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Snakefly

Snakeflies, belonging to the Raphidioptera, are a small group of holometabolous distinguished by their elongated that extends forward like a snake's neck, giving them a distinctive serpentine appearance, along with filiform antennae, chewing mouthparts, and two pairs of membranous wings held tent-like over the abdomen when at rest. Females possess a prominent, elongated used for laying eggs, which is not a , as snakeflies are harmless to humans. The comprises approximately 250 extant worldwide as of 2024, divided into two families: Raphidiidae (about 210 ) and Inocelliidae (about 45 ), placing them within the superorder alongside lacewings and antlions. These undergo complete , with a featuring eggs laid in crevices or leaf litter, followed by campodeiform larvae that develop over 1 to 3 years (up to 6 years in some cases) through 10 to 15 or more instars, and an active pupal stage lasting from days to several months, often requiring cold temperatures for . Both larvae and adults are predominantly predatory, with larvae hunting small arthropods such as eggs, mites, springtails, and soft-bodied prey like in detritus, under , or leaf litter, while adults target or occasionally feed on , , , or other sugary substances. Snakeflies are weak fliers and exhibit behaviors like rapid forward or backward locomotion in larvae and ovipositor-wagging in feeding females. Primarily distributed in the Holarctic region, snakeflies inhabit temperate forests, shrublands, and arboreal environments, often at higher altitudes (1,000–3,000 meters in warmer areas), and are absent from tropical zones and much of the southern hemisphere, with only about 22 species recorded in the United States and Canada, mostly west of the Rocky Mountains. As a relict order with a rich fossil record dating back to the Jurassic, their diversity has declined since the Mesozoic, making them ecologically significant as natural predators that help control pest populations like aphids in orchards and woodlands.

Taxonomy

Families and genera

The order Raphidioptera represents a small lineage of holometabolous insects within the superorder , distinguished from the related orders and Megaloptera by its unique combination of primitive wing venation, elongated , and complete featuring campodeiform larvae. The order currently includes approximately 253 extant species distributed across 31 genera in two families, reflecting its relict status with a primarily Holarctic range, as of 2024. The predominant family is Raphidiidae, which encompasses the majority of snakefly diversity (approximately 206 species, as of 2023) and exhibits a broad distribution across temperate regions of the , including , , and . Females in this family possess a notably elongated adapted for oviposition in bark crevices or wood, alongside three prominent ocelli and distinct wing venation featuring a veinlet within the pterostigma. Key genera include Raphidia, which is widespread in and and serves as a model for studies of snakefly biology due to its abundance, and Agulla, endemic to with about 17 species primarily in western regions. In contrast, the family Inocelliidae is smaller, with approximately 46 (as of 2023) and a more restricted concentrated in temperate forests of and parts of , often showing disjunct patterns. This family is characterized by the absence of ocelli, a shorter relative compared to Raphidiidae, and wing venation lacking a proximal crossvein and pterostigmal veinlet, adaptations possibly linked to specialized habitats. Representative genera include Inocellia, found in Asian and temperate zones, highlighting the family's biogeographic focus on Palearctic regions. Historically, the taxonomic distinction between Raphidiidae and Inocelliidae emerged in the early through morphological analyses, with the separation formalized based on ocellar presence, ovipositor proportions, and subtle differences in forewing venation such as the configuration of the pterostigma and crossveins; these revisions, notably advanced by workers like Navás and later Aspöck, elevated Inocelliidae to family status from a subfamilial grouping within Raphidiidae.

Species diversity

Snakeflies (order Raphidioptera) comprise approximately 252–253 extant worldwide, as of 2023–2024, distributed across two families: Raphidiidae with around 206 in 24 genera, and Inocelliidae with about 46 in 7 genera. This modest diversity reflects the order's status, with most concentrated in temperate regions of the . The highest species richness occurs in the temperate , particularly in with approximately 120 species, Europe and the with about 115 species, and with approximately 20 species, as of 2023. Asian diversity is driven by the extensive Raphidiidae fauna in eastern and central regions, while European species often exhibit high in Mediterranean islands and mountains; North American taxa are fewer and primarily western. Notable patterns of include the genus Mongoloraphidia (Raphidiidae), which is restricted to central and eastern with such as M. kaszabi endemic to and adjacent areas. In , rare insular like Mongoloraphidia (Japetoraphidia) occidentalis highlight localized on islands. Recent discoveries underscore ongoing taxonomic exploration, such as three new Inocellia species (Inocelliidae) described from the in in 2021, expanding known diversity in eastern near Indochina.

