Acromyrmex crassispinus

Acromyrmex crassispinus is the most common leaf-cutting ant species in southern Brazil (Rando & Forti, 2005). They prefer to nest in open areas (Nickele et al., 2009).

Distribution based on Regional Taxon Lists
Neotropical Region: Argentina, Brazil, Paraguay.

Biology
Nickele and Reis Filho (2015) studied the population dynamics of this species in São Mateus do Sul city (25°58’56,33”S, 50°23’49,26”W, alt. 766 m) state of Parana, Brazil. They worked in recently-planted Pinus taeda plantations (clear cut June-July, 2007 and replanted August, 2007). Initially the plantations contained no colonies of Acromyrmex crassispinus and within a few years the developing canopy created enough shade that none of the incipient colonies initially found, and studied, remained. The initial open nature of the tree plantation was a good area for the initiation of incipient nests, despite the subsequent poor nature of the site over a longer time frame.

The presence of A. crassispinus nests was observed from 15 months after planting (Spring/2008), where there was one nest per hectare, on average. Nest density rose to 26 nests per hectare at 30 months after planting (Summer/2010), then declined through time, Fifty-four months after planting, the forest canopy closed and at 72 months after planting, there was only 0.33 nests per hectare, on average. The few nests observed after 54 months after planting were located near tree gaps in the middle of planting.

In the spring of 2009, winged male or female ants were not observed in the nests sampled. In the spring of 2010, winged ants were observed in 50, 20, 20 and 10% of the nests sampled in September, October, November and December, respectively). Males emerge earlier than females. In several nests, while males were already adult, females were still in the pupal stage. Reproductives only occurred in the largest sized nests sampled. The presence of reproductive ants in sampled colonies only from the spring of 2010 (three years after planting) suggests the first nuptial flight of an A. crassispinus colony also occurs after the third year of the colony foundation.

Foraging
Nickele et al. (2015) studied this species in Paraná, Brazil, both in the field and lab, to elucidate details of their leaf transport. Some of their findings: In Acromyrmex crassispinus cutting and carrying of fragments were clearly separated activities performed by distinct worker groups differing in body size. Cutters were larger than carriers. In addition, the behavior of foragers of differed significantly according to variation in trail distances. On short trails (1 m), cutters frequently transported the fragments directly to the nest, whereas on long trails (more than 10 m), most cutters transferred the fragments to other workers. Transport chains (fragments found on the trail or directly received from nestmates are transported consecutively by different carriers) happened more frequently when workers harvested plants far from the nest. Transfer was mostly indirect, in other words, fragments were dropped on the ground and collected by outgoing workers that turned back and returned to the nest. Direct fragment transfers between workers were not observed under laboratory conditions. It was observed only on long trails in the field. Lopes et al. (2003) also did not observe direct fragment transfers for this species under laboratory conditions. These results demonstrate that Acromyrmex species display both division of labor between cutters and carriers, and task partitioning during leaf transport, with trail lengths showing marked effects on the likelihood of sequential transport. Furthermore, the results of this study provide support for the hypothesis that the behavioral response of transferring fragments in Acromyrmex species would have been selected for because of its positive effect on the information flow between workers.

Nomenclature

 *  crassispinus. Atta (Acromyrmex) mesonotalis var. crassispina Forel, 1909a: 257 (w.) PARAGUAY. Santschi, 1925a: 374 (q.). Combination in Acromyrmex: Emery, 1924d: 349. Subspecies of mesonotalis: Forel, 1914e: 11. Raised to species: Santschi, 1925a: 374. Senior synonym of: atratus, diabolica, insularis, mediocris, rusticus and material of the unavailable name rufescens referred here: Gonçalves, 1961: 139.
 * diabolica. Acromyrmex nigrosetosa var. diabolica Santschi, 1922b: 362 (w.) BRAZIL. Santschi, 1925d: 240 (m.). Subspecies of crassispinus: Santschi, 1925a: 375. Raised to species: Santschi, 1925d: 240. Junior synonym of crassispinus: Gonçalves, 1961: 139.
 * atratus. Acromyrmex hispidus st. atratus Santschi, 1925a: 376 (w.q.) ARGENTINA. Junior synonym of crassispinus: Gonçalves, 1961: 139.
 * insularis. Acromyrmex aspersus var. insularis Santschi, 1925d: 242 (w.) BRAZIL. Junior synonym of crassispinus: Gonçalves, 1961: 139.
 * mediocris. Acromyrmex diabolicus var. mediocris Santschi, 1925d: 241 (w.) BRAZIL. Junior synonym of crassispinus: Gonçalves, 1961: 139.
 * rusticus. Acromyrmex crassispinus st. rusticus Santschi, 1925a: 375 (w.q.) BRAZIL. Junior synonym of crassispinus: Gonçalves, 1961: 139.