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.

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

Biology
Yamada et al. (2018) - Colonies of A. careoscrobis can produce both alate and ergatoid queens. Ergatoid queens may function as cheap secondary reproductive females, whereas it is at present unclear whether they engage in dependent colony foundation or not. Ergatoid queens have the same number of ovarioles as alate queens, and show variable intermediate external morphology between major workers and alate queens. The colony redistributes nutrients via trophic eggs among colony members. Reviewing current available data for other Acanthomyrmex species, this study highlights the great prevalence and diversity of ergatoid queens in this poorly-studied genus.

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}}

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).

Taxonomic Notes
Yamada et al. (2018) - External morphology of ergatoid queens A total of eight ergatoid queens was examined. Ergatoid queens resembled majors in general appearance due to their enlarged head and reduced mesosoma, but exhibited variable characteristics intermediate between majors and alate queens. In comparison to majors, ergatoid queens had smaller head size [HW × HL = 4.60 ± 0.48 in ergatoid queens (n = 8), 5.53 ± 0.41 in majors (n = 11), average ± SD; Welch’s t-test, t = 4.41, df = 13.6, P < 0.001] and larger mesosomal length [ML= 0.67 ± 0.03 mm in ergatoid queens (n = 8), 0.58 ± 0.04 mm in majors (n = 11), average ± SD; Welch’s t-test, t = 5.07, df = 16.6, P < 0.001], although there was overlap. All ergatoid queens had ocelli (at least median ocellus) except for a single individual from FI09-103 (Colony Code). The individual lacking ocelli also resembled majors most closely, with a comparable head size, but was distinguished from majors by the structure of the mesoscutellum, which was vestigial but recognizable as a small process in lateral view. Six of the other ergatoid queens from FI11-88, FI98-77, FI99-418, and FI09-103 were distinguished from majors by the combinations of the following characteristics: i) head size almost comparable with majors or reduced to intermediate values between majors and alate queens; ii) presence of at least a median ocellus; and iii) mesoscutellum vestigial or conspicuously raised. One ergatoid queen from FI99-418 was particularly distinct from majors and rather close to alate queens in morphology in having the following characteristics: head greatly reduced in size, with broadly concave posterior margin in full-face view; both median and lateral ocelli present (black arrows in Fig. 17); ML largest among all ergatoid queens measured; pronotum larger than in other ergatoid queens; forewing rudiments present (black arrows in Fig. 19); and mesopleuron separated from metapleuron.

Taxonomic remarks The minor from Ulu Gombak was differentiated from the holotype (minor) from Gunung Mulu National Park (given in AntWeb: https://www.antweb.org/specimen/CASENT0901703) as follows: the former has normal standing hairs on the abdominal tergite III (gastral tergite I) and IV, but the latter lacks these; the former has a little shorter pronotal spine (PnSI 51 – 60) and more elongate petiole (PI 120 – 133) than the latter (PnSI 65; PI 110).

Worker
Minor. Holotype 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 Acanthomyrmex 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.

Yamada et al. (2018) - HW 2.03 – 2.27 mm; HL 2.43 – 2.73 mm; EL 0.21 – 0.27 mm; SL 0.97 – 1.05 mm; ML 0.55 – 0.68 mm; PdSL 0.49 – 0.62 mm; PL 0.60 – 0.69 mm; PH 0.44 – 0.52 mm; PPL 0.29 – 0.34 mm; PPH 0.39 – 0.47 mm; CI 82 – 88; EI 10 – 12; SI 44 – 50; PdSI 84 – 99; PI 131 – 141; PPI 72 – 77 (n = 11).

Structure. Cranium in full-face view with posterior margin strongly and narrowly concave medially; dorsal margin in lateral view strongly raised dorsad in frons and faintly impressed in vertex. Frons and vertex medially with an inconspicuous narrow longitudinal sulcus accompanied with a shallow depression on vertex. Funicular scrobe conspicuous but short. Antennal scape when folded back not reaching midlength of cranium in full-face view. Median part of clypeus produced anteriad, with anterior margin concave medially. Promesonotal suture present dorsally as a weak groove. Propodeal spine in lateral view broad, weakly downcurved at basal part, faintly upcurved near apex. Anterior peduncle of petiole in lateral view approximately a little longer than half length of entire petiole. Petiolar node in lateral view inverted V-shaped; in posterior view dorsal margin between dorsolateral angles concave; apical width measured between the angles approximately as large as or a little larger than basal width of the node. Postpetiolar tergite in lateral view just slightly convex without forming prominent dorsal uplift.

Sculpture. Dorsal and lateral faces of cranium foveolate-reticulate; foveae shallower and sparser on vertex and ventrolateral face than on frons where virtually reticulated; gena below antennal scrobe roughly reticulate. Pronotum smooth. Mesonotum with sparse large foveae. Mesopleuron, metapleuron, and lateral face of propodeum coarsely rugose. Petiole largely smooth. Postpetiolar tergite smooth on anterodorsal face and with two longitudinal rugae on lateral face (but often coarse and ambiguous).