Morphology

Adult characteristics

Adult snakeflies exhibit a distinctive characterized by a narrowly elongate that is often nearly twice the length of the head, creating a pronounced "neck-like" structure that allows mobility and contributes to their snake-like appearance when the head is raised. This feature, combined with a prognathous head, sets them apart from related neuropterid and provides a mantid-like appearance for predation. The overall body length ranges from 8 to 25 mm, with the prothorax typically larger than the meso- and metathorax. The wings consist of two pairs of subequal, membranous structures with similar shapes and net-like venation featuring extensive branching and abundant crossveins, particularly along the ; these wings measure 5–20 mm in length and are often held roof-like over the at rest, though adults are generally weak fliers. The head bears large, separated compound eyes and filiform antennae with approximately 30 segments, alongside mandibulate mouthparts adapted for predation. Sexual dimorphism is prominent in the abdominal region, where females possess an elongated —up to the length of the body in species of the family Raphidiidae—for egg deposition, while males have claspers for . Coloration is typically mottled brown or gray, providing cryptic of tree bark for concealment in forested habitats. Morphological differences distinguish the two families: Raphidiidae species have three ocelli and a kite-shaped head, while Inocelliidae lack ocelli and feature a sub-rectangular head with parallel sides.

Immature stages

Snakefly eggs are elongated and cylindrical, typically measuring around 1 mm in length, and feature a small at one end. They are laid in clusters within bark crevices or similar concealed sites, facilitating protection in humid, sheltered microhabitats that support embryonic development. This placement and shape aid adhesion to substrates, reducing dislodgement in the cryptic environments where snakeflies thrive, such as under loose tree . The larvae exhibit a campodeiform , characterized by an elongated, flattened body that enhances mobility in confined spaces like bark layers or leaf litter. Ranging from 12 to 25 mm in length at maturity, they possess a prognathous head with prominent mandibles adapted for capturing small arthropods, alongside a well-sclerotized pronotum for protection during predatory activities. The body comprises three thoracic segments and ten abdominal segments, supporting a flexible, worm-like locomotion suited to navigating tight, dark habitats. Their flattened form and rapid backward wriggling provide evasion from threats. These adaptations underscore their cryptic lifestyle, prioritizing stealth and endurance in resource-scarce, predator-prone niches, where they prey on soft-bodied . Pupae are exarate, with free legs, visible antennae, and developing wings, enabling limited mobility within their protective enclosure. They form in silken-free chambers excavated under or in leaf litter rather than true cocoons, reflecting an absence of silk-spinning capability that aligns with their reliance on physical concealment for . Non-feeding during this stage, the pupae overwinter in these hidden sites, emerging as adults after a cold period breaks , thus integrating seamlessly into the cryptic progression from larval concealment. This supports a transitional phase focused on internal reorganization while minimizing exposure in bark-protected refugia.

Distribution and habitat

Geographic range

Snakeflies, belonging to the order Raphidioptera, exhibit a primarily Holarctic distribution, spanning temperate regions of Europe, , and , while being notably absent from tropical zones and the . This pattern reflects their preference for cooler climates, with the order comprising approximately 253 extant across two families, Raphidiidae and Inocelliidae. The families show overlapping ranges in the but no native presence in equatorial or southern latitudes, where environmental conditions are unsuitable for their arboreal and predatory lifestyles. In , around 31 species are documented, with the majority concentrated in the and , particularly in coniferous forests of the and ; eastern regions lack native populations. 's fauna includes eight species, underscoring the limited eastward extension. Europe supports approximately 75 species, distributed from the northern limits in southward to the , where diversity peaks in mountainous and forested areas; the western Palaearctic serves as a key center of . Asia harbors the greatest species richness, with roughly 150 species, predominantly in temperate eastern and central zones extending to , , and the Himalayan region; rare southern extensions occur in northern Indochina, such as , marking the order's southernmost limits. No native snakefly species occur in , sub-Saharan Africa, or , with distributions confined to in that continent; attempts to introduce North American species to and have not resulted in established populations.

