This is only the second report of the life history, and first on ontogenic nymphal development, for any of the 22 western species of North American Sweltsa. Concurrent monthly collections of nymphs, with bi-weekly collections of adults from emergence traps, from an intermittent Oregon stream over 10 years yielded an interesting assemblage of stoneflies, including Sweltsa adamantea with no congeners.
Adults were taken from emergence traps from 2001 to 2007, and the head-capsule widths of 1,159 nymphs from 1999 to 2007 collections were individually measured. Counts of antennal and cereal segments, windpad presence and backward projecting length, presence of dark, thoracic sternocoxal hairs ( a diagnostic character in late instar Sweltsa), diameter of compound eyes, general presence and length of body hairs, and sexual dimorphism were determined for 68 nymphs representing an array of 0.27 to 1.86mm head capsule widths from 2004 to 2007 collections. Eggs were studied and photographed with SEM.
The life cycle was semivoltine; adult emergence was from late March to late July, with a peak from May 1 to June 15. The recruitment of a new cohort of nymphs before early emergence indicated an egg diapause for at least much of the first year. Growth of nymphs was slow through the warm, dry summer, accelerated in fall beginning about September, and rapid from December until emergence. Smallest field-sampled nymphs (0.27 to 0.50mm head capsule width) had 16–18 antennal segments, 5–6 cereal segments, 0.03mm diameter compound eyes, and no wingpads, dark sternocoxal hairs or sexual dimorphism. Progressive development of all these and other noted setal characters occurred until pre-emergent size. Antennal and cereal segments more than doubled in number, and wingpads, dark sternocoxal hairs and sexual dimorphism became discernible at specific hcw sizes. Eggs were oval, measured 234×340µm, had no collar, and had a finely punctuate chorion. Numerous nymphs of medium size range 0.51 to 1.08mm hcw had parts of chironomid larvae protruding from their mouthparts indicating that they were important predators.