Length: 2.50–2.90 mm; Width: 1.70–2.10 mm; TL/TW: 1.33–1.40; PL/PW: 0.52–0.58; EL/EW: 0.97–1.05; EW/PW: 1.34–1.43. Male: Body outline (Fig. 1e) elongate oval, broadest around middle of elytra, elytra apically somewhat broadly rounded to subtruncate; dorsum densely pubescent with silvery white hairs. Head dark pitchy brown to black, anterior clypeal margin slightly lighter; pronotum black except anterior and lateral margins narrowly reddish testaceous; elytra black, apical one-fourth reddish brown-testaceous. Ventral side with mouthparts, legs and abdominal ventrites dark brown to testaceous, middle of abdominal ventrite 1 and remaining areas dark pitchy brown. Head with interocular distance about 1.7x as wide as an eye; punctures dense in posterior half, slightly more widely spaced towards clypeal margin, separated by <0.5–1 diameter. Antenna 11-segmented with a distinct club. Pronotum with punctures on disc shallow, separated by 1–4 diameters, denser and more closely spaced on lateral sides, separated by <1–2 diameters. Elytra with shallowly impressed punctures, slightly larger and denser than that on pronotum, separated by 1–2 diameters on disc, slightly coarser, denser and more closely spaced around anterolateral margins, interspaces between punctures coriaceous; with a row of slightly larger punctures on either side of suture in anterior half. Prosternal process (Fig. 2a) with a pair of apically divergent carinae. Abdominal postcoxal lines (Fig. 2c) complete, broadly semicircular to boat-shaped, area enclosed densely punctate in anterior half, punctures coarser and fewer in posterior half, apical one-fourth adjacent to postcoxal line more or less smooth, devoid of punctures. Ventrite 5 with posterior margin weakly emarginate, ventrite 6 truncate. Tarsi pseudotrimerous. Tarsal claws bifid with a basal tooth (Fig. 2b). Male genitalia (Fig. 2d, e, f) with penis guide lanceolate in ventral view (Fig. 2e), parameres shorter than penis guide in lateral view (Fig. 2d), bilobate with triangular / subconical inner expansions in anterior half, inner expansion much shorter than outer, with long apical hairs reaching beyond apex of outer lobe, apices of outer lobes of parameres with much longer, denser hairs reaching beyond apex of penis guide. Penis (Fig. 2f) coiled with a prominent capsule having a distinctly longer outer arm than inner arm, penis ­­­apex membranous with a short, hook-like projection.

Female: Similar to male. Tarsal claws (Fig. 2b) with a more transverse basal tooth than in male, anterior process distinctly shorter than posterior (in male basal tooth more distinctly quadrate, posterior process only slightly longer than anterior). Last abdominal ventrites not showing any marked dimorphism, ventrite 5 with posterior margin broadly arcuate, ventrite 6 subtruncate. Female genitalia (Fig. 1f) with spermatheca having well differentiated cornu, nodulus and ramus and a sclerotised rod-like projection on bursa.

Scores of Oriental species of Scymnini are externally similar to S. (P.) rajeshwariae sp. n. in having black elytra with reddish brown / testaceous apices and are difficult to identify without examination of male genitalia. But the habits and male genitalia of this species appear to be unique. The male genitalia are very distinctive with bilobed parameres and the penis guide is apically lanceolate.

This species is named for Ms. S.K. Rajeshwari, Technical Officer, NBAIR, who has been a great help and support in my work on Coccinellidae.

India: Karnataka.

It appears to be a highly specific predator of the bamboo woolly aphid, Pseudoregma bambusicola (Takahashi) and is collected always in association with this species only. The larvae are greyish with white waxy dusting and lack the usual dense waxy filaments in many other Scymnus spp. (Fig. 1a, b, c​). The larvae are commonly seen in aphid colonies on bamboo stem, but the adults are more cryptic in their habits and found feeding only under the leaf sheaths. There are several common predators of P. bambusicola which occur along with this species. These include the giant bamboo ladybirds, Synonycha grandis (Thunberg) and Megalocaria dilatata (F.) (Coccinellidae), Dipha aphidivora (Meyrick) (Lepidoptera: Pyralidae), and Dideopsis aegrota (F.) (Diptera: Syrphidae). In view of its specificity, this species could be a potential biological control agent of P. bambusicola on bamboo.

