Caddisflies, also called sedge-flies or rail-flies, are a group of insects identified by long, thread-like antennae and a pair of hairy, membranous wings. They constitute the order Trichoptera, which is closely related to moths and butterflies of the order Lepidoptera. As of 2019, there are around 16,000 extant species of caddisflies found worldwide, except in Antarctica.
They are holometabolous insects, having four distinct life cycle stages: egg, larva, pupa, and adult. While the larvae are typically aquatic, living mostly in freshwater habitats, the adults are terrestrial and short-lived. Many caddisfly larvae build protective cases using silk produced from salivary glands and materials like sand, plant matter, or small stones.
These insects are excellent bioindicators of water quality, as their larvae are highly sensitive to chemical pollution.
Depending on the species, caddisfly adults typically measure 0.1 to 0.6 in (3 to 15 mm). Their larvae may reach around 1 in (25 mm), including the case.1
Their forewings are usually 0.15 to 0.7 in (4 to 20 mm) long, providing a total wingspan of 0.3 to 1.5 in (8 to 40 mm).
Their body is divided into three broad regions typical of all insects: head, thorax, and abdomen.
The head bears slender, thread-like antennae (as long as the body in some species), a pair of compound eyes, and mouthparts that are reduced or vestigial in adults.
The thoracic region is subdivided into three segments: pro-, meso, and meta-thorax, of which the mesothorax is usually the largest. Each segment bears a pair of legs, with each leg having a five-segmented tarsus.
The mesothorax and metathorax bear a pair of membranous, hairy wings that are held in a tent-like fashion when the insect is at rest. The forewings are longer and narrower than the hindwings.
The abdominal region is long and typically subdivided into 10 segments, with the last segments modified into genitalia. In males, the terminal segment bears a pair of claspers that help grasp the female during mating. In females, the terminal segment bears an ovipositor, which is specialized for egg-laying.
The name ‘Trichoptera’ comes from the Greek words thrix (hair) and pteron (wing), referring to the characteristic hairy wings of these insects. All species are divided into three suborders: Annulipalpia, Integripalpia, and Spicipalpia, having 46 extant families. However, the placement of the third suborder is still under study.
The earliest known fossils of these insects date to the Triassic Period, about 230 million years ago, and consist mainly of larval cases rather than preserved body remains. However, fossils of larval cases found in Lower Permian marine deposits in southern Brazil suggest that caddisflies may have originated as early as the Permian Period.2
These insects are found on all continents except Antarctica. Numerically, the Oriental Biogeographical Region (Southern Asia) has the maximum number of species (5,854 species) with the highest species density (405 species per gigameter²). In contrast, the East Palearctic Region (Northeast Asia, the Korean peninsula, Japan, China, Mongolia, and parts of the Himalayas) has the fewest number of species (around 1,200) and the lowest density (around 43 species per gigameter²).3
They occupy different habitats during their life cycle, with almost entirely aquatic larvae and terrestrial adults.
The larvae of most species occupy freshwater habitats, including lakes, streams, seasonal pools, wetlands, and marshes. However, the larvae of the species Philanisus plebeius are marine, occurring in intertidal zones of New Zealand and southern Australian coasts.4 In contrast, larvae of the land caddis (Enoicyla pusilla) are terrestrial and found in damp mosses of deciduous forests and rock crevices.5
Being terrestrial, caddisfly adults are found near the aquatic habitats where they developed as larvae.
The diet of caddisflies varies considerably between their larval and adult stages. While many larvae feed on fragments of plant substances, algae, diatoms, detritus, and organic matter, others, especially late instars, may consume insect larvae and small crustaceans. Gumaga nigricula larvae have been observed scavenging fish carcasses.6
Most adults are short-lived and do not feed, but those that do typically consume nectar, plant sap, or other plant fluids.
The adults of caddisflies are nocturnal and are easily attracted to artificial light sources. While many species are weak fliers, some are experts at flying and can disperse to new localities. For instance, in Finland, at least 17 caddisfly species were estimated to travel distances of 600 m (or more) from their larval aquatic habitats.7
Depending on the species, caddisfly larvae may take to different feeding strategies. For instance, collector-gatherers pick up organic fragments from the stream or lake bottom, whereas collector-filterers capture suspended particles from the water using silk nets or leg hairs. Some species are scrapers, consuming algae and periphyton growing on submerged surfaces, while others are shredder-herbivores that chew fragments off aquatic plants. Another group, the shredder-detritivores, consumes decaying wood or leaves that have been softened by bacterial and fungal activity.
Predatory species may actively seek and hunt their prey, or they may lurk and catch their prey unalarmed.
Larvae of the suborder Integripalpia have a unique ability to build protective cases using leaves, seeds, bark, sticks, grains of sand, larger fragments of rock, as well as mollusk shells. They produce silk from salivary glands near their mouths. Using this silk, they form a tube-like structure on which the materials are added and neatly assembled. Both ends of the built cases are open, allowing oxygenated water to pass through. The larvae move continuously inside the tube, maintaining the flow of water.
Members of the suborder Annulipalpia build fixed retreats within which they remain stationary, waiting for their food to come. The families Psychomyiidae, Ecnomidae, and Xiphocentronidae construct tubes from sand and other particles, gluing them together with silk and anchoring them to the substrate. Larvae of the family Polycentropodidae build short, flattened nests within rock hollows, whereas those of Philopotamidae make netted sacs in which microflora gets trapped.
Larvae of the family Glossosomatidae (within suborder Spicipalpia) build dome-shaped silk structures, which they carry around.
While the adults of most species live for around one or two weeks (primarily just for mating), those that feed on nectar or plant sap may survive around a month.
These insects reproduce sexually, with males and females mating near water or on surrounding vegetation, typically at night. After mating, the female usually lays between 100 and 1,000 eggs, placing them either directly on the water’s surface or on emergent twigs and vegetation above it, depending on the species. Uniquely, females of Philanisus plebeius lay their eggs in the coelomic cavity of intertidal starfish.
In most species, the eggs are laid in clusters or masses covered in a gelatinous cover, which protects them from predators and desiccation. Although most caddisflies are oviparous, a few species in the genus Triplectides are ovoviviparous, giving birth to live young.8
Within a couple of weeks, the eggs hatch into roughly cylindrical, aquatic larvae with short antennae and six pairs of tracheal gills (instead of spiracles) on the underside of the abdomen. All larvae are equipped with chewing-type mandibles for feeding, and most bear a pair of hooked appendages (anal prolegs) at the final abdominal segment.
The larva undergoes 5 to 7 molts over several weeks to months, transforming into an aquatic pupa. When ready to pupate, the larva constructs a protective case or a specialized silky cocoon, sealing itself in. After 1 to 2 weeks, the pupa uses its specialized mandibles to break open the case or cocoon, then relies on the shed exuvia to help it float to the surface as an adult. The adults of all species generally emerge at the same time, usually once per year (univoltine). While the entire developmental process is completed within a year in warmer regions, it often takes more than a year in higher latitudes.
Caddisfly larvae fall prey to various fish, including trout, salmon, and bass, as well as birds, such as waterfowl and dippers. They also become targets of amphibians and the larvae of other insects, including flies, stoneflies, and dobsonflies.
Adults are preyed upon by insectivorous birds, such as swallows and flycatchers. They also get trapped in spider webs. Since many adults are active at night, they are often preyed upon by bats.9
Welcome to AnimalFact - your go-to source for fascinating insights into the amazing world of animals!