Acromyrmex echinatior

Acromyrmex echinatior is a host species of the social parasite Acromyrmex insinuator.

Identification
Schultz et al. (1998) - Acromyrmex octospinosus var. echinatior was described by Forel (1900) and elevated to subspecies status by Wheeler (1937). These authors, as well as Santschi (1925), recognized A. octospinosus echinatior principally by the sculpture of the head and gaster of the major worker: “the tubercles on the posterior corners of the head, the pedicel, and the gaster are more developed and subspiniform, and some of those on the sides of the head are distinctly curved forward” (Wheeler, 1937).

WORKERS: In the fifty nests excavated in Panama, the largest major workers of A. echinatior are the same size as those of Acromyrmex octospinosus. However, a previous study by Bot and Boomsma (1997) found that pronotum width in A. echinatior (species 1 in that study) was significantly smaller than pronotum width in A. octospinosus (species 2 in that study). Qualitatively, major workers of the two species differ in the following ways: In A. echinatior the lateral pronotal spines are nearly vertical and parallel in frontal view, the vertical angle noticeably different from the angle of the anterior mesonotal spines, which diverge. In A. octospinosus, the anterior spines are not vertical and both pairs of spines diverge at approximately the same angle. Major workers of A. echinatior are hairier than those of A. octospinosus; e. g., at least some setae are present on the face of the propodeal dorsum in addition to those associated with the propodeal spines and with the anterior tubercles, whereas in A. octospinosus such setae are absent. In general, tubercles on the gaster of A. echinatior workers are sharp and dentiform to subspiniform, whereas those in A. octospinosus are low and blunted. Likewise, tubercles on the head of A. echinatior are sharp and spiniform, whereas those of A. octospinosus are shorter and blunter. We caution, however, that there is overlap between the two species in the form of the spines of the gaster and head and that these commonly cited characters are therefore not entirely reliable. Worker color is quite variable, ranging from yellow in callows to yellowish-ferrugineous to ferrugineous, with, as noted by Wheeler (1937) some workers acquiring a “bluish bloom.”

When workers from the entire range of both species are taken into account, the most constant distinguishing characters are the form of the spines on the head and gaster and to a lesser extent the differing angles of the lateral pronotal vs. anterior mesonotal spines. Major workers of A. echinatior from Costa Rica, Nicaragua, Guatemala, and Mexico, including the lectotype, frequently lack setae on the propodeal dorsum and are often much larger than those from the Panamanian nests.

FEMALES: In the Panamanian nests, A. echinatior females are smaller than Acromyrmex octospinosus females, and differ from them in the presence of a pigment spot entirely surrounding the ocelli (absent in the Panamanian A. octospinosus), the presence of setae on the propodeal dorsum (absent in A. octospinosus), and the presence of a broadly convex median anteroventral postpetiolar extension (variable in A. octospinosus). The occipital tubercle is thin and sharp and the tubercles on the first gastric tergite are sharp and dentiform; in A. octospinosus the occipital tubercle is thick and blunt, and the gastric tubercles are blunt and rounded. Color variation in females corresponds to that in workers. Over the whole of the species’ Central American range, A. echinatior females are more variable in size than in the Panamanian sample, tending to be larger, and variable in the presence/absence of the ocellar pigment spot.

MALES: In the Panamanian nests, A. echinatior males are the same size as A. octospinosus males, but differ from them by the presence of a pigmented frontal triangle that is entirely delineated by rugae (unpigmented and ill-defined in A. octospinosus), the presence of setae on the propodeal dorsum (absent in A. octospinosus), and the presence of a broadly convex median anteroventral postpetiolar extension (variable in A. octospinosus). Color is yellow-ferrugineous. A. echinatior males over the rest of the species’ range are variable in the characters of the frontal triangle.

