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The Mygalomorphae, or mygalomorphs, are an
infraorder Order ( la, ordo) is one of the eight major hierarchical taxonomic ranks in Linnaean taxonomy. It is classified between family and class. In biological classification, the order is a taxonomic rank used in the classification of organisms and ...
of
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 div ...
s, and comprise one of three major groups of living spiders with over 3000 species, found on all continents except Antarctica. Many members are known as
trapdoor spider Trapdoor spider is a common name that is used to refer to various spiders from several different groups that create burrows with a silk-hinged trapdoor to help them ambush prey. Several families within the infraorder Mygalomorphae contain trapdoo ...
s due to them forming trapdoors over their burrows. Other prominent groups include Australian funnel web spiders and
tarantula Tarantulas comprise a group of large and often hairy spiders of the family Theraphosidae. , 1,040 species have been identified, with 156 genera. The term "tarantula" is usually used to describe members of the family Theraphosidae, although m ...
s, with the latter accounting for around one third of all mygalomorphs.


Description

This group of spiders comprises mostly heavy-bodied, stout-legged spiders including
tarantula Tarantulas comprise a group of large and often hairy spiders of the family Theraphosidae. , 1,040 species have been identified, with 156 genera. The term "tarantula" is usually used to describe members of the family Theraphosidae, although m ...
s, Australian funnel-web spiders,
mouse spider ''Missulena'' is a genus of mygalomorph spiders in the family Actinopodidae. It was first described by Charles Walckenaer in 1805, and is a senior synonym of ''Eriodon''. ''M. tussulena'' is found in Chile, but the rest are indigenous to Aus ...
s, and various families of spiders commonly called
trapdoor spider Trapdoor spider is a common name that is used to refer to various spiders from several different groups that create burrows with a silk-hinged trapdoor to help them ambush prey. Several families within the infraorder Mygalomorphae contain trapdoo ...
s. Like the " primitive" suborder of spiders
Mesothelae The Mesothelae are a suborder of spiders (order Araneae) that includes a single extant family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and to retain ancestral ...
, they have two pairs of book lungs, and downward-pointing
chelicerae The chelicerae () are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as " jaws", chelicerae may be shaped as either articulated fangs, or similarl ...
. Because of this, the two groups were once believed to be closely related. Later it was realized that the common ancestors of all spiders had these features (a state known as
symplesiomorphy In phylogenetics, a plesiomorphy ("near form") and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades. Plesiomorphy, symplesiomorphy, apomorphy, ...
). Following the branching into the suborders of Mesothelae and
Opisthothelae The Opisthothelae are spiders within the order Araneae, consisting of the Mygalomorphae and the Araneomorphae, but excluding the Mesothelae. The Opisthothelae are sometimes presented as an unranked clade and sometimes as a suborder of the Arane ...
, the mygalomorphs retained them, while their fellow Opisthothelae members, the
araneomorph The Araneomorphae (also called the Labidognatha) are an infraorder of spiders. They are distinguishable by chelicerae (fangs) that point diagonally forward and cross in a pinching action, in contrast to the Mygalomorphae (tarantulas and their cl ...
s, evolved new "modern" features, including a
cribellum Cribellum literally means "little sieve", and in biology the term generally applies to anatomical structures in the form of tiny perforated plates. In certain groups of diatoms it refers to microscopically punctured regions of the frustule, or o ...
and cross-acting fangs. Mesotheles retain the external abdominal segmentation of ancestral arachnids and have at least vestiges of four pairs of spinnerets, whereas mygalomorphs lack abdominal segmentation (like other opistotheles) and have a reduced number of spinnerets, often only two pairs. Like spiders in general, most species of Mygalomorphae have eight eyes, one pair of principal and three pairs of secondary eyes. Their chelicerae and fangs are large and powerful and have ample
venom Venom or zootoxin is a type of toxin produced by an animal that is actively delivered through a wound by means of a bite, sting, or similar action. The toxin is delivered through a specially evolved ''venom apparatus'', such as fangs or a sti ...
glands that lie entirely within their
chelicerae The chelicerae () are the mouthparts of the subphylum Chelicerata, an arthropod group that includes arachnids, horseshoe crabs, and sea spiders. Commonly referred to as " jaws", chelicerae may be shaped as either articulated fangs, or similarl ...
. These weapons, combined with their size and strength, make Mygalomorph spiders powerful predators. Many of these spiders are well adapted to killing other large arthropods and will also sometimes kill small
mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...
s, birds, and
reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates ( lizards and snakes) and rhynchoce ...
s. Despite their fearsome appearance and reputation, most mygalomorph spiders are not harmful to humans, with the exception of the Australian funnel-web spiders, especially those of the genus '' Atrax''. While the world's biggest spiders are mygalomorphs – '' Theraphosa blondi'' has a body length of and a leg span of – some species are less than long. Mygalomorphs are capable of spinning at least slightly adhesive silk, and some build elaborate capture webs that approach a meter in diameter. Unlike
Araneomorphae The Araneomorphae (also called the Labidognatha) are an infraorder of spiders. They are distinguishable by chelicerae (fangs) that point diagonally forward and cross in a pinching action, in contrast to the Mygalomorphae (tarantulas and their cl ...
, which die after about a year, Mygalomorphae can live for up to 25 years, and some do not reach maturity until they are about six years old. Some flies in the family
Acroceridae The Acroceridae are a small family of odd-looking flies. They have a hump-backed appearance with a strikingly small head, generally with a long proboscis for accessing nectar. They are rare and not widely known. The most frequently applied common ...
that are
endoparasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has c ...
s of mygalomorphs may remain dormant in their book lungs for as long as 20 years before beginning their development and consuming the spider. One female trapdoor spider, first recorded in a survey in 1974 in Western Australia, is known to have lived for 43 years.


