Table 1. The major aquatic insect lineages, their character, and their value for diversification research.

radiation a		species b	tax c	lot d	diversity and ecology	diversification research potential
Ephemeroptera (mayflies)		3,046	+	+	Rivals Plecoptera as 3rd largest purely aquatic order (15). Differs notably from sister-group Odonata in that larvae feed mainly on algae and fine detritus and are morphologically diverse; adults do not feed, emerge synchronously, live for a short time, disperse poorly and are morphologically uniform.	Comparatively well studied taxonomically; few species-level phylogenies available. Palaeoptera provide interesting comparison of two ecologically very different radiations. Suitable for studying the role of life history in diversification, e.g. parthenogenesis and temporal isolation through reproductive synchronicity.
Odonata (dragon- & damselflies)		5,952	++	−	2nd largest purely aquatic order; 4th largest aquatic insect radiation (61). All species are predators due to excellent sight, adult flight and extrudable mouthparts of larva. Broad range of dispersal capacities rivaled only by some Coleoptera. Most visible sexual behavior among aquatic insects.	Best-researched group relative to species numbers, with most work on sexual selection and conflict by sperm-displacement (secondary genitalia), male visual courtship displays (often with colored wings and body) and female color forms. Taxonomy and distribution best known of any aquatic group. Only insects with global overview of species’ threat status (26).
Heteroptera	Nepomorpha (water bugs)	2,404	++	−	Two radiations, both largely predatory, in the otherwise strictly terrestrial order Hemiptera: in contrast to complex invasion history and extreme ecological diversity of Diptera and Coleoptera, All life stages bound to water. Unlike most freshwater insects, ecology of larvae and adults notably similar and with a high frequency of flightless forms in adults (5). Gerromorpha were main animal group to invade freshwater surface.	Moderately well studied, but the potential to study sexual conflict in groups with sexual dimorphism remains unexploited (6). Good potential for studying historical biogeography: despite many lentic species, allopatric diversification is thought to predominate due to hololimnic life cycle and low dispersal, which also offers unique potential for radiation in old lakes (100).
	Gerromorpha (water striders)	2,021	+	−
Plecoptera (stoneflies)		3,497	−	++	3rd largest purely aquatic order, most notable coldwater radiation with distinct northern and southern hemisphere radiations (42).	Very underworked. Ecologically sensitive and relatively uniform, with limited dispersal: mostly allopatric speciation, group suitable mainly for studies of historical biogeography, but also for temporal isolation.
Diptera (flies)	Culicomorpha (mosquitoes, midges, black flies)	19,618	−	−	Only mainly terrestrial order with large freshwater proportion, including 1st, 2nd and 5th aquatic insect radiations (but see Trichoptera). With >20 freshwater invasions up to 50% of aquatic insects may be Diptera and 30% of Diptera aquatic. Greatest ecological diversity and flexibility of any aquatic order. However, species’ ecologies often unknown (most notably in Tipulomorpha) and vast majority may live in moist substrates rather than in water (133). Specific pre-adaptation for multiple freshwater invasions and radiations unclear, but likely related to exceptionally diverse functional morphology, physiological adaptability (e.g. to extreme chemical and physical environments) and numerous feeding modes.	A large body of work focuses on disease vectors and medical applications, e.g. Simuliidae and Culicidae: excellent process work on Anopheles gambiae (see Sidebar), but notable lack of studies on patterns. Strong correlation of aquatic larvae with blood-sucking adults, perhaps due to pre-adapted mouthparts and host concentration near water. Research on most families hampered by lack of taxonomic and ecological knowledge. Only reasonably studied non-vectors are Chironomidae, which dominate aquatic communities in individual and species numbers, show extreme ecological diversity, e.g. survive from 5600 m height to 1000 m depth and −20°C in air to 40° in water; 7 day to 7 year life cycles; marine and Antarctic species (39). Often preserved as subfossils, making them an ideal group for geographic and diversification research (101).
	Tipulomorpha (crane flies)	15,770	− −	?
	Tabanomorpha (horse flies etc.)	5,373	?	?
	Psychodomorpha (moth flies etc.)	3,412	?	?
	Ephydridae (shore flies)	1,994	?	?
Trichoptera (caddisflies)		14,291	− −	+	Largest purely aquatic order, currently 3rd most diverse taxonomically, but may be the largest aquatic insect radiation with up to 50,000 species (33). Great diversity due to micro-habitat specialization, full array of feeding modes (probably second only to Diptera) facilitated by great diversity in silk-spinning strategies and case construction, and relatively low dispersal.	Very underworked, especially relative to species richness, as only 25% of species may be described. Few species level phylogenies. Well-known sister-group Lepidoptera offers good aquatic/terrestrial comparison. Adaptive significance of case-building and feeding behavior only poorly studied at high taxonomic resolution, though both likely present key innovations.
Megaloptera (fish-, dobson- & alderflies)		328	+	+	Very small purely aquatic order. Relict distribution, mostly in Americas and Asia (28). Monophyly has been contested, but most recently supported (134).	Low species and ecological diversity, and limited distribution, suitable for historical biogeography
Coleoptera (beetles)	Hydradephaga (diving b., whirligigs)	5,126	+	−	Largest group of animals on Earth is 97% terrestrial, with >20 freshwater invasions, but diverse life histories hard to define, aquatic and terrestrial behavior merge at shoreline. Sealed ‘air tank’ under elytra was major pre-adaptation for frequent and flexible invasion of freshwater: individuals could ‘reinvade’ freshwater daily. Only 8% of aquatic species have a typically amphibiotic life cycle (submerged larva, shorter-lived terrestrial adult) as most groups aquatic in all life stages, some with terrestrial larva and aquatic adult (56).	Prominent in the literature, perhaps second only to Odonata, with most work on diversification of any aquatic group. Much focus on habitat stability (105), providing good comparisons with strongly lotic (Gyrinidae, Elmidae), lentic and dispersive (Dytiscoidea) or specialized groups (Hydraenidae), as well as one of the best recent fossil records (1, 40). Ecology often linked to notable key innovations like adult surface dwelling (Gyrinidae), swimming by simultaneous stroke of adult middle and hind legs (Dytiscidae), and antimicrobial exocrine secretion (Hydraenidae).
	Hydrophyloidea (water scavenger b.)	2,205	+	−
	Scirticidae (marsh b.)	1,330	−	−
	Hydraenidae (minute moss b.)	1,380	−	+
	Elmidae (riffle b.)	900	+	++

