Plecoptera (Perlaria: Stoneflies)
Soft-bodied insects of moderate to rather large size with elongate, setaceous antennae. Mouthparts weak, of the biting type; mandibles normal or vestigial, ligula 4-lobed. Wings membranous, held flat over the back in repose, hind pair usually the larger,
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PLECOPTERA (PERLARIA: STONEFLIES) Soft-bodied insects of moderate to rather large size with elongate, setaceous antennae. Mouthparts weak, of the biting type; mandibles normal or vestigial, ligula 4-lobed. Wings membranous, held flat over the back in repose, hind pair usually the larger, with well-developed anal lobes. Venation variable, often considerably specialized; vein M 2-branched. Tarsi 3-segmented. Abdomen usually terminated by long multi-articulate cerci; ovipositor wanting. Metamorphosis hemimetabolous; nymphs aquatic, campodeiform, with the antennae and usually the cerci elongate; tracheal gills, which are variable in position, commonly present.
The Plecoptera are a small order, whose members are of certain interest on account of the archaic features in their structure, and the aquatic habits of their nymphs (Hynes, 1976). The adults share some features with the Orthoptera, but the mouthparts are weaker, there is never more than a slight. difference in texture between the fore and hind wings, and the coxae are small. They are poor fliers, and do not wander far from the margins of streams and lakes: they are commonly found resting upon stones, tree-trunks or palings near the water's edge, while the green forms frequent herbage. The larger species are well known to anglers as a bait for trout. The nymphs are almost exclusively aquatic, living beneath stones in clear water, particularly in streams with stony beds, and places where there are waterfalls, or where the water is otherwise well aerated. They do not live in polluted streams and few species occur in standing water. The adults of many of the species with well-developed mandibles feed on lichens and unicellular algae (Frison, 1935; Hynes, 1942; Brinck, 1949). External Anatomy - There is no detailed account of the anatomy of the order as a whole, but the works of Schoenemund (1912), Wu (1923), Clark (1934), Hanson (1946), Grandi (1948-so) and Nelson and Hanson (1969, 1971) are useful. The head-capsule (Hoke, 1924; Chisholm, 1962; Moulins, 1968) resembles that of the Orthoptera but is prognathous, the epicranial suture' may be reduced or even absent and the frontoclypeal sulcus is wanting in many species. The antennae are long and setaceous, with a large O. W. Richards et al., Imms’ General Textbook of Entomology © O. W. Richards and R. G. Davies 1977
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GENERAL TEXTBOOK OF ENTOMOLOGY
number of small segments. Compound eyes are well developed, and there are three (more rarely two) ocelli. The mouthparts (Fig. 243), although completely formed, are sometimes weak structures: the mandibles are normally developed in most families, but in the Perlidae and related families they are vestigial flexible lamellae. The maxillae consist of the typical sclerites and their palps are 5-segmented. In the labium, the submentum is large, the prementum is sometimes divided, and both glossae and paraglossae are evident; the labial palps are 3-segmented. The whole of the trunk is somewhat flattened, none of the parts are strongly sclerotized, and much
FIG. 242
Perl
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