Mygalomorphs

Mygalomorphae, or mygalomorph spiders, form one of the two major lineages of spiders, the other being the Araneomorphae. They represent an ancient radiation that includes around two thousand described species across several families, such as tarantulas (Theraphosidae), trapdoor spiders (Ctenizidae and related groups), and funnel‑web spiders. Most are terrestrial, with many species adapted to burrowing or hiding in retreats.

A hallmark of mygalomorphs is the orientation of their chelicerae. Their fangs point downward and operate in a vertical plane, moving parallel to each other rather than crossing. This orthognathous arrangement contrasts with the cross‑biting fangs of araneomorph spiders. Mygalomorphs typically have two pairs of book lungs, a robust body, and relatively large chelicerae used to seize prey.

Ecology and behavior: Many mygalomorphs are fossorial, constructing silk‑lined burrows or trap‑door retreats in soil or rock. Others are ground‑dwelling or arboreal. They tend to have slow growth and long lifespans, with females often living many years. Reproduction usually involves elaborate courtship and transfer of sperm via the male’s reproductive structures; eggs are laid in silk sacs and guarded by females.

Distribution and evolution: Mygalomorph spiders occur worldwide, with highest diversity in tropical regions. They are an ancient lineage, retaining several primitive features compared with araneomorphs. Because of their secretive habits and relatively low visibility, they are less studied than many araneomorphs, though some species can be venomous to humans.