Assigning the Panamanian specimens to A. echinatior is not without problems. The largest Panamanian workers are smaller than the lectotype and smaller than other worker specimens from Costa Rica, Nicaragua, Guatemala, and Mexico. Differences also exist in the characters of the propodeal setae (all castes), the ocellar pigment spot in females, and the form of the frontal triangle in males. Although the conservative position taken here is that these differences fall within the normal range of variation expected from a species distributed over a wide area, we would not be at all surprised to find that both Acromyrmex octospinosus sensu lato and Acromyrmex echinatior are composed of a number of cryptic species. If this is established by future research, then the Panamanian host of Acromyrmex insinuator may require species status separate from A. echinatior.

Distribution based on Regional Taxon Lists
Neotropical Region: Costa Rica, El Salvador, Guatemala, Honduras, Mexico, Nicaragua, Panama.

Biology
Nehring et al. (2015) - This species and the sympatric Acromyrmex octospinosus are both parasitized by Acromyrmex insinuator but the parasitic queens are only able to successfully reproduce in A. echinatior nests.

Reproduction
Liberti et al. (2018) studied sperm competition in this species. Queens of A. echinatior mate with multiple males. An earlier study (Liberti et al. 2016) found that queens produce reproductive tract fluid that enhances sperm motility. This increases the possibility that the subsequently stored sperm is viable. This current study found sperm motility increased when exposed to other male ejaculates. This increased activity was similar to what was observed with exposure to reproductive tract fluid in queens. Liberti et al. concluded "Our results suggest that ant sperm respond via a self–non-self recognition mechanism to similar or shared molecules expressed in the reproductive secretions of both sexes. Lower sperm motility in the presence of own seminal fluid indicates that enhanced motility is costly and may trade-off with sperm viability during sperm storage, consistent with studies in vertebrates. Our results imply that ant spermatozoa have evolved to adjust their energetic expenditure during insemination depending on the perceived level of sperm competition."

Genetics
Acromyrmex echinatior has had their entire genome sequenced.

Palomeque et al. (2015) found class II mariner elements, a form of transposable elements, in the genome of this ant.

Nomenclature

 *  echinatior. Atta (Acromyrmex) octospinosa var. echinatior Forel, 1899c: 34 (w.q.) MEXICO. Wheeler, W.M. 1937c: 72 (m.). Combination in Acromyrmex: Emery, 1924d: 350. Subspecies of octospinosus: Wheeler, W.M. 1937c: 71. Raised to species: Schultz, Bekkevold & Boomsma, 1998: 460.

Type Material
Schultz et al. (1998) - LECTOTYPE: Major worker. Guatemala: Senahuen Vera Paz, El Reposo, Zapote, 800 ft. (Champion). A. Forel Collection, Muséum d’Histoire naturelle, Geneva, Switzerland. Paratypes examined: 2 minor workers: Guatemala: Senahuen Vera Paz, El Reposo, 800 ft. (Champion). 1 alate and 1 dealate female: Panama: Volcan de Chiriquí, 25–1000 ft. (Champion). 2 alate females: Panama: Bugaba (Champion).

The list of specimens published in Forel’s (1899) description includes two workers, one from Guatemala and one from Costa Rica, the latter now apparently lost. Forel (1899) did not designate a holotype; five pins (seven specimens) in the syntype series bear the designation “type” written in Forel’s hand. Affixed to two of Forel’s syntype pins, one bearing a single major worker, the other bearing two females, are red “Typus” labels, but these may have been added subsequently and at any rate have no formal standing. Wheeler (1937) designated the type locality of A. octospinosus echinatior as Volcan de Chiriquí, Panama, the collection locality of two of Forel’s syntype females, but did not designate a lectotype. We have chosen to ignore this action and to designate the only remaining major worker in Forel’s syntype series as the lectotype for two important reasons: (1) species concepts in Acromyrmex are based entirely upon the characters of major workers and (2) Forel’s syntype females vary in size and collection locality, raising the possibility that they represent multiple species.

Worker
Schultz et al. (1998) - LECTOTYPE: HL = 2.16; HW = 2.64; WL = 3.72; SL = 2.64; maximum diameter of eye = 0.43.