Based on Kerremans 1898, D. modesta differs from D. semiobscura in having the elytra purplish black (vs. purple) with traces of grey mottling (vs. light grey speckles), the antennae & tarsi black (vs. 'obscure' [= dark]), and the pronotum and scutellar region a shiny copper purple (vs. purple). Also the head is green (vs. purple), but this may simply be a result of different sexes being described. Structural differences apparent in the accounts of the two species seem slight and inconsequential: dimensions 5 x 1.7 mm (vs. 5 x 1.8 mm); pronotum sides arched (vs. fairly arched); pronotum base with median lobe a little indented (vs. barely indented); elytra with small transverse wrinkles (vs. with rugosities resembling scales).

An extremely common and variable species for which the limits of variation are difficult to define. It is closely related to Diphucrania semiobscura (q.v.) which has an identical aedeagus and may not be separable as a distinct species.

The original description of this species (as Cisseis modesta) by Kerremans 1898 was followed on the same page by an adjacent protologue for C. semiobscura. Kerrremans introduced C. semiobscura stating that it had the appearance of C. modesta but was different in regard to certain details of structure and colouration but did not elaborate further. The differences given under 'Distinctive features' above are derived from a comparison of his brief descriptions for the two species.

Analysis of preliminary DNA barcode sequences (mitochondrial CO-1) divided D. modesta and D. semiobscura samples into two tightly clustered and well separated sister groups: a sample of six individuals from the Mt Lofty Ranges (Belair, Mt Barker, Lobethal and Kapunda areas) and a sample of nine individuals from the Murray Mallee, southern Flinders Ranges and Eyre Peninsula (Monarto, Quorn, Kimba and Edilillie areas). The first group includes some 'typical' D. modesta forms with blackish elytra and coppery pronotum, and the latter some with more purplish elytra and pronotum that fit C. semiobscura. However, forms from the southern Mt Lofty Ranges (including some samples that grouped with typical C. modesta) often have the elytra dark coppery bronze to purplish rather than blackish. There is much variation, and it is not possible to find consistent colour or morphological differences corresponding to the two groups, nor any distinction based on adult host plants. A more detailed study with nuclear DNA may help resolve the taxonomy of this complex.