Proceratium gibbosum

This species nests in the forest floor and the colonies are probably small. It can be found in wet evergreen and secondary tropical rainforests, nesting in the interphase of soil and leaf litter or in the debris along sheltered edges of decaying logs on floor (Image 6). Workers are solitary foragers and move at a slow pace. They feign dead when disturbed, camouflaging against the soil (Image 5D). In captivity, the workers readily accepted spider eggs as food (Image 5E) and built a nest chamber with spider silk and soil. Workers were slow in movement, looked generally uncoordinated and were averse to light. Other species that were found in the same microhabitat of Proceratium gibbosum were Tyrannomyrmex alii, Protanilla sp., Discothyrea sp., and Recurvidris sp. So far, this species is restricted to the mid-elevation tropical evergreen jungles of the Periyar Tiger Reserve, in Kerala, India.

Identification
Proceraitum gibbosum differs from the other members of the stictum species group by the following character combination: mesonotum with a small rounded dorsal hump, and petiole lacking ventral projections. Proceratium gibbosum also presents a pedunculate petiole with its dorsal margin convex in profile; all tbiae with pectinate spur, calcar of strigil with a basal spine; eyes composed of a single large convex ommatidium; propodeum unarmed but angulate, convex in profile, propodeum with a robust spine on each side, propodeal lobes broad lamellaceous expansions; head, mesosoma, petiole and postpetiole irregularly foveolate; frst gastral tergite convex in profile; antennal funicles wider than long; total length <4.8 mm; propodeum with a robust spine on each side, the propodeal lobes with broad lamellaceous expansions.

The other Proceratium species from India are Proceratium bhutanense de Andrade, 2003, described from Phuntsholing in Bhutan, Darjeeling in West Bengal, Kumaon in Uttar Pradesh (Uttarakhand), and Khasi Hills in Meghalaya (Urbani & De Andrade 2003). Bhart and Wachkoo (2014) found P. bhutanense to be conspecific with P. williamsi Tiwari, 2000 and hence is now treated as the junior synonym of the later. The species P. williamsi belongs to the itoi species group with the fourth abdominal segment sternite protruding over the third abdominal sternite (Urbani & De Andrade 2003).

According to the identification key from Urbani & De Andrade (2003), the closest known species in the stictum species group seems to be P. deelemani. However, P. deelemani lacks the distinct small rounded dorsal hump present on the new species. In addition, the petiole of the new species lacks any ventral projectons, while in P. deelemani it has a distnct ventral tooth. To P. stictum, the new species differs in the cephalic sculpture, deeply impressed on P. gibbosum and shallow on the former. Additonally, the frontal carinae of the new species diverge posteriorly, where in P. stictum they are not as divergent. Anteriorly, the frontal carinae are closer to each other in P. gibbosum, while they are farther away in P. deelemani. The frontal carinae run to a level almost midway between the anterior clypeal margin and the level of the eyes, but they extend only one third the same distance in P. deelemani (the frontal carinae are shorter in P. deelemani). The species is differentiated from P. foveolatum by the first gastral tergite being angulate on dorsum, while it is round on the curvature in P. gibbosum. The new species is diagnosed from P. shohei by the head being widest midway between the eyes and vertex, while the head is widest at the level of eyes in P. shohei. The petiolar node is relatively compressed dorsoventrally in P. shohei, while P. gibbosum has a pedunculate petiole, convex in profile.

Distribution based on Regional Taxon Lists
Oriental Region: India.

Nomenclature

 * . Proceratium gibbosum Sadasivan & Kripakaran, 2022: 21371, figs. 2A–C (w.) INDIA (Kerala).

Type Material

 * Holotype: NRC-AA-3758, 23 May 2016, Worker, Vallakadavu, Periyar Tiger Reserve, Idukki District, Kerala State, India, at 900 m, coll. by Kalesh Sadasivan, tray-sifting loose soil under a decaying log, in forest floor of a primary evergreen forest, deposited in the insect collection facility of the NCBS (National Centre for Biological Sciences), Tata Institute of Fundamental Research, GKVK, Bellary Road, Bengaluru, Karnataka 560065, India. Earlier, the holotype was with number TARG-1007, mounted for morphological study and later removed & preserved as wet specimen in absolute alcohol, deposited in the research collections facility at the Travancore Nature History Society (TNHS), Thiruvananthapuram, Kerala.
 * Paratype workers (n = 3) (Images 3–5):
 * NRC-AA-3759, 28 March 2021, Worker, Vallakadavu, Periyar Tiger Reserve, Idukky District, Kerala State, India, at 930 m, coll. Kalesh Sadasivan, tray-sifting leaflitter, in forest floor of a primary evergreen forest, deposited in the insect collection facility of the NCBS (National Centre for Biological Sciences), Tata Institute of Fundamental Research, GKVK, Bellary Road, Bengaluru, Karnataka 560065, India. Earlier, paratype number TARG-1008, preserved in absolute alcohol and currently deposited in the research collections facility at the TNHS, Trivandrum, Kerala.
 * Two other paratype workers, same data as paratype above. One paratype (TARG-1009) and (TARG-1010) both in absolute alcohol, to be deposited in the insect collection of Zoological Survey of India (ZSI), Kozhikode, Kerala.

