Description (from Thorne, 1974)
Female body slightly arcuate, male straighter but arcuate in region of supplements. Lateral field 1/3 - 1/4 body width without visible cellular structure. Lip region angular, set off by deep constriction, 1/3 width of neck base. Spear about 13 um long with aperture 1/2 its length. Cephalic papillae prominent. Esophagus enlarged by rather abrupt expansion near middle. Intestinal cells with scattered, rather coarse brown granules. Prerectum length about twice body width. Rectum longer than anal body diameter. Vulva transverse with sclerotized labia. Ovaries reflexed halfway to vulva. Eggs 45 x 80 um. Sperms present in uterine tracts, indicating that males are functional. Tail elongate hemispheroid with thick cuticle and two pairs of pores. Male similar to female with slightly arcuate tail bearing 9 supplements arranged as illustrated. Spicula about 50 um long with simple lateral guiding pieces.
Thonus major is immediately distinguished by its length, set off lip region and 9 supplements.
Habitat: Alfalfa field near Bear Butte, South Dakota