biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of cells that process hereditary ...

, a tagma (Greek: τάγμα, plural tagmata – τάγματα - body of soldiers; battalion) is a specialized grouping of multiple segments or metameres into a coherently functional morphological unit. Familiar examples are the head, the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the c ...

, and the

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

of insects. The segments within a tagma may be either fused (such as in the head of an insect) or so jointed as to be independently moveable (such as in the abdomen of most insects). Usually the term is taken to refer to tagmata in the morphology of members of the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...

Arthropod 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 chiti ...

a, but it applies equally validly in other phyla, such as the

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fi ...

. In a given taxon the names assigned to particular tagmata are in some sense informal and arbitrary; for example, not all the tagmata of species within a given subphylum of the Arthropoda are homologous to those of species in other subphyla; for one thing they do not all comprise corresponding somites, and for another, not all the tagmata have closely analogous functions or anatomy. In some cases this has led to earlier names for tagmata being more or less successfully superseded. For example, the one-time terms "

cephalothorax The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...

" and "

" of the Araneae, though not yet strictly regarded as invalid, are giving way to

prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...

and

opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to ...

. The latter two terms carry less of a suggestion of homology with the significantly different tagmata of insects.

Tagmosis

The development of distinct tagmata is believed to be a feature of the evolution of segmented animals, especially

arthropod 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 chiti ...

s. In the ancestral arthropod, the body was made up of repeated segments, each with similar internal organs and appendages. One evolutionary trend is the grouping together of some segments into larger units, the tagmata. The evolutionary process of grouping is called tagmosis (or tagmatization). The first and simplest stage was a division into two tagmata: an anterior "head" (cephalon) and a posterior "trunk". The head contained the brain and carried sensory and feeding appendages. The trunk bore the appendages responsible for locomotion and respiration (

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...

s in aquatic species). In almost all modern arthropods, the trunk is further divided into a "thorax" and an "abdomen", with the thorax bearing the main locomotory appendages. In some groups, such as arachnids, the cephalon (head) and thorax are hardly distinct externally and form a single tagma, the "cephalothorax" or "prosoma".

Mite Mites are small arachnids (eight-legged arthropods). Mites span two large orders of arachnids, the Acariformes and the Parasitiformes, which were historically grouped together in the subclass Acari, but genetic analysis does not show clear e ...

s appear to have a single tagma with no obvious external signs of either segments or separate tagmata. Tagmosis proceeded differently in different groups of arthropods, so that the tagmata are not derived from corresponding (homologous) segments, even though the same names may be used for the tagmata. Copepods (a kind of

crustacean Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapoda, decapods, ostracoda, seed shrimp, branchiopoda, branchiopods, argulidae, fish lice, krill, remipedes, isopoda, isopods, barnacles, copepods, ...

) provide an example. The basic copepod body consists of a head, a thorax with six segments, ancestrally each with a swimming leg, and an abdomen with five appendageless segments. Except in parasitic species, the body is divided functionally into two tagmata, that may be called a "prosome" and a "urosome", with an articulation between them allowing the body to flex. Different groups of copepods have the articulation at different places. In the

Calanoida Calanoida is an order of copepods, a group of arthropods commonly found as zooplankton. The order includes around 46 families with about 1800 species of both marine and freshwater copepods between them. Description Calanoids can be distinguis ...

, the articulation is between the thoracic and abdominal segments, so that the boundary between the prosome and urosome corresponds to the boundary between thoracic and abdominal segments. However, in the Harpacticoida, the articulation is between the fifth and sixth thoracic segments, so that the sixth thoracic segment is in the urosome (see the diagram). Tagmosis is an extreme form of heteronomy, mediated by ''Hox'' genes and the other developmental genes they influence.

Terminology

Trilobite sections-en.svg, The tagmata of a

trilobite Trilobites (; meaning "three lobes") are extinct marine arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the ...

: cephalon, thorax and pygidium File:Tagma.png, Tagmata of an insect: head (cephalon), thorax and abdomen Tagmata and major appendages of spider IMG 7639A.JPG, Tagmata and major appendages of a

spider Spiders (order Araneae) are air-breathing arthropods that have eight legs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude silk. They are the largest order of arachnids and rank seventh in total species ...

: cephalothorax or prosoma and abdomen or opisthosoma The number of tagma and their names vary among taxa. For example, the extinct

s had three tagmata: the

cephalon Cephalon, Inc. was an American biopharmaceutical company co-founded in 1987 by pharmacologist Frank Baldino, Jr., neuroscientist Michael Lewis, and organic chemist James C. Kauer—all three former scientists with the DuPont Company. Baldino s ...

(meaning head), the thorax (literally meaning chest, but in this application referring to the mid-portion of the body), and the pygidium (meaning rump). The

Hexapoda The subphylum Hexapoda (from Greek for 'six legs') comprises most species of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura (all of these were once considere ...

, including insects, also have three tagmata, usually termed the head, thorax, and abdomen. The bodies of many

arachnids Arachnida () is a class of joint-legged invertebrate animals (arthropods), in the subphylum Chelicerata. Arachnida includes, among others, spiders, scorpions, ticks, mites, pseudoscorpions, harvestmen, camel spiders, whip spiders and vinegar ...

, such as

s, have two tagmata, as do the bodies of some

s: in both groups the anterior tagma may be called the

(meaning head plus chest) or the

or prosome (meaning "fore-part of body"). The posterior tagma may be called the abdomen. In those arachnids that have two tagmata, the abdomen is also called the

. In crustaceans, the posterior tagma is also called the pleon or the urosome (meaning the tail part); alternatively, "pleon" may refer only to the abdominal segments incorporated into the posterior tagma, the thoracic segments in this tagma being called the "pereon".

References

Bibliography

* *{{Cite book , last1=Ruppert , first1=E.E. , last2=Fox , first2=R.S. , last3=Barnes , first3=R.D. , year=2004 , title=Invertebrate Zoology , edition=7th , publisher=Brooks/Cole , isbn=978-0-03-025982-1 , name-list-style=amp , url-access=registration , url=https://archive.org/details/isbn_9780030259821 Arthropod anatomy

Tagmosis

Terminology

See also

References

Bibliography