Taxonomy

The name is derived from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
''mygalē'', meaning "shrew", plus ''morphē'' meaning form or shape. An older name for the group is Orthognatha, derived from the orientation of the
fang A fang is a long, pointed tooth. In mammals, a fang is a modified maxillary tooth, used for biting and tearing flesh. In snakes, it is a specialized tooth that is associated with a venom gland (see snake venom). Spiders also have external fangs ...
s which point straight down and do not cross each other (as they do in the araneomorphs).


Evolution

'' Megarachne servinei'' was thought to be a giant mygalomorph from the
Upper Carboniferous Upper may refer to: * Shoe upper or ''vamp'', the part of a shoe on the top of the foot * Stimulant Stimulants (also often referred to as psychostimulants or colloquially as uppers) is an overarching term that covers many drugs including thos ...
(about 350 million years ago), but was later found to be a
eurypterid Eurypterids, often informally called sea scorpions, are a group of extinct arthropods that form the order Eurypterida. The earliest known eurypterids date to the Darriwilian stage of the Ordovician period 467.3 million years ago. The group is ...
. The oldest known mygalomorph is '' Rosamygale grauvogeli'' , an avicularoid from the
Triassic The Triassic ( ) is a geologic period and system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago ( Mya), to the beginning of the Jurassic Period 201.36 Mya. The Triassic is the first and shortest period ...
of northeastern France. No mygalomorphs from the Jurassic have yet been found. The number of families and their relationships have both been undergoing substantial changes since a cladogram showing family relationships was published in 2005, with two significant studies in 2018. The division of Mygalomorphae into two superfamilies, Atypoidea and Avicularioidea, has been established in many studies. The Atypoidea retain some vestiges of abdominal segmentation in the form of dorsal
tergite A ''tergum'' (Latin for "the back"; plural ''terga'', associated adjective tergal) is the dorsal ('upper') portion of an arthropod segment other than the head. The anterior edge is called the 'base' and posterior edge is called the 'apex' or 'ma ...
s; the Avicularioidea lack these.
Molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
studies undertaken between 2012 and 2017 have found somewhat different relationships within the Avicularioidea. Some families appear not to be
monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...
and further changes are possible in the future. ''Rosamygale'' belongs to Avicularioidea, based on the absence of an abdominal scutum and well-separated posterior lateral spinnerets. Mygalomorphae tend to be highly morphologically conserved, which makes it difficult to find reliable morphological features to use for taxonomy. It has been hypothesized that because Mygalomorphae all tend to be
fossorial A fossorial () animal is one adapted to digging which lives primarily, but not solely, underground. Some examples are badgers, naked mole-rats, clams, meerkats, and mole salamanders, as well as many beetles, wasps, and bees. Prehistoric evid ...
and live in tubular webs, they are subjected to similar selective pressures, so most species should evolve in similar ways. Additionally, this may also mean that
homoplasies Homoplasy, in biology and phylogenetics, is the term used to describe a feature that has been gained or lost independently in separate lineages over the course of evolution. This is different from homology, which is the term used to characterize ...
are more likely to occur, further complicating taxonomy based on morphology.


