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Mature larvae of the tribe Asidini are characterized by an unsclerotized body, dorsally concave mandibles with preapical gibbosity and submarginal setose area, granulate cranium, swollen preepipleurum, and huge granulate forelegs with contiguous coxae surrounded by an enlarged sternellum. The pygidial terminus is rounded in Philolithus densicollis (Horn) and bifurcate in Stenomorpha puncticollis (LeConte). First-instar larvae lack granules on cranium and forelegs, have 2 setae on the labrum, possess egg bursters, and are very similar in both species. Eggs are large, averaging 2.5 mm in length in P. densicollis and 3.1 mm in S. puncticollis. Pupae are not known. The two species are sympatric in eastern Washington state. Adults occur in autumn, living only one month. A female deposits about 1 egg per day. Eggs of P. densicollis hatch quickly and the first winter is passed in an early larval stage. Eggs of S. puncticollis apparently overwinter and hatch in early spring. The full life cycle probably takes two years. Cold temperature of the second winter breaks a developmental diapause and leads to adult eclosion the following autumn; S. puncticollis emerges somewhat later than P. densicollis. The prolonged immature phase as a subterranean scavenger is seen as a major adaptation to xeric environments.