Insects have conquered virtually every ecosystem on the planet, including water. From still ponds to mountain streamsThousands of species live totally or partially associated with the aquatic environment, with breathing, feeding and reproductive strategies as varied as they are surprising.
In this practical yet detailed guide you will find What are aquatic insects?, how they breathe, their life cycles, how to identify them in juvenile and adult stages, which orders and families stand out, examples with scientific names and their role as bioindicators for assessing water quality. If you've ever heard someone talk about mayflies, caddisflies, nymphs, or emergents and it sounded like a different language to you, here's a clear, useful approach for naturalists, educators, and fly fishers.
What are aquatic insects?
Aquatic insects are invertebrate arthropods. that develop all or a significant part of their lives in freshwater (rivers, lagoons, streams, ponds, estuaries). A considerable portion of insect diversity has aquatic juvenile stages (larvae or nymphs) and aerial adults, while others remain aquatic for almost their entire life cycle.
It is estimated that around 3% of insects They are aquatic, which translates to approximately 25.000–30.000 species worldwide, and there are sources that raise the total number of described types to over 76.000 forms when subgroups and categories are considered. This wealth is distributed among multiple orders such as Odonata, Ephemeroptera, Plecoptera, Trichoptera, Diptera, various Coleoptera and Heteroptera, along with less common but fascinating groups such as Megaloptera, some Neuroptera, pyralid Lepidoptera, and certain Hymenoptera.
In the aquatic landscape they occupy very different microhabitats: under stones, in oxygenated rapids, backwaters, pools, amidst submerged vegetation or gliding across the surface of the water. Many are strictly aquatic as juveniles and aerial as adults; others are semi-aquatic, such as the slipper gliders (Gerridae) and their relatives, which take advantage of surface tension to skid across the surface.
In ponds and small bodies of water it is common to find a varied community with boatmen, water scorpions, dragonfly larvae and water beetlesEach of these creatures contributes key functions to the food chain: from predators that control populations to decomposers that recycle organic matter.
How do aquatic insects breathe?
Breathing in aquatic environments has driven extraordinary adaptationsSome larvae exchange gases by diffusion through the integument, others use tracheal gills, and some use air bubbles or actual "physical gills."
- Air bubble and physical gillCertain Heteroptera and Coleoptera trap air between hydrophobic hairs or under the elytra. When oxygen is consumed, its partial pressure drops, and dissolved O2 diffuses from the water into the bubble, maintaining the supply for minutes or hours. In some cases, this stable film is called plastron, a layer supported by microsetae that does not need to be continually renewed.
- Breathing tubes (siphons): other species come to the surface using a “snorkel” to obtain atmospheric air. This allows us to inhabit waters poor in oxygen. where many other organisms would not thrive.
- Tracheal gills (tracheobranchial tubes): thin extensions of the tracheal system that facilitate water exchange. They are very common in nymphs of mayflies, stoneflies and odonatesTheir effectiveness depends on water renewal, so many species produce ventilatory movements, especially in calm waters.
- Integumentary respiration: by increasing hemolymphatic irrigation and body extensions (blood gills), the body surface acts as a respiratory organ. In several Diptera, the hemolymph may contain pigments with high affinity for oxygen, which makes life easier in anoxic environments.
- Utilization of aerenchyma: larvae of some beetles (e.g., Donation) and Diptera obtain oxygen from the aerated tissue of aquatic plants, inserting respiratory structures into the plant.
In addition to the mechanism, the type of tracheal system It matters: there are apneustic larvae (without functional spiracles) that depend entirely on dissolved oxygen exchange, while others have functioning spiracles to capture air when necessary. This diversity allows colonization from cold, oxygenated mountain rivers even warm or low-quality ponds, as long as there is a niche available.
Life cycles and metamorphosis: nymph, larva, pupa, subimago and imago
In aquatic insects relevant to naturalists and fishermen, there are two major development models. Incomplete metamorphosis (hemimetabola) It has egg, nymph and adult stages; complete metamorphosis (holometabola) adds the pupal stage between the larva and the imago.
– Hemimetabolos: orders such as Ephemeroptera (mayflies), Plecoptera (stoneflies) and Odonata (dragonflies and damselflies) are aquatic nymphs that molt several times until they reach the winged adult. Mayflies are special because they go through a subimago (first winged state) before the definitive imago.
– Holometabolous: Diptera, Trichoptera, Coleoptera, Megaloptera, Neuroptera, Lepidoptera and some aquatic Hymenoptera develop frequently worm-like larvae, then pupate (sometimes in cocoons or puparium) and finally emerge as winged adults.
For those who observe in the water, it is useful to recognize the stages: nymph (developing wings visible as plates; well-formed legs), larva (without wing outlines and often worm-like in appearance), pupa (developed wings but limbs attached to the body or inside a cocoon), emergent (moment of ascent and transformation close to the surface) and adult (functional wings and aerial activity).
