Solenopsis gnoma

Queens and workers of the type series were collected among a nest of Tetramorium bicarinatum in a rotten trunk. This species has also been found from as low as 170 m to 864 m at the top of Cerro Crocker (Crocker Hill) on Santa Cruz Island, between the Transition Zone and Humid Zone. It is common to find it foraging on rocks and on litter. It is most commonly found in humid places; nevertheless it has been found in dry localities like Albany, Bowditch South and Espanola Island. (Pacheco and Mackay 2013)

Identification
A New World thief ant that is a member of the molesta species complex. (Key to New World Solenopsis Species Complexes)

Pacheco and Mackay (2013) – Worker - The worker is bicolored, with a golden-brown body and brown gaster. The head is rectangular and is semi-coarsely punctated. The lateral clypeal teeth are well-developed and the extralateral teeth are angulate. The frontal lobes are weakly, vertically striated. The pronotum has semi-coarse punctation. The propodeal margin is rounded, lacking a defined dorsopropodeum. The petiole forms a triangular node viewed laterally. Queen - The queen is minute (2 mm in total length) and is concolorous dark brown, with golden-brown appendages. The clypeal carinae are well defined and converge posteriorly. The frontal lobes are vertically striated. The medial ocellus is very small (0.03 mm). The metapleuron is horizontally striated. The petiole and postpetiole have roughened sculpturing.

(Modified from Pacheco et al. 2007). Solenopsis gnoma is one of three species of Solenopsis present on the Galapagos Islands and is apparently endemic. The other two species are Solenopsis globularia, a member of the former subgenus Euophthalma and Solenopsis geminata, a fire ant from the geminata species complex. Queens and workers of S. gnoma  are small even within the minute thief ants of the genus and its small size easily distinguishes it. Solenopsis gnoma is monomorphic while S. globularia and S. geminata are both polymorphic. However, both the major and minor workers of both S. globularia and S. geminata are easily distinguished from S. gnoma.

Solenopsis globularia is distinguished by its greatly dilated, globose postpetiole. Additionally, the clypeal carinae are welgnomal developed with five teeth present on the anterior clypeal margin. The extralateral teeth are present as angles, the lateral teeth are well developed and a medial tooth is present as well. The workers have horizontal striae present on the mesopleuron as well as the metapleuron and this species varies in color from light to dark brown. The eyes are large, with 15-25 ommatidia. Solenopsis globularia is most often found on beaches under rocks or in logs.

Solenopsis geminata workers are considerably larger than both S. gnoma and S. globularia, with a head length ranging from 1.06-2.20 mm, a length often larger than the total length of the workers of S. gnoma and ''S. globularia. Solenopsis geminata'' is distinguished as both the minors and majors lack horizontal striae on the mesopleuron and metapleuron. The petiolar peduncle of the workers have a thin flange ventrally, a character lacking in both S. globularia and S. gnoma . Color is also variable within this species ranging from red-orange to dark brown (occasionally bicolored) (Trager 1991).

Solenopsis gnoma is similar to Solenopsis tenuis, a species found in Colombia and Ecuador, but it is markedly smaller in total length in both the worker and queen castes. Solenopsis tenuis has smaller cephalic punctures and less developed lateral clypeal teeth. Solenopsis gnoma is also similar to the mainland species Solenopsis subtilis and Solenopsis sulfurea. Solenopsis gnoma can be distinguished from S. subtilis (eastern part of South America) as it has an elongated clypeus while S. subtilis has a more compact clypeus. If a queen is present in the series, the petiole and postpetiole are horizontally striated when viewed laterally for S. subtilis and with roughened sculpturing for S. gnoma. Solenopsis sulfurea (also eastern part of South America) has an elongated clypeus as does S. gnoma but has weakly defined clypeal carinae; well-defined with S. gnoma. Additionally, S. gnoma has angulate extralateral teeth, while the extralateral teeth are absent with S. sulfurea. If a queen of S. sulfurea is collected, it can be distinguished as it has a conspicuously elongated head with four well developed clypeal teeth.

Distribution
Ecuador, Galapagos Islands, Albany; Bowditch South; Espanola; Floreana; Isabela and Santa Cruz, Bellavista (type locality).

Distribution based on Regional Taxon Lists
Neotropical Region: Ecuador, Galapagos Islands.

Nomenclature

 *  gnoma. Solenopsis gnoma Pacheco, Herrera & Mackay, 2007: 1075 (w.q.) ECUADOR (Galapagos Is.)

Worker
Pacheco and Mackay (2013) - Measurements (n=7). TL 1.08-1.20 (1.16); HL 0.330-0.360 (0.353); HW 0.300; EL 0.036; ED 0.030; SL 0.250-0.258 (0.255); FSL 0.102; CI 83.3-90.9 (85.2); SI 70.0-75.8 (72.3); PL 0.060-0.066 (0.061); PW 0.078-0.084 (0.081); PI 71.4-78.6 (75.9); PPL 0.084-0.090 (0.086); PPW 0.108-0.114 (0.109); PPI 77.9-78.9 (78.1); WL 0.240-0.270 (0.259); PSL 0.030; PSW 0.030.

