Temporal range: Triassic to present
|Missulena bradleyi, a mouse spider|
The Mygalomorphae (also called the Orthognatha) are an infraorder of spiders. The scientific name comes from the orientation of the fangs which point straight down and do not cross each other (as opposed to araneomorph).
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 (Coddington & Levy, 1991).
Almost all species of Mygalomorphae have eight eyes, however there are some with fewer (Masteria lewisi has only six eyes).
Their chelicerae and fangs are large and powerful, and have ample venom glands that lie entirely within their chelicerae. It is a notable indication of the effectiveness of these attributes that occasionally members of this suborder will even kill small fish, small mammals, and the like. However, only spiders of the Australian genus Atrax possess venom harmful to humans.
While the world's biggest spiders are mygalomorphs—Theraphosa blondi (Latreille, 1804) has a body length of 10 cm, and a leg span of 28 cm—some species are less than one millimeter long. Mygalomorphs are capable of spinning at least slightly adhesive silk, and some build elaborate capture webs that approach a meter in diameter (Coddington & Levy, 1991).
Unlike Araneomorphae, which die after about a year, Mygalomorphae can live for up to 25 years, and some don't reach maturity until they are about six years old. Some flies in the family Acroceridae which are endoparasites of mygalomorphs may remain dormant in the book lungs for as long as 20 years before beginning their development and consuming the spider.
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 (Selden et al., 2005a). Thus, the oldest known mygalomorph is Rosamygale grauvogeli Selden & Gall, 1992 (Hexathelidae) from the Triassic of north-east France. No mygalomorphs from the Jurassic have yet been found. (Selden et al., 2005b).
However, it is suggested that the Mygalomorphae were distributed world-wide before the breakup of Pangaea (Selden et al., 2005b).
|Cladogram of the group|
- About Spiders
- Coddington, J.A. 2005. Phylogeny and Classification of Spiders. In D. Ubick, P. Paquin, P. E. Cushing, and V. Roth (eds.) Spiders of North America: an identification manual, American Arachnological Society. 377 pages. Chapter 2, pp. 18-24.
- Raven, R.J. (1985). The spider infraorder Mygalomorphae: Cladistics and systematics. Bull. Am. Mus. Nat. Hist. 182:1-180.
- Coddington, J.A. & Levi, H.W. (1991). Systematics and Evolution of Spiders (Araneae). Annu. Rev. Ecol. Syst. 22:565-592.
- Goloboff, P.A. (1993). A Reanalysis of Mygalomorphae Spider Families (Araenae). American Museum Novitates 3056. PDF
- Selden, P.A., Corronca, J.A. & Hünicken, M.A. (2005). The true identity of the supposed giant fossil spider Megarachne. Biology Letters 1: 44-48. doi:10.1098/rsbl.2004.0272
- Selden, P.A., da Costa Casado, F. & Vianna Mesquita, M. (2005). Mygalomorph spiders (Araneae: Dipluridae) from the Lower Cretaceous Crato Lagerstätte, Araripe Basin, North-east Brazil. Palaeontology 49(4): 817-826. doi:10.1111/j.1475-4983.2006.00561.x
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- Suborder Mygalomorphae