Many species synchronize emergence with environmental conditions. Factors such as water temperature, photoperiod, rainfall and altitude influence maturation and reproductive timing. In high mountains, for example, emergence may be delayed compared to warmer lowland areas, and some species have extended summer flight periods when conditions permit.
Another piece of the puzzle is the diapause, a programmed physiological pause that can occur in eggs, larvae, pupae, or adults. This strategy "anticipates" unfavorable periods (extreme cold, drought, lack of food) and helps ensure the cycle continues in variable climates.
Practical identification: adults and immatures
With a careful look you can locate the main group. In adults, look at wings, tails (cerci), antennae and resting posture:
- Ephemera: 2–3 long tails; wings held vertically when perched.
- Tricopter: hairy wings with a “roof” over the abdomen; long antennae, sometimes as long as the body.
- Odonates: very large eyes; tapered abdomen; in dragonflies the wings rest extended laterally, in damselflies parallel to the body.
- houseflies: two tails, wings folded flat over the abdomen.
- Diptera: a single pair of visible wings and no tails on the abdomen.
In immatures, the visual cues change. Observe tails, gills, and head/abdomen shape to label the nymph or larva:
- Mayfly nymph: 2–3 tails; lateral gills on abdomen; legs with one claw.
- Stonefly nymph: without typical lateral abdominal gills; sometimes gill filaments on thorax; legs with two claws.
- Odonate nymphs: large eyes; elongated or robust-oval body; predatory extensible lip.
- Dipteran larvae: worm-like body; reduced or internal head; no developed true legs.
- Caddisfly larvae: “caterpillar” appearance with thoracic legs; they usually build cases of sand, twigs, or silk.
Main orders and prominent families
To mentally organize diversity, it helps to remember what metamorphosis each order makes:
- Hemimetabolous: Ephemeroptera (mayflies), Plecoptera (stoneflies), Odonata (dragonflies and seahorses).
- Holometabolous: Diptera (flies and mosquitoes), Trichoptera (phryganids), Coleoptera (water beetles), Megaloptera (sialine beetles), Neuroptera (some aquatic larvae), Lepidoptera (aquatic pyralids), Hymenoptera (Agriotypus, etc.).
- Odonata (dragonflies and seahorses): flying, predatory adults; aquatic nymphs with powerful hunting lips. Anisoptera (dragonflies) with pairs of unequal, robust wings; Zygoptera (seahorses) with equal wings and slimmer bodies.
- ephemeroptera: intermediate subimago before the imago; nymphs with varied shapes (depressed for fast currents, swimmers for slow waters, burrowers, marchers). Many scrape periphyton or filter particles, although there are specialized predators.
- Plecoptera: Flattened nymphs with two cerci and chewing mouthparts; excellent indicators of cold, oxygenated waters. Adults are poor flyers and, depending on the family, may feed little or not at all.
- Trichoptera: larvae that build cases with materials from the environment or silk; excellent bioindicators. Pupation in a chamber or within the case. Adults with hairy wings folded into the roof.
- dipter: Extreme diversity of larvae (saprophagous, phytophagous, predatory, parasitic); many strictly aquatic species in both larval and pupal stages. Varied reproductive strategies, from wedding dance to parthenogenesis.
- coleoptera (aquatic beetles): families such as Dytiscidae (divers) and Hydrophilidae (hydrophilids) with aquatic adults and larvae; others alternate phases. They use air reserves, plastrons, or tracheobranchs; trophic regimes from predators to detritivores and phytophages.
- Aquatic Heteroptera: Corixidae (boatmen) with paddle-like hind legs; Naucoridae of oxygenated waters; Gerridae (shoemakers) They skid across the surface thanks to water-repellent hairs. Nepidae (water scorpions) also breathe by siphoning.
- Megaloptera: Large, predatory, and pollution-sensitive larvae; short-lived adults, courting via vibrations or chemical signals. Excellent indicators of clean water.
- Neuroptera: some larvae are aquatic or semi-aquatic; Sisyridae depend on freshwater sponges, while Osmylidae hunt dipteran eggs in moist substrates.
- Lepidoptera (aquatic pyralids): phytophagous larvae in macrophytes, with respiratory mechanisms ranging from tegumentary respiration to the plastron in certain genera.
- Hymenoptera: the gender Agriotypus It stands out as a parasitoid of caddisfly pupae; the females can dive to lay eggs next to its host.
Examples of species and families (representative samples)
Amongst the aquatic beetles the water beetle stands out Hydrophilus piceus, of large relative size, and the diviscids , the Dytiscus marginalis y Dytiscus latissimusOther notable families: Gyrinidae (they swim by turning on the surface), Haliplidae, Noteridae, Elmidae e Hygrobiidae.