Description
Worker

Holotype Measurements: EL 0.05, HW 0.80, HL 0.90, HFeL 0.75, HTiL 0.55, HBaL 0.40, A3L 0.90 A4L 0.45, LS4 0.25 PeL 0.47, PeW 0.34, SL 0.50, WL 1.09, TL 3.91, CI 89, OI 6.25, SI 55.55, DPeI 72.34, ASI 50, IGR 55.56.

Head: In full-face view marginally longer than wide (CI 89). Vertexal margin almost straight with only very shallow concavity. Head wider at midway distance between the level of eyes and the lateral angle of the vertex. Clypeus narrow, not surrounding the antennal insertons and projecting inferiorly only in the area between the anterior margin of the frontal carinae. Anterior clypeal margin notched medially. The frontal carinae are well-separated, running in parallel anteriorly and then diverging posteriorly. The frontal carinae reaches up to midway between the anterior clypeal margin and the level of the eyes (Image 2C). Eyes simple (single ommatdium), located slightly below the mid-length of the head in full-face view. Ocelli absent. Antennal scape distally incrassate and not reaching the vertexal margin. Antennal scape as long as broad, all other segments broader than long. Length of last funicular segment equal to the sum of lengths of 7–11 funicles. Mandibles with three denticles before the apical tooth. Palp formula 4,3.

Mesosoma: In lateral view, slightly convex; mesonotum presenting a visible tumulus. Mesosoma slightly longer than the sum of HL and mandible length. Both the promesonotal suture and metanotal groove shallow and barely discernible. Propodeal margins with a well-defined tooth, lobes expanded into a broad lamella. In dorsal view, pronotal margin angulate, but lacking projectons, tooth or spines. The mesonotum bears on its mid-dorsal surface aspect a large tumulus (0.25 mm), occupying almost half of the area on dorsal side of the mesonotum (Images 2B). The propodeum has the tooth directed postero-laterally and the broad propodeal lobes. Propodeal declivity slightly concave, almost fat. The posterolateral aspect of metapleuron with a concavity bearing the opening of the metapleural gland. Petiole: In dorsal view, slightly longer than broad (PeL 0.47, PeW 0.34). The narrowest part of the petiole is its anterior end (peduncle). The sides of the node are diverging to about the beginning of distal third where it is the widest and the converges slightly towards the posterior end. The anterior margin of the peduncle is thickly marginated. In profle view, a mid-ventral keel extends till the end of the junction of the anterior and middle third of the length of the petiole. No tooth or spine present ventrally (Images 2A,B). Postpetiole: In lateral view, postpetiole is 2.5 times the length of the petiole. Dorsal profile broadly convex, ventrally the anterior half is slightly concave and distal half is convex in outline (Image 2A). The sides of the tergite are convex and the anterior end is produced as a small blunt triangular extension. In ventral view, the sternite has a mid-carina which is rudimentary. The anterior margin of the sternite extends as a broad triangular extension (Images 2A,B).

Gaster: Constriction between the post petiole (abdominal segment AIII) and first gastral segment (AIV) well defined and deep (Image 2A). Tergite of the AIII twice the length of the post petiole (AII). The tergite of AIII double the length of tergite of AIV. The first gastral segment recurved ventrally to almost a right angle and its curvature is smooth and convex. The distal edge of the AIII was marginated. Remaining gastral segments curved ventrally and telescoped inside the gaster. Sting present, robust (0.2 mm long). Legs: All tibiae with pectinate spur. Calcar of strigil with a basal spine. Hind basitarsi slightly longer than half the length of the hind tibia.

Sculpture and Pilosity: Head, mesosoma, petiole and AIII irregularly foveolate with sparse tiny nodules. The irregular edges of the foveolae gives a scabrous appearance to the surface. Area of the mesonotal tumulus finely granular. AIV almost scabrous in appearance. Legs covered in dense but shallow foveolae, giving them a reticular appearance. Body is covered in four types of hairs: 1) Very short decumbent hairs on the antennal funicles; 2) Short sub-decumbent hairs, which are denser on the legs and the mesonotal tumulus; 3) Long sub-erect hairs throughout the whole body; 4) Short appressed hairs on the apical antennal funicle. Short hairs on the mesonotal tumulus irregular, disposed with the tips pointing to the centre of the tumulus. Color: Live specimens dark brown. Petiole, the mesonotal tumulus and the propodeum darker. The pronotum, postpetiole and head slightly paler. Legs and antennae dark orange brown. Hairs pale amber brown.

