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Worker: Length 2-6 mm. - Anterior part of ridge of vertex low and faint, posterior half produced as high and acute ridge. Occipital spines long, acute, deflected apically, other tubercles of head large, distinct, acute. Inferior pronotal spines well developed, acute, directed anteriorly and slightly outward. Lateral pronotal spines exceeding mesonotal spines in length, acute, directed forward. Medial pronotal spines exceeding mesonotal spines in length, acute, directed forward. Medial pronotal spines very distinct, acute, rising abruptly. Anterior mesonotal spines large, stout, acutely conic. Posterior mesonotal spines much smaller, somewhat variable in size, slender, directed backward at a slight angle. Epinotal spines of length comparable with lateral pronotal spines but more slender and gently curving backward in most specimens. Some workers with spines straight and directed upward. Pedicel and gaster strongly tuberculate. Surface of body dull. Pubescence sparse and appressed. Sparse, recurved hairs on spines and appendages. - Dark ferrugineous; medial darker streak on gaster.
Two winged female taken by myself June 20, 1936, near the Oko River, tributary of the Cuyuni River, British Guiana, doubtless belong to this variety. They were captured singly about 9 a. m. as they flew through the bush. Since they do not belong to A. histrix HNS or A. octospinosa HNS , the two (other known species of this region, and since intensive collecting revealed none but these three forms, jthey are here described:
Female (undescribed): Length 7 mm. - Ridges of vertex distinct and complete. Occipital spines short, stout, acute. Inferior pronotal spines flattened, acute, straight, directed forward and outward. Superior pronotal spines long, acute, directed forward and outward. Epinotal spines stout, rising upward, then sharply deflected backward and outward. Gaster irregularly tuberculate. - Entire body densely vermiculate-rugulose. Hairs short, curved irregularly. Fine sparse pubescence. Colored strikingly in dark brown and yellowish brown patches. Mesonotum yellowish brown with an antero-median blotch and lateral elongate ellipsoid blotches of dark brown. Dorsum of first gastric segment with a sharply defined hastate patch of dark brown on yellowish brown background.
In Santschi's 1935 Revision of Acromyrmex HNS this variety (British Guiana - type locality) is described, from 4 workers, as having clearly recurved inferior pronotal spines, barely distinct medial pronotal spines, and a grayish pruinose surface. Four colonies taken by myself in 1935 near the Forest Settlement, Mazaruni River, and in 1936 near the Takutu River, British Guiana demonstrate the variability of these characters. Rather than introduce a separate name I have redescribed the worker of this variety on the basis of a large series.