The mesosoma has of seven segments. It is as broad as the prosoma anteriorly but slightly narrow posteriorly. Each segment of mesosoma is covered dorsally by a tergal plate or tergum and ventrally by a sternal plate or sternum and the two are joined with each other laterally by a pleural or arthrodial membrane.
The sternum of the first mesosomal segment: small and bears the median genital aperture which is covered by a plate-like, rounded, bifid and movable lid or genital operculum.
The sternum of second segment: pair of comb-like appendages, the pectines.
Pectine: consists of a three segmented stem or shaft or handle attached to the median line and along the posterior margin of which is a row of 4 to 36 narrow comb-like teeth.
The sterna of third, fourth, fifth and sixth pre-abdominal segments bear a pair of lateral, oblique, slit-like openings or apertures, the stigmata. Each stigmata leads into a pulmonary sac or book-lung. The sternum of the seventh segment has no appendage.