There are three subspecies (P. a. algidus, P. a. marcidus, and P. a. coquilletti).
13-28 mm (females>males, some overlap).
This wasp is in the subfamily Pompilinae, tribe Pompilini and therefore possesses the following characters:
Pocket in basioposterior corner of the third discal cell.
Spine pits on dorsal side of hind femur.
Spines at apex of hind tibia of unequal length and spacing.
Females of this species have a well-developed tarsal comb.
Relatively short pronotum (shorter than mesonotum) and depressed pronotal collar.
They are separated from other Pompiline genera by the following:
Postnotum broadened between the median line and the spiracles.
Posterior wing with anal vein meeting the median vein beyond the origin of the cubitus.
Lacks scale-like pubescence, possesses strongly converging eyes, and possesses a row of spines on the ventral side of the apical tarsal segments.
They are separated from the similar P. interruptus) by the following characters:
Females with three strong tarsal comb spines (4 moderate comb spines in interruptus).
Eyes much more convergent abouve. Third antennal segment about 1.3X the upper interocular distance (about .65-.75 in interruptus).
Males with the aedoaegus broadly bilobed (narrowly in interruptus).
Claws bifid in females (dentate in interruptus).
Coloration mainly of black and dark ferruginous. Coloration mainly of yellow and ferruginous in interruptus (except P. i. cressoni, but structure is sufficient to separate them from P. algidus).
Widespread. P. a. marcidus and P. a. coquilletti are restricted to the west. The nominal subspecies is widespread.
Psammophilous. The females have well-developed tarsal combs and are fossorial and therefore prefer sandy areas (although they may be found in areas of abundant vegetation).
Likely a univoltine mid-summer to late autumn species (pers. obs.)
Adults frequently visit flowers. Females provision nest burrows with Araneid spiders.
Univoltine (except possibly in the very southern parts of its range).
Evans, H.E. 1950. A taxonomic study of the nearctic spider wasps belonging to the tribe pompilini, Part I. Trans. Amer. Entomol. Soc., 75: 133-270. (descriptions, keys, biology, distribution).
Krombein, K.V. 1979. Pompilidae, pp. 1551. In Krombein, K.V., P.D. Hurd, Jr., D.R. Smith, and B.D. Burks, eds. Catalog of Hymenoptera in America North of Mexico. Vol. 2 Apocrita (Aculeata). Smithsonian Inst. Press, Washington, D.C. (distribution).