Insects of Britain and Ireland: orders

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L. Watson and M. J. Dallwitz



Adult insects. Nearly always found near water. Incapable of feeding. Small to medium sized; capable of flight (the reproductive behaviour commonly involves the males forming aerial swarms of a few individuals to hundreds, by day or perhaps in some species by night, usually near water); with one pair of propellant wings, or with two pairs of propellant wings. Mouthparts much reduced (reduced to asymmetric non-sclerotised vestiges, except sometimes the maxillary palps); non-functional for feeding (the adults taking no food). Antennae inconspicuous; simple (though with a short, thick scape); 15–35 segmented (small, but ‘many-segmented’). Ocelli 3. Wings two, or four (the hind-wings often much reduced, absent in the Caenidae); the single pair when only one pair, membranous; of similar texture in both pairs (membranous). Fore-wings membranous. Hind-wings smaller than the fore-wings, or similar in size to the fore-wings; markedly broader than the fore-wings, or no broader than the fore-wings; not folded in the resting insect. Wings with numerous cross-veins (usually), or with few cross-veins; more or less naked; fringed (in sub-imagines, also in adult Caenidae), or not fringed (mostly). Wings of the resting insect held above the body, with their upper surfaces more or less apposed. Tarsi (3–)5 segmented. Abdomen conspicuously appendaged at the rear; with long terminal bristles; with cerci clearly visible at its tip (constituting a pair of long, slender caudal styles, which are often associated with a third bristle representing an ‘appendix dorsalis’). Abdomen of females with an exserted ovipositor (Leptophlebiidae), or with no exserted ovipositor. Abdomen apparently 10 segmented (and a short ‘post-abdomen’ fused with segment 10).

Larvae. Larvae aquatic (with segmentally arranged tracheal gills); with three pairs of segmented thoracic legs; without ventral abdominal prolegs. Development of larva into adult gradual; exopterygote (with a unique, sub-imaginal winged stage); not involving a pupal stage.

Classification. Subclass Pterygota; Division Exopterygota.

British representation. Baetidae, Caenidae, Ephemerellidae, Ephemeridae, Heptageniidae, Leptophlebiidae, Potamanthidae, Siphlonuridae (the families described individually in the accompanying data set); genera 18; 46 species.

General comments. The cerci of the imago are represented by a pair of slender, elongated styles, and an ‘appendix dorsalis’ often constitutes a third, cf. Thysanura. The final moult of the nymph results in a fully winged ‘sub-imaginal’ stage, of duration from a few minutes to about 24 hours depending on the species; and the final ecdysis involves a process unique to Ephemeroptera, in which the sub-imago casts a delicate pellicle from its entire body including the wings. Sub-imagines (the ‘duns’ of anglers) are usually distinguishable from adults by their duller appearance, and their translucent rather than transparent, conspicuously hair-fringed wings. The Order is named after the ephemeral life of the imagines (the ‘spinners’ of anglers), which famously survive for only a few hours to a few days. The one-clawed tarsi of the nymphs readily distinguish these from the otherwise similar nymphs of Plecoptera, which are consistently two-clawed. Some species reproduce parthenogenetically.

Illustrations. • Ecdyonurus dispar Curtis (Ephemeridae. Dissimilar May-fly: B. Ent. 484). • Ecdyonurus dispar (B. Ent. 484, legend+text). • Ecdyonurus dispar (text, cont.: B. Ent. 484). • Ephemera vulgata: B. Ent. 708. • Ephemera vulgata: B. Ent. 708, legend+text. • Ephemera vulgata: B. Ent. 708, legend cont.. • Summer dance of the May-flies: Gilbert White, 1771.

To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.

Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2003 onwards. Insects of Britain and Ireland: orders. Version: 16th May 2016.’.