The mygalomorphs are an infraorder of spiders. Mygalomorph spiders have straight-pointing chelicerae which do not cross, unlike those of the araneomorph spiders. They also possess two pairs of book lungs. These two traits are in common with the more basal mesothelae, though the two infraorders do not appear to be closely related. Instead it appears that both are derived from an earlier common ancestor which had both characteristics.

Most mygalomorphs are heavy-bodied, fairly large spiders. Some, like the Goliath bird-eater (Theraphosa blondi) can exceed 100mm in body length, though there are smaller members, too. Among them are such spiders as Masteria lewisi, which is generally about 4mm long. It is also unique in that it has only six eyes - all other known mygalomorphs have eight.

All mygalomorphs are venomous, though among these, only a few Australian species are hazardous to humans.* Mygalomorphs are much more likely than araneomorphs to be active pursuit hunters, though many of them do spin web traps of various sorts, and nearly all can produce at least partially adhesive silk.

Mygalomorphs are found primarily in tropical and subtropical regions of the world, but some appear as far north as the central United States, and as far south as Tasmania. While they are found on all continents except Antarctica, they are particularly rare in Europe, with only a handful of species are known.

While araneomorphs seldom live more than about a year, mygalomorph spiders can be quite long-lived, up to 25 years in some cases.

*There are reports that the Chinese bird spider, Haplopelma schmidti, and several species in the Macrothele genus may be dangerous to humans as well, but there's no confirmation or reliable documentation of this.

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