Paratype Workers

Measurements: EL 0.05–0.06, HW 0.70–0.80, HL 0.80–0.90, HFeL 0.80–0.90, HTiL 0.50–0.60, HBaL 0.35– 0.45, A3L 0.90–1.00, A4L 0.45–0.50, LS4 0.20–0.30, PeL 0.47–0.50, PeW 0.34–0.40, SL 0.50–0.70, WL 1.09–1.30, TL 3.71–4.20, CI 87.5–89, OI 6.20–7.50, SI 55.55–77.78, DPeI 72–80, ASI 50.00, IGR 44.40-60.

Variation in workers. No variation except subtle differences in body measurements as given above.

Gyne Unknown

Male Unknown

Measurements

EL—Eye length: maximum length of eye measured in lateral view | HL—Head length: maximum measurable distance from the mid-point of the anterior clypeal margin to the mid-point of the posterior margin of head, measured in full-face view | HW—Head width: maximum head width directly behind the eyes, measured in full face view | SL—Scape length: maximum length of scape shaft excluding basal condyle | PH—Pronotal Height: the maximum height of the pronotum in profile | PW— Pronotal Width: the maximum width of the pronotum in dorsal view | DML—Dorsal Mesosoma Length: maximum length of mesosomal dorsum from anterio-dorsal margin of pronotum to dorsal margin of propodeal declivity | WL—Weber’s Length of Mesosoma: the maximum diagonal length of the mesosoma in profile, from the angle at which the pronotum meets the cervix to the posterior basal angle of the metapleuron | HFeL— Metafemur Length: the maximum straight-line length of the metafemur, measured in dorsal view | HTiL—Hind tbia length: maximum length of hind tibia measured on its external face | HBaL—Hind basitarsus length: maximum length of hind basitarsus measured along its external face | PeL—Abdominal Segment II (petiole) Length: the maximum length of abdominal segment II (petiole), measured in dorsal view | PeH—Abdominal Segment II (petiole) Height: the maximum height of the petiolar tergum in profile view, including laterotergite, excluding petiolar sternum | PeW—Abdominal Segment II (petiole) Width: the maximum width of abdominal segment II (petiole), measured in dorsal view | A3L— Abdominal Segment III Length: the maximum length of abdominal segment III, measured in dorsal view | A3W—Abdominal Segment III Width: the maximum width of abdominal segment III, measured in dorsal view | A3H—Postpetole Height: Maximum height of postpetiole in profile | A4L—Abdominal Segment IV Length: the maximum length of abdominal segment IV, measured in dorsal view | A4W—Abdominal Segment IV Width: the maximum width of abdominal segment IV, measured in dorsal view | LS4—Abdominal sternum IV length: maximum length of abdominal sternum IV in lateral view | A5L— Abdominal Segment V Length: the maximum length of abdominal segment V, measured in dorsal view | A5W— Abdominal Segment V Width: the maximum width of abdominal segment V, measured in dorsal view | A6L— Abdominal Segment VI Length: the maximum length of abdominal segment VI, measured in dorsal view | A6W— Abdominal Segment VI Width: the maximum width of abdominal segment VI, measured in dorsal view | WL— Weber’s length: diagonal length of mesosoma in lateral view from the anterior-most point of pronotal slope (excluding neck) to posterioventral margin of propodeal lamella or lobe | TL—Total body length: combined length of HL + WL + PeL + A3L + A4L for Proceratinae.

Indices

CI—Cephalic index: HW / HL × 100 | OI—Ocular index: EL / HW × 100 | SI—Scape index: SL / HL × 100 | DMI —Dorsal Mesosoma Index: PW / WL × 100 | DMI2 —Dorsal Mesosoma Index 2: DML / WL × 100 | LMI — Lateral Mesosoma Index: PH / WL × 100 | DPe (DPI)— Dorsal petole index: PeW / PeL × 100 | LPI —Lateral Petole Index: PeL / PeH × 100 | MFI —Metafemur Index: HFeL / HW × 100 | ASI—Abdominal segment index: A4L /A3L × 100 | IGR—Gastral reflection index: LS4 / A4L.

Etymology
The specific epithet gibbosum (from Latn ‘gibbosus’, meaning protruding or humpbacked) is a singular neuter adjective in the nominative case and refers to the hump-like protuberance on the mesonotum, characteristic of the species.