Species related with Citrogramma gedehanum and C. fascipleurum. Males differ from C. gedehanum only by the abdominal pattern and from C. fascipleurum by yellow facial and notopleural pilosity and black anepimeron. Females have lunule dark and frons with a medial dark macula, in comparison with females of C. fascipleurum, which have lunule yellow and frons yellow medially. Females of C. difficile and C. gedehanum only differ in their occipital pilosity, which is black on dorsal 1/3 in C. difficile.
From original description (Curran 1928).
MALE.
Head subtranslucent yellowish, the occiput behind the eyes and the vertical triangle, blackish-green, the latter thinly ochreous pollinose, thickly so behind the ocelli, the posterior orbits silvery greyish pollinose, becoming white on the gena and narrow facial orbits, the front wholly with opaque yellow pollen, only the lunule shining and slightly infuscated. Frontal triangle with long, not abundant, black pile to the level of the antennae; pile of vertical triangle black, on the vertex and dorsal third of the posterior orbits, yellow, the orbits elsewhere silvery yellowish pilose, the face and gena with fine white pile. Pile on posterior orbits inclined to be squamose. Facial profile as in variscutatus, the sides of the face distinctly diverging below, the tubercle a little more pronounced. Antennae similar.
Thorax with similar yellow markings, the scutum wholly dark and apparently entirely lacking the peculiar metallic colouring and pollen, although there are black pilose sublateral vittae and the apical third is similarly clothed; there are no black pile on the yellow ground above the root of the wing. The scutellum bears a shining deep black triangle which leaves only the broad margin pale honey yellow. The legs and abdomen appear to be similar, but the markings on the latter are not clearly discernible owing to slight discoloration. The genital surstyli are much more pilose.
Citrogramma difficile (Curran, 1928).
Curran, C.H. (1928) The Syrphidae of the Malay Peninsula. Journal of the Federal Malay States Museums 14, 141-324.
Curran (1928) questioned validity of C. difficile as a different species and pointed out the possibility that it could be a synonym of C. gedehanum.
Synonyms: Syrphus difficilis Curran, 1928, 1931a: 313 (key); Citrogramma difficile of Vockeroth, 1969: 95 (comb. nov.); Knutson et al., 1975: 311 (cat. cit.); Citrogramma difficilis of Wyatt, 1991: 155 (cit.), 158 (ident. key), 163 (descr.).
unless otherwise stated