Length: 2.20-2.65 mm; width: 1.65–1.80 mm. Male: Body (Fig. 3) elongate oval, moderately convex. Head and pronotum testaceous to dark brown, elytra dark brown with four yellowish spots – first pair transverse, roughly quadrate, located before middle in anterior half, second pair smaller, transverse, located around 4/5^th of elytra, both spots occasionally larger, almost touching lateral margins of elytra; head with elongate silvery white hairs, pronotum with yellowish white to brownish pubescence, elytra with a mixture of dark brown and yellowish white hairs, those on elytral spots paler, those on rest of elytra predominantly brown to black. Ventral side more or less uniform reddish brown, pro-, meso- and metasternites darker reddish brown, tarsi of legs slightly lighter, yellowish brown. Head with clypeal margin carinate, punctures separated by 2-3 diameters, denser and separated by less than their own diameter near eye margins. Pronotum densely punctate on posterior and lateral margins, slightly more widely spaced on disc, separated by 3-5 diameters. Elytral punctures denser and slightly larger than those on pronotum, separated by 2-5 diameters on disc, punctures on lateral sides and apices more closely spaced. Prosternal process with an inverted Y-shaped carina. Abdominal postcoxal line (Fig. 4a) complete and semicircular, area enclosed by postcoxal line sparsely punctate with few punctures along anterior margins. Ventrite 5 truncate, ventrite 6 barely emarginate. Tarsi trimerous. Male genitalia (Fig. 4b, c, d, e) with tegmen in ventral view (Fig. 4b, c) with penis guide shorter than parameres, apically very slightly asymmetrical, broadest at base, progressively narrowed to apex, apical one-fifth triangular; parameres longer than penis guide, apices with elongate hairs. Penis capsule (Fig. 4d) with a prominent inner arm, outer arm lacking; apex of penis (Fig. 4e) strongly flattened or spatulate, densely spotted. Female: Similar to male. Ventrite 6 apically weakly arcuate. Genitalia with spermatheca (Fig. 4f) with spermatheca tubular, long and intricately coiled, sperm duct elongate and progressively broader.

The external appearance of H. sororius sp. n. is very similar to that of Horniolus vietnamicus Miyatake. Horniolus hisamatsui Miyatake also has a similar elytral pattern, but it has reddish brown head and pronotum. But the male genitalia, particularly the enlarged siphonal apex, are diagnostic and unique to this species. Horniolus sororius sp. n. is also similar to two common Indian species, Scymnus (Pullus) latemaculatus Motschulsky (Fig. 5a), and Pharoscymnus horni (Weise) (Fig. 5d). The former is close to H. sororius sp. n. in general appearance, but can be distinguished by the presence of parallel, apically divergent carinae on the prosternal process and the male genitalia (Fig. 5b, c) are also diagnostic. Besides, it is one of the most common general predators of aphids in India. Pharoscymnus horni can be differentiated from H. sororius sp. n. by its distinctly more rounded body outline and other generic characters.

The specific epithet is a Latin adjective (“sororius” L.= of a sister, sisterly) in reference to its similarity to other common species.

India: Karnataka.

Collected in association with coffee mealybugs and on Eleusine coracana (label data). This species was recorded by Ramani et al. (2002) (as Horniolus sp.) as a predator of spiralling whitefly, Aleurodicus dispersus Russell (Hemiptera: Aleyrodidae).

Horniolus is an Oriental genus and only three species, H. dispar Weise (1900) (from Sri Lanka), H. nigripes Miyatake (1976) (from southern India) and H. hisamatsui (Nepal) are represented in the Indian subcontinent. Horniolus guimeti (Mulsant), distributed in Malaysia and Borneo, has been mentioned in literature from the Indian subcontinent, but is a doubtful record for India (R.G. Booth, BMNH, in litt.). Korschefsky (1931) includes India in its distribution range, but Indian records of H. guimeti are mostly misidentifications of Scymnus (Pullus) latemaculatus (Fig. 5a), and Pharoscymnus horni, which are externally similar.