cells. A primordium is the simp ...

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 such as an arm or a leg. Protrusions from single-celled bacteria and archaea are known as cell-surface appendages or surface app ...

. 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). Nerves have historically been considered the basic units of the peripheral nervous system. A nerve provides a common pathway for the electrochemical nerve impulses called ...

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 (from Latin meaning: "those with shells" or "crusted ones") are invertebrate animals that constitute one group of arthropods that are traditionally a part of the subphylum Crustacea (), a large, diverse group of mainly aquatic arthrop ...

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 follo ...

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 supraoesophageal ganglion, panarthropod brain. Recent studies suggest that it comprises two regions. Region associated with the expression of ''six3'' ''six3'' is a transcription factor that marks ...

, 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 Megacheira ("great hands", also historically great appendage arthropods) is an extinct class of predatory arthropods defined by their possession of spined "great appendages". Their taxonomic position is controversial, with studies either consider ...

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

onychophora Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, ''Peripatus''), is a phylum of el ...

ns which seem to be innervated from a very anterior part of the brain, in front of the eyes. 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 ...

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 ( ) are invertebrates in the phylum Arthropoda. They possess an arthropod exoskeleton, exoskeleton with a cuticle made of chitin, often Mineralization (biology), mineralised with calcium carbonate, a body with differentiated (Metam ...

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

of the labrum and of segmental appendages of the trunk. There do remain difficulties however; the labrum develops in

median The median of a set of numbers is the value separating the higher half from the lower half of a Sample (statistics), data sample, a statistical population, population, or a probability distribution. For a data set, it may be thought of as the “ ...

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 language, Greek: ''mecos'' = "long", ''ptera'' = "wings") is an Order (biology), order of insects in the superorder Holometabola with about six hundred species in nine Family (biology), families worldwide. Mecopterans a ...

, an order regarded as resembling the common ancestors of the

Endopterygota Holometabola (from Ancient Greek "complete" + "change"), also known as Endopterygota (from "inner" + "wing" + Neo-Latin "-having"), is a supra- ordinal clade of insects within the infraclass Neoptera that go through distinctive larval, pupal ...

. 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