Sialidae

Alderflies are distinguished from dobsonflies and fishflies (Corydalidae) by their smaller size, less robust mandibles (especially in males), and more delicate overall build. They differ from lacewings (Neuroptera) by wing venation patterns and antennae structure. Adults are distinguished from stoneflies (Plecoptera) by wing posture at rest (roof-like vs. flat or folded flat over body) and by the presence of a distinct pterostigmal crossvein in the forewing. Larvae are identified by the combination of lateral filamentous gills on all abdominal segments and two terminal filaments, versus stonefly larvae which have only two tail filaments and no lateral gills.

Larvae are strictly aquatic, inhabiting lentic (still water) environments including lakes, ponds, slow-moving streams, and marshes. They require fine substrates such as silt, mud, or detritus for burrowing. Adults are terrestrial but remain near water bodies, often found resting on vegetation along shorelines. Larval habitat specificity makes them sensitive to water quality and substrate disturbance.

Widespread in temperate regions of the Northern Hemisphere. Documented from North America (including Texas and Vermont in the United States), Europe (Denmark, Norway, Sweden), and Asia. The family has been introduced to some regions outside native range. Neotropical species in the genus Ilyobius occur in South America (Argentina, Brazil, Chile).

Adult emergence typically occurs in spring and early summer, with peak activity varying by latitude and local climate conditions. In temperate regions, adults are active from April through June. The flight period is brief, usually lasting only a few weeks for any given population. Larvae are present year-round in aquatic habitats, with multiple instars overlapping.

Larvae are predatory, feeding on small aquatic invertebrates including insect larvae, crustaceans, and oligochaete worms. They are generalist predators within their benthic habitat. Adult mouthparts are reduced and non-functional; adults do not feed.

Holometabolous with four stages: egg, larva, pupa, and adult. Eggs are laid in gelatinous masses on vegetation overhanging water. Larvae drop into water upon hatching and undergo multiple instars (typically 10–12) over one to two years. Pupation occurs in a chamber constructed in moist soil or mud near the water's edge; the pupal stage is non-feeding and relatively brief. Adults emerge from pupal chambers and live only a few days to weeks. Oogenesis begins during larval growth in at least some species, with germ cell proliferation and cluster formation in ovarioles documented in Sialis flavilatera.

Adults are weak fliers with limited dispersal ability, typically remaining within meters of emergence sites. They are most active during daylight hours, resting on vegetation near water. Mating occurs on vegetation, with males often searching for females. Adults are attracted to light. Larvae are benthic burrowers, constructing simple galleries in soft substrates and emerging to forage. They are poor swimmers and move primarily by crawling.

Larvae function as intermediate predators in freshwater benthic food webs, consuming smaller invertebrates and serving as prey for larger aquatic predators including fish and other insects. Their presence indicates lentic habitats with stable, fine substrates and relatively good water quality. As a group with specific habitat requirements, they contribute to nutrient cycling through bioturbation of sediments.

Alderflies have limited direct economic importance. They are occasionally used as bioindicators for lentic freshwater habitat quality. Larvae are not significant pests. Adults do not bite or sting. They are of minor interest to anglers as occasional fish food, though less important than caddisflies or mayflies. The family is subject to conservation concern in some regions due to habitat degradation; the Scarce Yellow Sally (Isogenus nubecula) in Wales represents a notable example of a threatened sialid species with targeted conservation efforts.

• Corydalidae (dobsonflies and fishflies)Larger body size, more robust build, and in males, greatly elongated mandibles; larvae have lateral gills only on first 7 abdominal segments versus all segments in Sialidae
• Plecoptera (stoneflies)Adults hold wings flat over body or folded around body, not roof-like; larvae have only two terminal filaments and lack lateral abdominal gills
• Neuroptera (lacewings, antlions, etc.)Different wing venation with fewer crossveins, often clubbed or thickened antennae in some families, and different larval morphology with piercing-sucking mouthparts

• BugGuide
• GBIF taxonomy match
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• The Wonder of Dragonflies | Bug Squad
• Fossils clarify the origins of wasps and their kin: alderfly ancestors, snakefly cousins | Blog
• The Stonefly Diaries ~ a blog Part 2 - Buglife Blog - Buglife
• Description of the larva of Ilyobius chilensis (McLachlan) (Megaloptera: Sialidae) and notes on the adult morphology
• Texas Alderflies (Megaloptera: Sialidae): New Records, Distributions, and a Key to Adults in Texas
• Germ cell proliferation and cluster behavior in ovarioles of Sialis flavilatera (Megaloptera: Sialidae) during larval growth
• Updates on the taxonomy of Ilyobius nubilus (Navás, 1933) (Megaloptera: Sialidae: Sialinae) and Ilyobius brasiliensis (Navás, 1936) from southeastern Brazil
• The fauna and distribution of alderfly larvae (Sialidae, Megaloptera) in Northern Fennoscandia and its dependence on natural and anthropogenous factors