What is "mesothelae"

Mesothelae is a suborder of spiders (order Araneae) that includes a single living (extant) family, Liphistiidae, and a number of extinct families. This suborder is thought to form the sister group to all other living spiders, and retain ancestral characters, such as a segmented abdomen with spinnerets in the middle and two pairs of book lungs. Members of Liphistiidae are medium to large spiders with eight eyes grouped on a tubercle. They are only found in China, Japan, and southeast Asia.

The Heptathelidae were once considered their own family; today they are considered a subfamily of the Liphistiidae (i.e. as Heptathelinae).