The Mygalomorphae or mygalomorphs are an infraorder of spiders. The name is derived from the Greek mugalē, meaning "shrew", plus morphē meaning form or shape. An older name for the group is Orthognatha, derived from the orientation of the fangs which point straight down and do not cross each other (as they do in the araneomorphs).
Temporal range: Triassic to present
|Missulena bradleyi, a mouse spider|
|About 20 families|
This group of spiders comprises mostly heavy-bodied, stout-legged spiders including tarantulas, Australian funnel-web spiders, mouse spiders, and various families of spiders commonly called trapdoor spiders.
Like the "primitive" suborder of spiders Mesothelae, they have two pairs of book lungs, and downward-pointing chelicerae. 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). Following the branching into the suborders of Mesothelae and Opisthothelae, the mygalomorphs retained them, while their fellow Opisthothelae members, the araneomorphs, evolved new "modern" features, including a cribellum 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. But there are also species with six, four, two or no eyes.
Their chelicerae and fangs are large and powerful and have ample venom glands that lie entirely within their chelicerae. 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 mammals, birds, and reptiles. 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 10 cm (3.9 in) and a leg span of 28 cm (11 in)—some species are less than one millimeter (0.039 in) 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, 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 that are endoparasites 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.
Megarachne servinei was thought to be a giant mygalomorph from the Upper Carboniferous (ca. 350 million years ago), but was later found to be an eurypterid. Thus, the oldest known mygalomorph is Rosamygale grauvogeli (Hexathelidae) from the Triassic of north-east 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 tergites; the Avicularioidea lack these. Molecular phylogenetic studies undertaken between 2012 and 2017 have found somewhat different relationships within the Avicularioidea. Some families appear not to be monophyletic and further changes are possible in future.