Opisthothelae is a taxon

Taxon

|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 within Order

Order (biology)

In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

 Araneae

Spider

Spiders are air-breathing arthropods that have eight legs, and chelicerae with fangs that inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all other groups of organisms...

, consisting of the Mygalomorphae

Mygalomorphae

The Mygalomorphae, , are an infraorder of spiders. The latter name comes from the orientation of the fangs which point straight down and do not cross each other .-Description:...

 and the Araneomorphae

Araneomorphae

The Araneomorphae are a suborder of spiders. They are distinguished by having fangs that oppose each other and cross in a pinching action, in contrast to the Mygalomorphae , which have fangs that are nearly parallel in alignment.- Distinguishing characteristics :Note the difference in the...

, but excluding the Mesothelae

Mesothelae

The Mesothelae are a suborder of spiders that includes the extinct families Arthrolycosidae and Arthromygalidae and the only extant family Liphistiidae....

. Opisthothelae is sometimes presented as an unranked clade

Clade

A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

 and sometimes as a suborder of the Araneae. In the latter case, Mygalomorphae and Araneomorphae are demoted to infraorders.

The fairly recent creation of this taxon has been justified by the requirement to distinguish these spiders from the Mesothelae

Mesothelae

The Mesothelae are a suborder of spiders that includes the extinct families Arthrolycosidae and Arthromygalidae and the only extant family Liphistiidae....

, which display many more primitive characteristics. Those that distinguish between Mesothelae and Opisthothelae are:

• the tergite
  Tergum
  A tergum is the dorsal portion of an arthropod segment other than the head. The anterior edge is called the base and posterior edge is called the apex or margin. A given tergum may be divided into hardened plates or sclerites commonly referred to as tergites...
  
   plates on the abdomen
  Opisthosoma
  The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma . It is a distinctive feature of the subphylum Chelicerata...
  
   of Mesothelae but absent in Opisthothelae;
• the almost total absence of ganglia
  Ganglion
  In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
  
   in the abdomen of Opisthothelae;
• the almost-median
  Median
  In probability theory and statistics, a median is described as the numerical value separating the higher half of a sample, a population, or a probability distribution, from the lower half. The median of a finite list of numbers can be found by arranging all the observations from lowest value to...
  
   position of the spinnerets in the Mesothelae compared to the hindmost position of those of the Opistothelae;
• the absence of venom glands in most Mesothelae species.

Among the Opisthothelae, the fangs of the Mygalomorphae point straight down in front of the mouth aperture and only allow the spider to grasp its prey from above and below, whereas in the Araneomorphae they face one another like pincers, allowing a firmer grip. It is difficult to distinguish araneomorphs and mygalomorphs on first inspection unless the specimens are large enough to permit immediate examination of the fangs.