Pilosity. Dorsal and lateral faces of cranium with relatively dense standing hairs. Clypeus with sparse standing hairs among short appressed hairs. Antennal scape with sparse standing hairs on leading edge and dorsoapical part. Pronotum with sparse, short appressed hairs, and just 1 – 2 standing hairs near dorsoposterolateral corner. Mesonotum with sparse standing hairs. Propodeal spine only with a few very short appressed hairs. Propodeal lobe with some standing hairs on outer face. Outer faces of femur and tibia of each leg with sparse vertically standing hairs among short appressed hairs. Petiole with 1 – 3 standing hairs on lateral face, and 3 – 5 standing hairs on posterior slope of node. Postpetiolar tergite with 13 – 16 standing hairs. Abdominal tergite III with sparse standing hairs among short appressed hairs. Abdominal tergite IV with a sparse and transverse series of standing hairs near the posterior margin.

Minor

HW 1.03 – 1.14 mm; HL 0.95 – 1.01 mm; EL 0.17 – 0.20 mm; SL 0.89 – 0.94 mm; ML 0.41 – 0.47 mm; PnSL 0.22 – 0.25 mm; PdSL 0.43 – 0.48 mm; PL 0.42 – 0.46 mm; PH 0.33 – 0.37 mm; PPL 0.20 – 0.23 mm; PPH 0.24 – 0.27 mm; CI 108 – 114; EI 17 – 18; SI 83 – 88; PnSI 51 – 60; PdSI 99 – 113; PI 120 – 133; PPI 77 – 90 (n = 8).

(redescription based on specimens from Ulu Gombak; see also “Taxonomic remarks” above the Description heading). Structure. Cranium in full-face view with posterior margin strongly and broadly concave medially; posteroventral margin in lateral view faintly concave. Funicular scrobe absent. Anterior margin of the median part of clypeus with just two weak submedian processes from which oblique-longitudinal submedian ridge run backward (the ridge occasionally coarsely divergent or distorted). Antennal scape when folded back extending a little beyond posterolateral corner of cranium in full-face view; basal corner of leading edge of scape shaft not forming a conspicuous longitudinal lobe. Pronotal spine in lateral view short, faintly curved anteriad. Propodeal spine in latetal view much longer than pronotal spine, entirely weakly downcurved. Anterior peduncle of petiole in lateral view approximately a little longer than half length of entire petiole. Petiolar node in lateral view inverted V-shaped; in posterior view dorsal margin between dorsolateral angles concave; apical width measured between the angles narrower than basal width of the node. Postpetiolar tergite in lateral view just slightly convex without forming prominent dorsal uplift.

Sculpture. Dorsal and lateral faces of cranium, anterodorsal and lateral faces of promesonotum foveolate-reticulate; remaining face of mesosoma (except for posterior face of propodeum) with rugae which run longitudinally in dorsal part but become irregularly in ventral part of lateral face. Petiole largely smooth. Postpetiolar tergite smooth on anterodorsal face, and with two longitudinal rugae on lateral face.

Pilosity. Dorsal and lateral faces of cranium with relatively dense standing hairs. Area between two submedian ridges of clypeus without standing hairs except for a long and thick median seta on the anterior margin. Antennal scape with sparse standing hairs among short appressed hairs on leading edge and dorsoapical part. Dorsolateral face of pronotum and dorsum of promesonotum with sparse standing hairs. Pronotal spine with a few short appressed hairs and 2 – 3 standing hairs. Propodeal spine without standing hairs but with a few short appressed hairs. Propodeal lobe with a few standing hairs on outer face. Outer faces of femur and tibia of each leg with sparse vertically standing hairs among short appressed hairs. Petiole with 1 – 2 standing hairs on lateral face and two standing hairs on posterior base of node. Postpetiolar tergite with 7 – 10 standing hairs. Abdominal tergite III with sparse standing hairs among short appressed hairs. Abdominal tergite IV with a sparse and transverse series of standing hairs near the posterior margin.

Queen
Yamada et al. (2018) - Structure. Cranium in full-face view with posterior margin weakly and broadly concave medially. Frons with a weak narrow median longitudinal sulcus running backward from posterior clypeal margin and ending at the fovea encircled median ocellus. Vertex medially with shallow depression. Funicular scrobe conspicuous but short. Mesoscutellum in lateral view roundly raised posterodorsad without prominent tapering posteriorly; its dorsal outline not exceeding imaginary line tangent to dorsal outline of mesoscutum. Mesoscutellum in dorsal view subtrapezoidal, with posteromedian margin weakly concave. Propodeal spine in lateral view, almost straight, faintly upcurved at apex. Petiole and postpetiole mostly as in the major.

Sculpture. Dorsal and lateral faces of cranium entirely foveolate-reticulate. Pronotum smooth with a few foveae. Mesoscutum with submedially rugose-foveolate. Mesoscutellum with sparse large foveae. Mesopleuron largely smooth just with some large foveae. Metapleuron and lateral face of propodeum coarsely rugose. Petiole and postpetiole as in the major.

