Box 10.3. Plecoptera (stoneflies)
The stoneflies constitute a minor and often cryptic order of 16 families, with more than 2000 species worldwide, predominantly in temperate and cool areas. They are hemimetabolous, with adults resembling winged nymphs. The adult (see Plate 4.3, facing here) is mandibulate with filiform antennae, bulging compound eyes, and two or three ocelli. The thoracic segments are subequal, and the fore and hind wings are membranous and similar (except the hind wings are broader), with the folded wings partly wrapping the abdomen and extending beyond the abdominal apex (as illustrated for an adult of the Australian gripopterygid, Illiesoperla); however, aptery and brachyptery are frequent. The legs are unspecialized, and the tarsi comprise three segments. The abdomen is soft and 10-segmented, with vestiges of segments 11 and 12 serving as paraprocts, cerci, and epiproct, a combination of which serve as male accessory copulatory structures, sometimes in conjunction with the abdominal sclerites of segments 9 and 10. The nymphs have 10–24, rarely as many as 33, aquatic instars, with fully developed mandibulate mouthparts; the wings pads are first visible in half- grown nymphs. The tracheal system is closed, with simple or plumose gills on the basal abdominal segments or near the anus (Fig. 10.1) — sometimes extrusible from the anus — or on the mouthparts, neck, or thorax, or lacking altogether. The cerci are usually multi- segmented, and there is no median terminal filament.
Stoneflies usually mate during daylight; some species drum the substrate with their abdomen prior to mating. Eggs are dropped into water, laid in a jelly on water, or laid underneath stones in water or into damp crevices near water. Eggs may diapause. Nymphal development may take several years in some species.
Nymphs may be omnivores, detritivores, herbivores, or predators. Adults feed on algae, lichen, higher plants, and/or rotten wood; some may not eat. Mature nymphs crawl to the water’s edge where adult emergence takes place. Nymphs occur predominantly on stony or gravelly substrates in cool water, mostly in well-aerated streams, with fewer species in lakes. Generally they are very intolerant of organic and thermal pollution.