In Heteroptera they abound Gerridae , the gerris lacustris y Aquarius remigis, Corixidae (e.g., Corixa punctata) and Belostomatidae (giant water bugs). odonates include dragonflies such as Anax imperator, Depressed dragonfly o Orthetrum cancellatum and little horses like Calopteryx virgo o Coenagrion mercuriale.
As a showcase of approximate reference sizes, these species illustrate the variety (usual total lengths): Acilius sulcatus (1,2–1,8cm), Aeshna cyanea (9–11cm), Anax imperator (11–15cm), Aquarius remigis (3,5–4,5cm), Colymbetes fuscus (1,8–2,2cm), Cordulegaster boltonii (14–16cm), Corixa punctata (1,3–1,5cm), Dytiscus marginalis (4–6cm), gerris lacustris (3,5–4,5cm), Gyrinus natator (0,5–1,5cm), Halobates sericeus (0,2–0,4cm), Hydrometra stagnorum (1–2cm), Hydrophilus piceus (5,5–6,5cm), Ilyocoris cimicoides (1,2–1,6cm), Lethocerus americanus (4,5–5,5cm), Ranatra linearis (4,5–5,5cm), Somatochlora metallica (5,5–6,5cm), Velia caprai (0,6–0,9cm)This sample gives an idea of the morphological breadth between orders and families.
Reproductive behavior and strategies
The flight period (adults) concentrates the dispersal, courtship and reproductionPhenology is modulated by water temperature, photoperiod, wind, or precipitation, with variations depending on latitude and altitude. High-mountain populations tend to delay emergence and, on occasion, lengthen the juvenile cycle.
To find and recognize each other, species use swarms —common in mayflies, diptera and caddisflies—, defense of mating territories (noticeable in odonates) and vibrational signals Substrate-borne signals (typical in stoneflies), in addition to visual cues and pheromones. These signals reduce unsuccessful copulations between species and facilitate successful pairing.
La intrasexual competition This has led to mate-guarding behaviors; for example, in odonates, the male remains in tandem with the female until laying (contact guarding) or close escort (noncontact). In some groups, structures have been described to remove previous sperm or block further copulations, all as part of an evolutionary race to ensure paternity.
In Megaloptera the spermatophore transfer, which the female can consume after mating, providing resources. Oviposition can occur on the water surface, emergent substrates or even a certain height in riparian vegetation, allowing the larvae to subsequently fall into the water.
In rivers, the downstream drift of juvenile forms is compensated by return flights upstream of adult females before laying eggs, maintaining populations in favorable sections of the riverbed.
Ecological functions and value for water quality
Aquatic insects form the core of the benthic macroinvertebrates along with other groups, and structure complex food webs: predators (e.g., dytiscids, odonates), scrapers and filter feeders (mayflies, caddisflies), detritivores and scavengers (various diptera and coleopterans). The simultaneous presence of current specialists, backwater generalists and surface semi-aquatics ensures the processing of energy and nutrients in almost any body of fresh water.
As bioindicators, macroinvertebrates allow the evaluation of biological quality with sensitivity and low cost. Indexes such as the BMWP (at the family level and with qualitative data on presence/absence) assign scores based on tolerance to organic contamination: highly sensitive families such as Perlidae or Oligoneuridae score high, while tolerant groups such as Tubificidae receive low values. A “healthy” community is recognized by the coherent combination of sensitive and moderately sensitive taxa according to the habitat.
This approach detects alterations that are difficult to capture with a single physicochemical measurement, since the biota integrates environmental stress over time. Furthermore, identification is usually feasible with a magnifying glass and basic guides, making it ideal for monitoring in rivers, streams, and wetlands.
A note for fly fishermen
The key to imitation is in get the order and stage rightIf you see wings spread upwards and two or three tails, they are probably mayflies; if the insect emerges with a case, think caddisfly; if you see an emergent just breaking the surface, it may be the sweet moment of hatchingIdentifying whether the abundant fly is a nymph, larva, pupa, emergent, subimago, or imago simplifies the choice of fly pattern and, with it, success.
In cold mountain waters they usually succeed mayfly and stonefly nymphs in the background, while at dusk a dance of subimagos reveals the mayflies. When the caddis flies touch, the females can lay eggs and return to the water, and recreating that moment with an emerging pattern is often definitive.
By the end of this tour you have seen that aquatic insects are not just “water bugs”, but a mosaic of respiratory adaptations, fascinating life cycles, courtship behaviors and oviposition strategies, with enormous ecological and applied importance: from indicate the health of rivers and lakes even inspiring fly-fishing decisions. Understanding orders, families, and life stages allows us to read the water through different eyes and appreciate the diversity that sustains our freshwater ecosystems.