This is a large family, including 24 genera and close to 100 species. They are known as riffle beetles, due to their common occurrence in habitats with fast-flowing water and high oxygen levels. Both larvae and adults are slow-moving clingers that feed by scraping or collecting and gatheringperiphyton and detritus from surfaces of rocks and other substrates. The larvae get their oxygen directly from gills that can be retracted into the end of the abdomen. Adults have an effective plastron and usually do not need to surface for air. Elmid larvae are unusual in that they have up to 8 instars, whereas most beetle larvae molt only 3 times.
Order:Larvae: Usually without lateral abdominal gills. If gills present, then 4 hooks clustered on segment 10. Thoracic legs each usually with 4 or 5 segments and with 1 or 2 claws; if without legs, head distinctly sclerotized and posterior body (thorax and abdomen) simple, without gills, hair brushes, suckers, or breathing tube. Eye spots usually present, but compound eyes absent.
Family:Labrum and clypeus separated by distinct suture. Antennae 2–3 segmented. Head with ocelli in groups of 5. Legs adapted for walking, sometimes small, each 3–4 segmented and with single claw. Thoracic tergites clearly defined. Abdomen 9-segmented, ventral gills absent from segments 1–8. Segment 9 (posterior-most segment) with ventral lid-like flap (operculum) concealing thread-like gills and pair of hooks. Body strongly sclerotized (hardened), dark brown or red-brown, roughly cylindrical or spindle-shaped, with head and legs visible in dorsal view.
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