Small, bicolored, golden-brown head, brown gaster; head rectangular, longer than wide, semi-coarsely punctated; lateral clypeal teeth well developed, extralateral teeth angulate; clypeal carinae well defined, converge posteriorly on clypeus; frontal lobes vertically striated (difficult to see); scapes reach 3/4 length to posterior lateral comer of head, semi-coarsely punctate; minor funicular segments 3-8 short; eye small, black, 3-5 ommatidia; pronotum semi-coarsely punctate, mesopleuron smooth and shiny; metapleuron horizontally striated; propodeal spiracle small; posterior propodeal margin rounded, lacking defined dorsopropodeum; petiole forming triangular node, anterior and posterior face approximately 50 degree angles, lacking subpeduncular tooth; postpetiolar node semicircular viewed laterally, wider than petiole and oval-shaped when viewed dorsally, lacking tooth ventrally; gaster semi-coarsely punctated.

Abundantly hairy, pilosity yellow; erect and suberect hairs of various lengths (0.03-0.09 mm) covering all body surfaces; long (0.072-0.09 mm) suberect hairs on first gastral tergum, curve posteriorly.

Queen
Pacheco and Mackay (2013) - Measurements (n=3). TL 1.92-2.16 (2.08); HL 0.408-0.432 (0.420); HW 0.360; EL 0.120; ED 0.090-0.096 (0.094); MOL 0.030-0.036 (0.032); MOD 0.030-0.036 (0.032); SL 0.300; FSL 0.132; CI 83.3-88.2 (85.8); SI 69.4-73.5 (71.5); PSL 0.036-0.042 (0.038); PSW 0.030; PL 0.096-0.120 (0.108); PW 0.150; PI 64.0-80.0 (72.0); PPL 0.120-0.150 (0.130); PPW 0.174-0.180 (0.178); PPI 66.7-83.3 (72.9); WL 0.460-0.480 (0.473).

Small; concolorous dark brown with golden-brown appendages; head rectangular, longer than wide, sides nearly straight, posterior border nearly straight, semi-coarsely punctated; lateral clypeal teeth well developed, extralateral teeth angulate; clypeal carinae well defined; frontal lobes vertically striated; scape extends past medial ocellus; eyes small, black, extend 0.036 mm past lateral margin of head; ocelli minute, without pigment; mesosoma smooth and shiny; metapleuron horizontally striated; petiole and postpetiole with roughened sculpturing, both lacking tooth or flange ventrally, but with minute angle; first gastral tergum semi-coarsely punctated.

Abundantly hairy, pilosity light brown and yellow; erect and suberect hairs of various lengths (0.03-0.12 mm) covering all body surfaces; hairs on petiole and postpetiole longer (0.132 mm) than those on mesosoma, curve posteriorly; first tergum of gaster abundantly hairy with subsequent tergum nearly without any pilosity.

Type Material
Holotype queen, Puerto Ayora, Santa Cruz-Galapagos ( # 104994), ECUADOR, Galapagos, Santa Cruz, Bellavista, 00°38'18.4" S, 090°25'44.6" W, 20-vii-2005, Colecta Manual, H. Herrera # HWH 137 (2 paratype workers CDRS and 1 worker CASC # 104995, 1 paratype queen and 2 paratype workers ; 1 paratype queen and 2 paratype workers ).

Etymology
From Latin, gnomus, meaning diminutive fabled being, referring to the minute size of the worker and queen of this species (a noun in apposition).

References based on Global Ant Biodiversity Informatics

 * Dekoninck W., F. Fernandez, H. W. Herrera, N. Wauters, G. Brito, L. Jumbo, D. Marin-Armijos, and T. Delsinne. 2014. Results of ant collections on Santa Cruz Island within the framework of the 2012 Global Taxonomy Initiative Ant Course at Galápagos (Hymenoptera: Formicidae). Bulletin de la Société royale belge d’Entomologie 150: 250-255.
 * Herrera H. W. 2013. CDF Checklist of Galapagos Ants - FCD Lista de especies de Hormigas Galápagos. In: Bungartz, F., Herrera, H., Jaramillo, P., Tirado, N., Jiménez-Uzcátegui, G., Ruiz, D., Guézou, A. & Ziemmeck, F. (eds.). Charles Darwin Foundation Galapagos Species Checklist - Lista de Especies de Galápagos de la Fundación Charles Darwin. Charles Darwin Foundation / Fundación Charles Darwin, Puerto Ayora, Galapagos: http://checklists.datazone.darwinfoundation.org/terrestrial-invertebrates/formicidae/ Last updated: 01 Mar 2013
 * Pacheco J. A., and W. P. Mackay. 2013. The systematics and biology of the New World thief ants of the genus Solenopsis (Hymenoptera: Formicidae). Lewiston, New York: Edwin Mellen Press, 501 pp.
 * Pacheco, J.; Herrera, H. W.; Mackay, W. 2007. A new species of thief ant of the genus Solenopsis from the Galapagos Islands (Hymenoptera: Formicidae). Sociobiology 50:1075-1086.
 * Wauters N., W. Dekoninck, F. Hendrickx, H.W. Herrera, and D. Fournier. 2016. Habitat a ssociation a nd coexistence of endemic and introduced ant species in the Galápagos Islands. Ecological Entomology 41, 40–50.