Key to Pogonomyrmex californicus species group

The following text and worker key is from: Johnson, R.A., Overson, R.P. & Moreau, C.S. 2013. A new species of seed-harvester ant, Pogonomyrmex hoelldobleri (Hymenoptera: Formicidae), from the Mohave and Sonoran deserts of North America. Zootaxa 3646, 201-227.

Key to the workers in the Pogonomyrmex californicus group from central and western North America. (P. anzensis is included, though it may not belong in this species group)

Identification of species in the P. californicus group can be challenging given the considerable degree of morphological variation, both within and among nests (see Cole, 1968; Ward, 2005). In the experience of RAJ, and as noted by Cole (1968), it is particularly important to observe these species in the field because characters such as nest structure, nest sites, microhabitat, and behavior provide valuable information to make an accurate identification. Such observations and collections are especially helpful when difficult to identify species occur in sympatry (e.g., P. magnacanthus and P. hoelldobleri, P. mohavensis and P. californicus, P. hoelldobleri and P. californicus, and P. maricopa and P. californicus).

Circumocular whorls, which are generally considered to be a diagnostic character for species in the P. californicus group (e.g., Cole, 1968), are an especially noteworthy character that causes problems during identification. The problem relates to the fact that circumocular whorls are lacking in two species (P. hoelldobleri and P. mohavensis), and they are highly variable and sometimes lacking in other species (e.g., P. maricopa and P. californicus).

Tooth number is also an important character for separating P. hoelldobleri and P. mohavensis, but most series of Pogonomyrmex consist of foragers, and these older workers typically have substantial mandibular wear that might negate using this character; pinning specimens such that teeth are not easily visible presents an additional problem. Moreover, at least several workers from each nest series should be examined when attempting identification. Collectors should also locate and excavate nests so as to collect series that include non-foragers. Lastly, it is recommended that collectors search sites to locate possible sympatric congeners, which will significantly reduce problems during identification.

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Pogonomyrmex

1

 * Basal tooth strongly offset from basal margin; diastema present between basal and sub-basal teeth, mandible sometimes with eight teeth when a very small tooth occurs in diastema . . . . . Pogonomyrmex snellingi


 * Basal tooth not offset, lacking diastema between basal and sub-basal teeth . . . . . 2

2
return to couplet #1
 * Dorsum of petiolar node, viewed from side, distinctly flattened, and viewed from above, with strong widely spaced wavy, subparallel rugae and usually a distinct, broad, shallow, longitudinal depression; propodeum armed with short to long spines; cephalic interrugal punctures prominent . . . . . Pogonomyrmex comanche


 * Dorsum of petiolar node, viewed from side, not flattened, and viewed from above, lacking strong, widely spaced, wavy, subparallel rugae and a broad, shallow longitudinal depression; propodeal armature present or absent; cephalic interrugal punctures absent to prominent . . . . . 3

3
return to couplet #2
 * Mandible with six teeth and circumocular whorls present; venter of postpetiole with a strong, triangular tooth . . . . . Pogonomyrmex anzensis


 * Mandible with seven teeth or mandible with six teeth and cephalic rugae extending to vertex in side view, not forming circumocular whorls; venter of postpetiole lacking a strong triangular tooth . . . . . 4

4
return to couplet #3
 * In side view, eye unusually large, OI usually > 28.5 and malar ratio usually < 1.0; circumocular whorls usually present, though sometimes indistinct . . . . . Pogonomyrmex magnacanthus


 * In side view, eye not unusually large, OI < 27.5 and malar ratio usually > 1.05; circumocular whorls present or absent . . . . . 5

5
return to couplet #4
 * Propodeal spines absent or with a pair of angles, denticles, or short to long spines; cephalic interrugal punctulation rather strong and interrugal punctulation of mesepisternum moderate to strong; rugae on mesepisternum often indistinct; interrugal spaces dull to weakly shining in both areas; circumocular whorls usually present; posterior declivity of propodeum with transverse rugae, not smooth and strongly shining . . . . . Pogonomyrmex maricopa


 * Propodeal spines absent; cephalic interrugal punctulation absent to weak and interrugal punctulation of mesepisternum absent to moderately strong; rugae on mesepisternum usually distinct; interrugal spaces moderately to strongly shining in both areas; circumocular whorls present or absent; posterior declivity of propodeum lacking transverse rugae, smooth and strongly shining . . . . . 6

6
return to couplet #5
 * Mandibles with six teeth, a seventh small (subequal) tooth sometimes present between basal and sub-basal teeth; cephalic rugae extending more or less directly to the vertex in side view, not converging posterior to eyes or forming circumocular whorls; gaster concolorous with head and mesosoma . . . . . Pogonomyrmex mohavensis


 * Mandibles with seven teeth; cephalic rugae forming circumocular whorls or rugae (sometimes faint) converge at or near vertex, but not forming circumocular whorls; gaster black or concolorous with head and mesosoma . . . . . 7

7
return to couplet #6
 * Circumocular whorls almost always present, occasional individuals with rugae converging toward vertex; area posterior to eyes always with distinct rugae; interrugal spaces on pronotal sides smooth and shining, rarely weakly granulate; gaster black or concolorous with head and mesosoma . . . . . Pogonomyrmex californicus


 * Circumocular whorls absent, cephalic rugae converging near vertex or area posterior to eye with very faint rugae; interrugal spaces on pronotal sides moderately to strongly granulate, dull to weakly shining; gaster concolorous with head and mesosoma . . . . . Pogonomyrmex hoelldobleri