Labrum (arthropod mouthpart)

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Modifications of the labrum (in red) in assorted insects. (A) grasshopper, (B) honey bee, (C) butterfly (D) mosquito. Evolution insect mouthparts coloured.png
Modifications of the labrum (in red) in assorted insects. (A) grasshopper, (B) honey bee, (C) butterfly (D) mosquito.
Bembix rostrata female using its labrum in sucking the blood out of a fly. Wasp August 2007-23.jpg
Bembix rostrata female using its labrum in sucking the blood out of a fly.

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.

Sea spider Marine arthropod of class Pycnogonida

Sea spiders, also called Pantopoda or pycnogonids, are marine arthropods of class Pycnogonida. They are cosmopolitan, found in oceans around the world. There are over 1300 known species, with a leg span ranging from 1 mm (0.04 in) to over 70 cm (2.3 ft). Most are toward the smaller end of this range in relatively shallow depths; however, they can grow to be quite large in Antarctic and deep waters.


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.

Evolution Change in the heritable characteristics of biological populations over successive generations

Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes that are passed on from parent to offspring during reproduction. Different characteristics tend to exist within any given population as a result of mutation, genetic recombination and other sources of genetic variation. Evolution occurs when evolutionary processes such as natural selection and genetic drift act on this variation, resulting in certain characteristics becoming more common or rare within a population. It is this process of evolution that has given rise to biodiversity at every level of biological organisation, including the levels of species, individual organisms and molecules.

Morphology (biology) In biology, the form and structure of organisms

Morphology is a branch of biology dealing with the study of the form and structure of organisms and their specific structural features.

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 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. [1] This evidence has been used to suggest that the labrum is in fact a highly reduced appendage. Its tritocerebral innervation 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 crustaceans in this segment (the second antenna, corresponding to the intercalary segment of insects) rules this out.

Primordium organ in the earliest recognizable stage of embryonic development

A primordium 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 triggering growth of the would-be organ and the initial foundation from which an organ is able to grow. In flowering plants, a floral primordium gives rise to a flower.

Appendage external body part, or natural prolongation, that protrudes from an organisms body (in vertebrate biology, an example would be a vertebrates limbs); any of the homologous body parts that may extend from a body segment

In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body. An appendage is any of the homologous body parts that may extend from a body segment. These include antennae, mouthparts, gills, walking legs (pereiopods), swimming legs (pleopods), sexual organs (gonopods), and parts of the tail (uropods). Typically, each body segment carries one pair of appendages.

Crustacean subphylum of arthropods

Crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill, woodlice, and barnacles. The crustacean group is usually treated as a subphylum, and because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, and comprises all animals in the Pancrustacea clade other than hexapods. Some crustaceans are more closely related to insects and other hexapods than they are to certain other crustaceans.

The most obvious choice for this is the segment whose ganglion is the protocerebrum, 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 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. [2] 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. Evidence of varied origin and nature, though not qualitatively uniform in all Arthropoda, 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. [3]

Arthropod phylum of animals

An arthropod is an invertebrate animal having an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids, myriapods, and crustaceans. The term Arthropoda as originally proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin, often mineralised with calcium carbonate. The arthropod body plan consists of segments, each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton. Some species have wings.

Anatomical terms of location Standard terms for unambiguous description of relative placement of body parts

Standard anatomical terms of location deal unambiguously with the anatomy of animals, including humans.

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. It too suggests that the labrum derives from paired appendages. [4]

Mecoptera order of insects

Mecoptera are an order of insects in the superorder Endopterygota with about six hundred species in nine families worldwide. Mecopterans are sometimes called scorpionflies after their largest family, Panorpidae, in which the males have enlarged genitals that look similar to the stingers of scorpions, and long beaklike rostra. The Bittacidae, or hangingflies, are another prominent family and are known for their elaborate mating rituals, in which females choose mates based on the quality of gift prey offered to them by the males. A smaller group is the snow scorpionflies, family Boreidae, adults of which are sometimes seen walking on snowfields. In contrast, the majority of species in the order inhabit moist environments in tropical locations.

Endopterygota clade of insects

Endopterygota, also known as Holometabola, is a superorder of insects within the infraclass Neoptera that go through distinctive larval, pupal, and adult stages. They undergo a radical metamorphosis, with the larval and adult stages differing considerably in their structure and behaviour. This is called holometabolism, or complete metamorphism.

See also

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Arthropod mouthparts

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Arthropod head problem Uncertainty regarding the evolutionary realtionship of the segmental composition of the head in various arthropod groups

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Labrum Latin, defined as "having the edge"

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Rhombic lip

The rhombic lip is a posterior section of the developing metencephalon which can be recognized transiently within the vertebrate embryo. It extends posteriorly from the roof of the fourth ventricle to dorsal neuroepithelial cells. The rhombic lip can be divided into eight structural units based on rhombomeres 1-8 (r1-r8), which can be recognized at early stages of hindbrain development. Producing granule cells and five brainstem nuclei, the rhombic lip plays an important role in developing a complex cerebellar neural system.

The evolution of nervous systems dates back to the first development of nervous systems in animals. Neurons developed as specialized electrical signaling cells in multicellular animals, adapting the mechanism of action potentials present in motile single-celled and colonial eukaryotes. Simple nerve nets seen in animals like Cnidaria (jellyfish) evolved first, consisted of polymodal neurons which serve a dual purpose in motor and sensory functions. Cnidarians can be compared to Ctenophores, which although are both jellyfish, have very different nervous systems. Unlike Cnidarians, Ctenophores have neurons that use electrochemical signaling. This was perplexing because the phylum Ctenophora was considered to be more ancient than that of Porifera (sponges), which have no nervous system at all. This led to the rise of two theories which described how the early nervous system came about. One theory stated that the nervous system came about in an ancestor basal to all of these phylum, however was lost in Porifera. The other theory states that the nervous system arose independently twice, one basal to Cnidarians and one basal to Ctenophores. Bilateral animals – ventral nerve cords in invertebrates and dorsal nerve cords supported by a notochord in chordates-- evolved with a central nervous system that was around a central region, a process known as cephalization.

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Hexapoda subphylum of arthropods

The subphylum Hexapoda constitutes the largest number of species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura. The Collembola are very abundant in terrestrial environments. Hexapods are named for their most distinctive feature: a consolidated thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.

The protocerebrum is the first segment of the panarthropod brain.


  1. Haas, M. S.; Brown, S. J.; Beeman, R. W. (2001). "Pondering the Procephalon: the Segmental Origin of the Labrum". Development, Genes and Evolution. 211 (2): 89–95. doi:10.1007/s004270000129.
  2. Roonwal [ citation needed ]
  3. Posnien, Nico; Bashasab, Fakrudin; Bucher, Gregor (2009). "The insect upper lip (labrum) is a nonsegmental appendage-like structure". Evolution & Development. 11 (5): 480–488. doi:10.1111/j.1525-142X.2009.00356.x. PMID   19754705.
  4. Du, Xiaoliang; Yue, Chao; Hua, Baozhen (2009). "Embryonic development of the scorpionfly Panorpa emarginata Cheng with special reference to external morphology (Mecoptera: Panorpidae)". Journal of Morphology. 270 (8): 984–995. doi:10.1002/jmor.10736. PMID   19274641.