Opisthothelae

Opisthothelae is a taxon within Order Araneae, consisting of the Mygalomorphae and the Araneomorphae, but excluding the Mesothelae. Opisthothelae is sometimes presented as an unranked clade^[1][2] and sometimes as a suborder of the Araneae.^[1] In the latter case, Mygalomorphae and Araneomorphae are demoted to infraorders.

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

• the tergite plates on the abdomen of Mesothelae but absent in Opisthothelae;
• the almost total absence of ganglia in the abdomen of Opisthothelae;
• the almost-median position of the spinnerets in the Mesothelae compared to the hindmost position of those of the Opistothelae;
• the absence of venom glands in most Mesothelae species.

Among the Opisthothelae, the fangs of the Mygalomorphae point straight down in front of the mouth aperture and only allow the spider to grasp its prey from above and below, whereas in the Araneomorphae they face one another like pincers, allowing a firmer grip. It is difficult to distinguish araneomorphs and mygalomorphs on first inspection unless the specimens are large enough to permit immediate examination of the fangs.

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