^aOnly groups of >300 species resulting from single freshwater invasion are shown; Culicomorpha and Psychodomorpha probably form one lineage of over 23,000 known species (137), as do Ephemeroptera and Odonata (Palaeoptera) of over 9,000 (126). Many smaller or partly aquatic groups are excluded, all in mostly terrestrial orders, thus only seventeen of over fifty aquatic invasions are considered. Groups tied closely to freshwater but lacking aquatic life stages are also excluded, e.g. Leptopodomorpha (shore bugs). The list of included Diptera is tentative, because the number of actually aquatic species (and thus separate invasions) is unclear. Other families with hundreds and possibly thousands of aquatic species are Dolichopodidae, Muscidae, Stratiomyidae and Syrphidae.

^bNumbers of described species taken from stated sources and updates (50, 92). Major (>300 species) constituents of Culicomorpha are Chironomidae (7,290), Ceratopogonidae (5,902), Culicidae (3,725) and Simuliidae (2,121); of Tipulomorpha are Limoniidae (10,777), Tipulidae (4,415) and Pediciidae (496); of Tabanomorpha are Tabanidae (4,434) and Rhagionidae (756); of Psychodomorpha are Psychodidae (3,026) and Blephariceridae (331); of Hydradephaga are Dytiscidae (3,908) and Gyrinidae (750); of Hydrophyloidea is Hydrophylidae (1,800).

^cState of taxonomy is inferred from estimates of species described: under 30% (−−), and over 50% (−), 70% (+) or 80% (++), or unknown (?) and likely very low.

^dProportion of lotic species is based on estimates from North America and Europe: almost all species strictly lotic (++), most lotic but good number lentic (+), >25% lentic (−), or knowledge deficient but many edge species and lotic/lentic distinction often unclear (?).