Tetramorium spinosum (Pergande) Bolton, B., 1979

Bolton, B., 1979, The ant tribe Tetramoriini (Hymenoptera: Formicidae). The genus Tetramorium Mayr in the Malagasy region and in the New World., Bulletin of the British Museum (Natural History) Entomology 38, pp. 129-181 : 163-164

Tetramorium spinosum (Pergande) comb. n.

(Fig. 55)

Xiphomyrmex spinosus Pergande, 1896: 894 . LECTOTYPE and three paralectotype workers, Mexico: Baja California, Sierra San Lazaro, Cape Region (Eisen & Vaslii) (USNM, Washington), here designated [examined]. Tetramorium {Xiphomyrmex) wheeleri Forel, 1901: 128 . Syntype workers, Mexico: Zacatecas, Pacheco (W. M. Wheeler) (MHN, Geneva) [examined]. Syn. n. Xiphomyrmex spinosus subsp. insons Wheeler, 1915: 416. Syntype workers, U. S. A.: Texas, Austin (W. M. Wheeler) (MCZ, Cambridge; BMNH) [examined]. Syn. n.

Worker. TL 3.6 - 5.1, HL 0.84 - 1.12, HW 0.77 - 1.04, CI 88 - 96, SL 0.62 - 0.88, SI 79 - 90, PW 0.58 - 0.82, AL 0.98 - 1.42 (100 measured).

Mandibles strongly longitudinally striate. Anterior clypeal margin usually with a median impression but this may be very shallow or vestigial in some samples. Frontal carinae strongly developed, running back well beyond the level of the posterior margins of the eyes but towards the occipital corners fading out and blending into the occipital rugoreticulum. Throughout their length the frontal carinae with a raised, semitranslucent ridge which is highest anteriorly and gradually becomes lower posteriorly. Eyes moderate to fairly large, maximum diameter 0.18 - 0.24, about 0.20 - 0.25 x HW but with relatively few samples in the upper range. Propodeal spines varying from elongate-triangular to long-spiniform, with all intermediates. Similarly, the metapleural lobes varying from low, broadly triangular structures to elongate spiniform teeth. Dorsum of head longitudinally rugulose, the rugulae irregular or sinuate along their length. Reticular cross-meshes usually present in western samples but tending to be reduced or absent in those from the east, but a rugoreticulum present occipitally in all cases. Dorsal alitrunk rugose, predominantly longitudinally so but with a rugoreticulum present at least on the pronotum; sometimes everywhere reticulate-rugose. Dorsum of petiole and postpetiole irregularly rugose or rugulose, the former more strongly so than the latter. First gastral tergite varying from completely smooth to strongly punctulate basally, with all intermediate phases apparent. Pilosity on all dorsal surfaces of head and body elongate, fine and dense, acute apically; the longest hairs on the dorsal alitrunk longer than the maximum diameter of the eye. Hairs on leading edge of antennal scapes and on dorsal (outer) surfaces of hind tibiae varying from erect to subdecumbent but always shorter than the maximum diameter of the appendage from which they arise. Colour reddish yellow to reddish brown, often with the gaster lighter in shade than the head and alitrunk.

This is the most common, most widely distributed and most variable species of the spinosumcomplex in North America. Variation in spinosum takes the form of a rough double cline, one running from west to east and the other from north to south. Predominant variation on the west-east axis, which runs from Baja California across to Texas, is the reduction of sculpture on the first gastral tergite. In specimens from Baja California the base of the tergite is usually distinctly sculptured, but further east in Arizona it is reduced to fainter markings and in Texan material the gaster is smooth. Exceptions to the trend occur in Jalisco where specimens without gastral sculpture are quite common, but despite this it is now obvious that gastral sculpture is of no use in separating the species of this complex. On the north-south axis the most obvious variation is in the length of the propodeal spines, which start off quite short in Arizona and other northern areas and show an overall gradual increase in length as one moves south, ending up long and narrow in Jalisco and Zacatecas. To a lesser degree the metapleural lobes share in this trend as specimens from Jalisco tend to have them much longer and more definitely spiniform than do specimens from further north where they tend to be more markedly triangular. In some specimens from Arizona the metapleural lobes are low and very broad, approaching the condition seen in hispidum .

The above discussion shows trends in variation, but it should be noted that here and there odd samples form exceptions to the rule and other variation, which appears to be sporadic, is also present. This includes the density and degree of elevation of tibial pilosity and intensity of sculpture. The second of these does not appear to have any pattern to it, but the tibial pilosity can be summarized thus: specimens from Texas and Nuevo Leon tend to have numerous fine hairs on the dorsal (outer) tibial surface which are subdecumbent, gently curved along their length and inclined towards the tibial apex. Material from Jalisco, on the other hand, tends to have fewer hairs on the outer tibial surface and those present are generally suberect and straight. In intermediate zones (Arizona to W. Texas) and in Baja California both forms occur as do intergrades between the two extremes.

The three other species of the complex which occur inside the vast range of spinosum are best separated from it by reference to characters which the central species does not possess, such as long pilosity and elongate antennal scapes in mexicanum , small size and unsculptured postpetiole in placidum and large eyes and short stubbly pilosity in hispidum .

Material examined

U. S. A.: Texas, Austin (W. M. Wheeler); Austin {R. A. Cushmari); Texas, Bulverde (D. H. Bixby); Brownsville {H. S. Barber); Brownsville {W. S. Ross); Bexar Co., Helotes; Texas, Ozona (A. C. Cole); Texas, Junction {S. E. Aldous); Beeville {Pergande); San Diego {Pergande?); Texas, Del Rio {W. M. Mann); Bracketville (M. P. Creighton); Arizona, Cochise Co., Huachuca Mts, Ash Canyon (R. R. Snelling); Carr Canyon {R. R. Snelling); Ramsey Canyon {W. S. Creighton); Pinaleno Mts, Post Canyon {W. M. Wheeler); Santa Cniz Co., Pena Blanca Spring {Bryan). Mexico: Nuevo Leon, nr Linares (JE. M. & J. L. Fisher); Sonora, Nogales; Baja California, Los Parras {R. R. Snelling); Los Parras {W. M. Mann); Baja California, Purissima (If. M. Mann); Loreta {W. M. Mann); Jalisco, Atenquique {Dixon & Heyer); Nevado de Colima {A. Newton); S. of Mazamitin (E. S. Ross); Jalisco, Cocula {W. M. Mann); Cocula, San Diego {W. M. Mann); Michoacan, Uruapan {W. M. Mann); Nayarit, Tepic, Santiago (T. Pergande).