Pronotum strongly rounded at the sides and tapered towards its base; elytra with 1 to 2 very shallow to weakly defined abbreviated ridges (not reaching base), and suture raised in its posterior half; silvery-white hairs dense on frons of head, and mid-dense on much of the ventral side; ‘prosternum gently and shortly bent over or depressed along its anterior margin’ (Blackburn 1887). The body shape is more slender (narrower and thinner) than in M. rothei and the silvery hairs more conspicuous. Differs from M. nervosa in SA by its much reduced elytral ribs, smaller size and different aedeagus. Readily distinguishable from M. sordida by the puncta of the medial pronotum not being transversely elongated.

Blackburn 1887 described Melobasis soror from an insect he received from 'Mr. Rothe of Sedan', along with 'several specimens in the South Australian Museum devoid of any record of capture'.

Previously this species had been often misidentified in SA as Melobasis rothei, and it was included on these pages under that name until November 2020. However, in the key to species groups of Levey 2012, M. rothei is in the M. melanura group, whereas this species clearly belongs in the M. nervosa species group.

Using the key in the revision of the M. nervosa species group by Levey 2018, I determined my collections as being closest to the endemic WA species M. barkeri but differing in several features. This view was supported by Levey (pers. comm. Jul. 2020) who regarded it as 'probably an undescribed species' based on a specimen image I sent to him. Consequently, it was treated on this website as Melobasis sp. Senna from November 2020 to December 2023.

I was baffled by M. soror being a long-established and widely distributed SA species that I had seemingly not encountered. In December 2023 I undertook a critical examination of the two old and darkened paralectotypes, which, along with four other SA specimens at the SA Museum (two old and two more recent), had determinations as M. soror made by Brian Levey in 1983 or 1984. All were found to display the salient features in Blackburn’s description, and the two more recent collections also closely resemble M. sp. Senna in coloration and surface texture. Furthermore, the aedeagus on specimens of M. sp. Senna in my collection displayed the key features described and illustrated for M. soror in the Levey 2018 treatment. It became clear that M. sp. Senna was synonymous with M. soror, and more particularly with subspecies soror of Levey 2018.

A contributing factor to the past confusion is the rather atypical specimen from Beverley in WA used to illustrate the habit of M. soror soror in the treatment of Levey 2018. It differs from SA specimens by its shorter shape, and it appears to have more prominent ridges on the elytra than the very subtle ones usual in SA. Levey compared M. soror to the prominently ribbed M. nervosa, stating that subspecies soror looked ‘very like small specimens of M. nervosa which might be the most closely related species', and was 'easily distinguished from this species by the form of the aedeagus'. Blackburn 1887, considered M. soror as 'somewhat allied to M. nervosa' and emphasised the reduction in number and prominence of elytral ridges.

As with M. rothei, adults are strongly associated with Senna artemisioides and their appearance coincides with the flowering time of these bushes. Recently, however, I encountered it in large numbers on flowering Acacia calamifolia in the southern Flinders Ranges.