Acanthomyrmex careoscrobis

Nests are found in cavities in dead twigs or rotten wood, under or between stones and in litter on the forest floor. They usually contain tiny fig seeds

Identification
Moffett (1986) - Similar to Acanthomyrmex mindanao, but lacking even a poorly developed funicular scrobe; without strongly projecting medial clypeal lobes. Major worker unknown.

Distribution based on Regional Taxon Lists
Indo-Australian Region: Borneo, Indonesia, Malaysia.

Biology
Yamada et al. (2018) - Eight colonies contained 22.6 ±11.1 workers and 1.8 ±0.9 soldiers. Six colonies contained a single dealate queen, whereas two colonies had a mated ergatoid queen instead. Alate queens (n = 4), ergatoid queens (n = 8), and majors (n = 7) always had six ovarioles per individual (three ovarioles per ovary). Minors (n = 23) always had two ovarioles per individual(one ovariole per ovary).

Ninety-six hours of observations of egg-laying behavior of a laboratory colony with 1 ergatoid queen, 1 major, and 20 minors: A total of three ovipositions by the ergatoid queen, four by the major and 21 by the minors were recorded during 96 hours of observation. There were two morphologically distinguishable types of eggs (constant-sized and bean-shaped reproductive eggs vs. variable-sized and round trophic eggs), as also reported for Acanthomyrmex ferox (Gobin & Ito 2000). In total, egg-laying of two reproductive and 26 trophic eggs was observed. Reproductive eggs were laid by the ergatoid queen only, whereas trophic eggs were laid by the ergatoid queen, major and minors. The majority of the trophic eggs were given to larvae, but the ergatoid queen and major also fed on trophic eggs laid by minors.

There were 7-250 fig seeds found inside the nests, confirming the prevalence of this specialized diet in the genus. The holotype was collected from a mixed dipterocarp forest.

Castes
Colonies of A. careoscrobis can produce both winged and ergatoid queens (queen polymorphism). Ergatoid queens may function as cheaper secondary reproductive females. Both queen types have the same number of ovarioles, however ergatoid queens show variable intermediate external morphology between soldiers and winged queens.

Nomenclature

 * . Acanthomyrmex careoscrobis Moffett, 1986c: 78, figs. 39A, 40-43 (w.) BORNEO (East Malaysia: Sarawak).
 * Yamada, Ito, et al. 2018: 7, 18 (s. ergatoid q., m.).
 * Status as species: Bolton, 1995b: 53; Pfeiffer, et al. 2011: 44; Yamada, Ito, et al. 2018: 15 (redescription).{nomenplus}}

Worker
Minor. Holotype measures HW 0.94, HL 0.79 (CI 118), ML 0.70 (MI 88), SL 0.80 (SI 86), EL 0.16, HFL 0.83 (FLI 105; FWI 24) mm. Head shape similar to A. mindanao, but slightly more deeply concave across posterior margin. Maximum head width virtually the same above and below the level of the eyes. Head with rounded foveae having thick walls; dorsally the longest hairs extend about 0.10 mm. Without any trace of grooves to retain retracted funiculi adjacent to scrobes for each scape. Scapes relatively short, barely overreaching back margin of head when retracted to scrobes. Clypeal index 178. Lobe to each side of medial clypeal hair feeble (essentially absent) and widely separated from the hair; lateral clypeal hairs apparently not in a completely enclosed fovea, but sculpture difficult to interpret. Smooth medial area of clypeus bordered by longitudinal rugae laterally. Mandibles lacking a ventral tooth.

Spines on trunk relatively longer than in A. mindanao, with pronotal spines in particular not as reduced in length. Pronotal spines with one hair or none. Pronotal angle not forming a feeble tooth. Propodeal declivity bordered on each side by a single rugum.

Node of petiole as in figure, PWI narrow (ca. 49). Anterior peduncle of petiole shorter and deeper than in A. mindanao and other species in the genus, and with lateral petiolar hairs present and sublateral hairs lacking; subpetiolar declivity present. Postpetiole similar to A. mindanao; with two pairs of hairs dorsally and two lateral pairs. Femora with scattered hairs throughout length, and with ventral surfaces distinctly concave; hind femora relatively short (FLI < 110). Orange yellow, with legs yellow.

Type Material
Holotype. Minor worker deposited in from Malaysia: Sarawak: 4th Div., Gunung Mulu National Park, mixed dipterocarp forest, 200 m, pitfall trap, 14/III/1978 (N. M. Collins).

Etymology
Name derived from Latin careo + scrobis, referring to the lack of scrobes for retracted funiculi.

References based on Global Ant Biodiversity Informatics

 * Moffett, M. 1986. Revision of the myrmicinae genus Acanthomyrmex (Hymenoptera: Formicidae). Bulletin of the Museum of Comparative Zoology 151(2):55-89.
 * Moffett, M. W. 1986. Revision of the myrmicine genus Acanthomyrmex (Hymenoptera: Formicidae). Bulletin of the Museum of Comparative Zoology 151:55-89.
 * Pfeiffer M.; Mezger, D.; Hosoishi, S.; Bakhtiar, E. Y.; Kohout, R. J. 2011. The Formicidae of Borneo (Insecta: Hymenoptera): a preliminary species list. Asian Myrmecology 4:9-58