Pilosity. Cranium, antenna, clypeus, and pronotum as in the major. Mesoscutum, mesoscutellum with sparse standing hairs. Katepisternum, metapleuron, and lateral face of propodeum with a few standing hairs respectively. Propodeal spine with 1 – 3 standing hairs and a few very short appressed hairs. Propodeal lobe and legs as in the major. Petiole with 2 – 5 standing hairs on lateral face, and 2 – 7 standing hairs on posterior slope of the node. Postpetiolar tergite with 19 – 21 standing hairs. Abdominal tergite III and IV as in the major.

Ergatoid (See “External morphology of ergatoid queens” above the Description heading). Measurement and indices. HW 1.94 – 2.18 mm; HL 2.08 – 2.54 mm; EL 0.23 – 0.28 mm; OL 0.06 – 0.08 mm; SL 0.94 – 0.99 mm; ML 0.62 – 0.73 mm; PdSL 0.50 – 0.64 mm; PL 0.64 – 0.72 mm; PH 0.45 – 0.51 mm; PPL 0.26 – 0.33 mm; PPH 0.40 – 0.55 mm; CI 84 – 94; EI 12 – 14; OI 2 – 4; SI 47 – 51; PdSI 74 – 98; PI 132 – 145; PPI 46 – 81 (n = 7 for OL and OI, n = 8 for the others).

Male
Yamada et al. (2018) - HW 0.94 – 1.02 mm; HL 0.66 – 0.70 mm; EL 0.32 – 0.34 mm; OL 0.07 – 0.09 mm; SL 0.13 – 0.16 mm; ML 0.55 – 0.64 mm; PL 0.42 – 0.48 mm; PH 0.19 – 0.23 mm; PPL 0.20 – 0.26 mm; PPH 0.20 – 0.24 mm; PPW 0.20 – 0.22; CI 137 – 145; EI 33 – 35; OI 8 – 9; SI 14 – 16; PI 204 – 224; PPI 96 – 117 (n = 5).

Color. Body dark brown to black; antennae, mandibles, legs and gaster yellowish. Structure. Cranium in full-face view subrectangular, wider than long, with posterior margin slightly and broadly concave. Frontal carina virtually absent or present just as rugulae. Antennal scrobe present as a short, broad and shallow depression. Median part of clypeus produced anteriad, with anterior margin weakly convex. Antennal scape very short, when folded back, not reaching transverse line spanning the posteriormost points of compound eyes. Antennal segment II almost as long as broad, much shorter than antennal scape; III – VIII much longer than broad; III longest, slightly flattened dorsoventrally, in dorsal view with almost straight lateral outline; IV – VIII almost same length; each of them longer than IX; each of IV – VII slightly to weakly bent; VIII almost straight or slightly bent; IX – XII almost same length; IX relatively strongly bent; XIII a little longer than IX, almost as long as VIII. Masticatory margin of mandible with 3 – 4 distinct teeth between a relatively large apical tooth and basal angle. Mesoscutellum in lateral view roundly raised posterodorsad, in dorsal view subtrapezoidal with posteromedian margin almost straight. Posterodorsolateral part of propodeum with a short lobate process directed posteroventrad. Venation of wings as in. Petiolar node in lateral view very low and weakly roundly raised, in posterior view with its dorsal margin almost straight; posterior slope behind the apex of node very gently descend. Dorsal margin of postpetiolar tergite in lateral view almost straight.

Sculpture. Cranium weakly and coarsely foveolate-reticulate. Pronotum largely smooth. Mesoscutum coarsely microrugose, and overlaid with relatively shallow and irregular foveae on the submedian face. Mesoscutellum coarsely reticulate. Anepisternum relatively smooth, but weakly microreticulate on posterior part. Katepisternum coarsely and irregularly reticulate. Metapleuron and lateral face of propodeum coarsely and irregularly rugose-reticulate. Petiole and postpetiolar tergite largely smooth, but weakly and coarsely rugose on a part of lateral face. Pilosity. Highly variable among individuals even in a single colony: cranium, dorsum of mesosoma, abdominal tergite III with few standing hairs, or with standing and/or decumbent hairs relatively dense; femur and tibia of each leg with sparse decumbent hairs but with few standing hairs, or with sparse decumbent to standing hairs; petiole and postpetiole with few hairs, or with several decumbent to standing hairs.

Genitalia. Pygostyle digitiform with long hairs on the apical half. Abdominal sternite IX a little wider than long to a little longer than wide (including spiculum); spiculum long, 0.37 – 0.51 times as long as entire length of the sternite IX when spiculum length measured from transverse line spanning the posteriormost points of each anterolateral margin; posterior part gently tapering posteriad, forming lobe subparabolic in outline, with several short to long hairs on outer face of apical part of the lobe. Cupula wider than long. Paramere with acute gonostipital arm, in lateral view with distinctly concave ventral outline around the articulation between basimere and telomere. Telomere in lateral view longer than high. Cuspis virtually absent or present just as small lobe in inner lateral view. Digitus in lateral view long, with round apex, entirely downcurved and necked basally. Valviceps in lateral view almost as long as wide and gently tapering posteriad, forming a broadly rounded apex; anteroventral corner weakly produced anteriad; ventral margin with 11 – 18 denticles.

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