The order Megaloptera is supposedly the sister group of the Raphidioptera and is often considered to be the most primitive group of endopterygote insects. About 300 species have been described. The order is widely distributed and diverse through temperate regions, with fewer species in the tropics.
The interactive LUCID key available on this website (see the 'Identify Families' link) was written by Shaun Winterton of the QDPI and the University of Queensland.
Megalopteran larvae are inhabitants of flowing freshwater streams, although some are also found in swamps (Theischinger 1991). They are predatory on other aquatic invertebrates.
Van der Weele's (1910) treatment of the world fauna is now outdated but Tillyard (1919), Riek (1954), Theischinger (1983), and Theischinger and Houston (1988) have provided comprehensive accounts of the Australian fauna. Theischinger and Houston (1988) catalogued the Australian species.
The wing-span of adults ranges from less than 20 mm to 175 mm. They resemble some broad-winged lacewings (Neuroptera) but in Megaloptera there is generally much less end-twigging of longitudinal veins and the abdomen is more soft and very flexible. The aquatic larvae of Megaloptera are recognisable from those of Neuroptera by not having the mouth-parts modified into a sucking tube and by having lateral abdominal gills. Superficially, Megaloptera larvae are also very similar to the larvae of some Gyrinidae (Coleoptera). The labrum and clypeus are distinct in Megaloptera larvae, whereas only a single sclerite is discernible in gyrinid larvae.
Larval Megaloptera are most similar to certain aquatic Polyphaga, and the Corydalidae in particular are easily confused. However, the presence of a free labrum, porrect, two-segmented abdominal gills, and hooked posterior prolegs, should serve to distinguish them from most beetle larvae.