Bolton, B., 1979:
Holotype worker. TL 2.8, HL 0.68, HW 0.58, CI 85, SL 0.48, SI 83, PW 0.43, AL 0.78.
Antennae with 12 segments. Mandibles unsculptured, smooth and shining with scattered hair-pits. Anterior clypeal margin with a shallow median impression. Frontal carinae strong and running almost to occipital corners before blending into the occipital rugoreticulum, and surmounted to this point by a narrow raised rim or flange. Maximum diameter of eye 0.17, about 0.29 x HW. Propodeal spines in profile elongate and narrow. Metapleural lobes triangular, broad basally but rapidly tapering to an acute apex. Petiole node in profile shaped as in insolens (Fig. 45, this paper), longer than broad in dorsal view. Clypeus sculptured only with the three longitudinal carinae typical of this group. Dorsum of head in front of the level of the eyes with 5 longitudinal rugulae between the frontal carinae, occipitally with a strong rugoreticulum and between these two zones with an area in which the longitudinal rugulae continue but which also has a few cross-meshes. Dorsal alitrunk reticulate-rugose and with a strong transverse carina at the junction of pro- and mesonotum. Petiole dorsum reticulate-rugulose, postpetiole dorsum similarly but less strongly sculptured. First gastral tergite without basal costulae. All dorsal surfaces of head and body with stout, stiff erect or suberect hairs. Colour pale yellow.
Holotype worker, Borneo: Sarawak, 4 th Division, Gunong Mulu Nat. Pk, R. G. S. Expd. Long Pala, lowl. rainfor., leaf litter, 28. ix. 77 (B. Bolton) (BMNH).
Also placed in this species is a single worker from Thailand: Nong Hoi, 21. vii. 1975 (D. Jackson) which closely resembles the holotype but has the anterodorsal petiole angle rather more acutely developed. It has almost the same measurements as the holotype (TL 3.0, HL 0.70, HW 0.61, CI 87, SL 0.50, SI 82, PW 0.44, AL 0.80) and shares the diagnostic characters of the holotype. The strong transverse carina on the dorsal alitrunk quickly separates adelphon from related species in the Oriental and Indo-Australian regions, but this character also occurs in the small yellow African species related to phasias Forel, which may turn out to be more closely related to adelphon than either obtusidens or kydelphon .