Phylogeny

The relationships of taxa in the Mygalomorphae were restructured based on a comprehensive phylogenetic study by Opatova ''et al''. (2020) The generic composition of the families Ctenizidae, Cyrtaucheniidae, Dipluridae, and Nemesiidae were relimited. Five subfamilies were raised to the rank of family: Anamidae, Euagridae, Ischnothelidae, Pycnothelidae, and Bemmeridae. Three new families were created: Entypesidae, Microhexuridae, and Stasimopidae. Lastly, a new subfamily, Australothelinae, was generated and placed in the family Euagridae. The preferred
cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...
from Optova et al. (2020) is:


Families


Historical classification

In 1802, C. A. Walckenaer separated mygalomorph spiders into a separate
genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomencla ...
, ''Mygale'', leaving all other spiders in ''Aranea'' In 1985, Robert Raven published a monograph of the Mygalomorphae in which he proposed an internal classification for the Mygalomorphae, based on morphological features. Opatova ''et al''. (2020) commented "In short, much of today’s classification scheme dates back to Raven (1985)". Raven used various compound ranks, such as "gigapicoorder" and "hyperpicoorder". Ignoring these unusual rank names, his classification can be shown diagrammatically: Subsequent research, largely based on
molecular phylogenetic Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...
studies, has not upheld some of Raven's groupings. In particular his primary division between Tuberculotae and Fornicephalae has been replaced by a very different division between Atypoidea (expanded from Raven's Atypoidina) and Avicularioidea, which has no counterpart in his system. As another example, the families Mecicobothriidae and Microstigmatidae, which Raven placed in Mecicobothrioidina (a "gigapicoorder"), are now placed very far apart in the Atypoidea and Avicularioidea respectively. Other groups, such as Crassitarsae and Domiothelina, are more recognizable, allowing for some changes in family circumscriptions.


Distribution

Most members of this infraorder occur in the
tropics The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...
and
subtropics The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...
, but their range can extend farther north, e.g. into the southern and western regions of the
United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territo ...
. Only a few occur in Europe: 12 species from the families Atypidae,
Nemesiidae Nemesiidae, also known as funnel-web trapdoor spiders, is a family of mygalomorph spiders first described by Eugène Simon in 1889, and raised to family status in 1985. Before becoming its own family, it was considered part of " Dipluridae". Desc ...
, Ctenizidae, Macrothelidae,
Theraphosidae Tarantulas comprise a group of large and often hairy spiders of the family Theraphosidae. , 1,040 species have been identified, with 156 genera. The term "tarantula" is usually used to describe members of the family Theraphosidae, although m ...
, and Cyrtaucheniidae. Despite their limited range, currently, it is suggested that the Mygalomorphae were distributed worldwide before the breakup of
Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...
.


Notes


References


Bibliography

* *


External links

* * * {{Authority control Extant Triassic first appearances