Opisthothelae

Opisthothelae is a suborder of spiders within the order Araneae, containing Mygalomorphae and Araneomorphae, but excluding Mesothelae. The Opisthothelae are sometimes presented as an unranked clade and sometimes as a suborder of Araneae. In the latter case, Mygalomorphae and Araneomorphae are treated as infraorders.

The creation of this taxon has been justified by the need to distinguish these spiders from the Mesothelae, which display more primitive characteristics. The characteristics that distinguish between the Mesothelae and Opisthothelae are:

• Mesothelae have a segmented abdomen with tergite plates, whereas Opisthothelae have fused abdominal segments and lack tergite plates.
• The almost total absence of ganglia in the abdomen of Opisthothelae.
• The almost median position of the spinnerets in the Mesothelae compared with the hindmost position of those of the Opistothelae.