Color dark brown to black, marked with white, testaceous, and piceous scales as follows. Thorax usually with a central dark fusiform area; elytra often with apices light and with an oblique light spot on basal third at sides. Overall color varies from largely fuscous with distinct markings to largely white.

Scales granular with a pronounced central tubercle; not margined; contiguous or not. When in large clusters, the white scales often imbricated posteriorly. Setae of dorsal surface very fine, recurved, inconspicuous.

Beak long, rather thin. Sides of head and beak gently convergent from base to apex; sides of beak nearly vertical toward apex, so that only the basal portion of the scrobes are visible from above and the pregenae are visible in front of the eyes. Dorsal surface of beak flattened, quite variable; usually with a deep fovea between anterior margin of eyes, median line vaguely to deeply impressed, basal two-thirds of beak slightly to strongly concave, with a faint transverse ridge across beak between insertion of antennae. Apex of beak deeply acutely triangularly emarginate. Anterior margin of epistoma triangularly emarginate. Scrobes long, arcuate in front of insertion of antennae, straight behind insertion, descending obliquely and reaching undersurface of beak below front margin of eyes. Antennal funicle usually six-segmented, but sometimes five- or seven-segmented (see discussion following description). Middle segments of funicle moniliform, becoming wider than long.

Thorax wider than long, strongly rounded at sides, broadest near middle. Basal and apical constrictions strong laterally, moderate to weak dorsally. Disc flattened, with or without vague median line and with scattered deep punctures. An oblique depression often faintly visible on disc on each side of middle.

Elytra across humeri 1.1 to 1.3 times length of thorax. Elytra of male 2.6 to 2.9 times length of thorax; elytra of female 3 to 3.5 times length of thorax. Humeri prominent. Elytra parallel-sided on basal sixth, inflated behind basal sixth, more so in female. Sixth interval prominent at its termination below declivity. Striae composed of small elongate punctures. Intervals equal, slightly convex with scattered, very inconspicuous setae.

Fore femora greatly enlarged. Fore femora each with a short obsolete glabrous groove on inner edge near apex, marked dorsally by a crescentic projected margin. Fore tibiae longer than their femora, inner edge of each with five to nine small but conspicuous, blunt, irregularly-spaced teeth scattered minute denticles between teeth. Apical tooth of fore tibiae long and acute. All tibiae usually straight, but in a few specimens from Arizona and Mexico, the tibiae are gently bowed inward, the reverse direction of the bowed tibiae in other species.

Males: Length 3.6 to 4.5 mm., width 1.4 to 1.9 mm. Last segment of abdomen apically deflected and emarginate. Fore and middle coxae narrowly and equally separated. Aedeagus (plate 2, fig. 16) in lateral profile thick medially, extreme apex upturned; apical opening elongate-oval.

Females: Length 4.1 to 6.6 mm., width 1.7 to 2.7 mm. Last segment of abdomen often slightly directed downward, bluntly rounded apically, with a shallow dark indentation at the base either side of the midline. Fore coxae separated by twice the distance of middle coxae.

This species was seen from the mountains of south-central Arizona; Cloudcroft and Las Vegas, New Mexico; the Davis and Chisos mountains of western Texas; Tejupilco, Temascaltepec, Mexico; and Gaborachic, and 35 miles west of Balleza in Chihuahua, Mexico. Over 100 specimens were examined.

Host records include oak, wild walnut, and pine, and many specimens were taken by the author at light at the Southwest Research Station in the Chiricahuas. Intensive collecting in the Chiricahuas in the summer of 1956 did not turn up any specimens of P. rotundicollis until after the first shower of the rainy season. Three days after the first rain, the first specimen (a female) was collected. It still had both mandibular cusps in place and appeared freshly emerged.

Pandeleteius rotundicollis is highly variable species, but may be at once distinguished from other species of Pandeleteius in the United States by the glabrous groove with projected margin on the fore femora; usually six-segmented funicle, greatly enlarged fore legs, tuberculate scales, and short thorax with strongly constricted sides and flattened disc.

The funicle of P. rotundicollis is usually six-segmented, but specimens were seen with seven- and five-segmented funicles. In a series of 19 specimens (all females) in the United States National Museum collection and all bearing the label “Chiricahua Mts.,” June 1 to 7, seven specimens have seven-segmented funicles, nine have six-segmented funicles, two have an intermediate condition in which the third and fourth segments appear fused, and one specimen has a six-segmented funicle on the right side and a five-segmented funicle on the left side. Pierce’s type material of P. depressus is from this series, but only five of his paratypes were seen. Of the five paratypes, three have seven-segmented funicles, one has a six-segmented funicle and one is “intermediate.” All the specimens of the series are uniformly large with many pale scales, but no characters could be found that distinguish seven-segmented specimens (P. depressus) from others.

Another female with one five-segmented funicle and one six-segmented funicle was collected by the author at the Southwest Research Station in the Chiricahua Mountains in 1956. Of the nine Mexican specimens examined, seven have seven-segmented funicles, one has the intermediate condition, and one has six-segmented funicles.

Two paratypes and 10 topotypes of P. bryanti Tanner were studied and compared with large series of P. rotundicollis, including two topotypes of the latter. The two paratypes (one male, one female) of P. bryanti (Ft. Davis, Texas) and the two topotypes (males) of P. rotundicollis (Cloudcroft, New Mexico) were compared with particular reference to the distinguishing characters listed for P. bryanti with the following results. One paratype is smaller, one larger than the specimens with a flattened beak and one specimen with the concavity quite pronounced. Segments three to five of the funicle are equally moniliform, prothorax equally dilated, thoracic punctures deeper in P. bryanti, prothoracic tibiae equally bent at apex; tarsi, claws and femora similar; scales not fewer not “less compact”; color brown in all four specimens; scale pattern as distinct as in P. rotundicollis. In view of the above comparisons, P. bryanti is here considered a synonym of P. rotundicollis.