Habitat preferences

Snakeflies primarily inhabit temperate forests and woodlands across the Holarctic region, favoring environments with a pronounced winter period that supports their cold-tolerant life stages. They are most abundant in coniferous and forests, where they exploit arboreal microhabitats such as loose on trees with rough or peeling exteriors. In , species like those in the family Raphidiidae are commonly associated with and woodlands, while in eastern Asia and , they occur in similar mixed pine- stands. These preferences stem from the need for shaded, moist conditions that maintain under and in , avoiding hot and dry areas that limit their distribution to higher elevations in warmer zones. Larvae exhibit a strong association with dead or dying wood, developing in crevices under loose or in the layer near , where they prey on small arthropods in decaying . For instance, larvae of such as Phaeostigma notata are found under the of both coniferous and trees, often on weakened or decomposing hosts like . Adults, in contrast, perch on tree trunks, branches, or in the canopy of these s, seeking out shaded sites for resting and oviposition in bark fissures. This specialization on arboreal decay makes snakeflies indicators of mature forest health, though some tolerate edge habitats. Their altitudinal range spans from to the timberline in temperate areas, extending up to 3,000 m in mountainous regions of the Mediterranean and , reflecting adaptations to cooler, humid climates over arid lowlands. In the Nearctic, they are confined to western forests up to similar elevations, underscoring a broad but ecologically constrained niche within forested landscapes.

Life cycle

Developmental stages

Snakeflies (order Raphidioptera) undergo complete , or holometaboly, characterized by four distinct developmental stages: egg, , , and , which sets them apart from hemimetabolous insect orders like that lack a pupal stage. This transformation involves profound morphological changes, with immature stages differing markedly from the winged adults. The egg stage begins when females use a long ovipositor to deposit elongate eggs in groups within crevices or slits of tree bark, often under loose bark or in soil detritus, providing protection for embryonic development. Embryonic development proceeds internally, with eggs typically hatching in a few days to three weeks, depending on temperature and species. Larvae emerge as campodeiform predators, featuring flattened, elongated bodies with prominent heads, long antennae, and well-developed thoracic legs adapted for active under or in . They pass through multiple instars, typically 10 to 11 but varying from 7 to 15 or more, during which they actively hunt soft-bodied arthropods such as eggs, , and small larvae. The pupal stage occurs in a protected site like under bark or in an earthen chamber, where reshapes the body for form. This stage lasts a few days to three weeks in most , culminating in that reveals the fully formed . The pupa is exarate, with free appendages, and may exhibit mobility in some cases prior to emergence. Adults are short-lived, typically surviving for a few weeks, with their primary focus on and oviposition rather than prolonged feeding, though many , especially in the family Raphidiidae, actively prey on and other small arthropods or consume to sustain reproductive efforts.

Duration and environmental influences

The snakefly typically spans 2 to 3 years and is univoltine, producing one generation per year in most . The larval stage dominates this period, lasting 1 to 3 years across 10–11 s, with overwintering usually occurring once as a quiescence (not true ) in the final instar for over 95% of . Some , such as those in the Agulla, complete the cycle in 1 year, while others require 3 years or more, extending up to 6 years under experimental conditions with multiple overwinterings. Environmental factors strongly modulate these timelines. Temperature exerts the primary influence: cooler conditions slow overall development, while a mandatory chilling period at low temperatures (approximately 4°C for at least 20 weeks) is essential to terminate larval quiescence and initiate proper pupation in spring. Without sufficient chilling, larvae or pupae may enter metathetely—a disordered developmental state—leading to high mortality rates. Photoperiod plays no significant role in inducing or regulating quiescence. In warmer regions of the Northern Hemisphere, such as the Mediterranean or Central America, snakeflies occupy higher altitudes (1,000–3,000 m) to access cooler microclimates suitable for their chilling requirements, potentially resulting in longer cycles compared to temperate zones. Mortality during is influenced by these factors and pressures. Insufficient low-temperature exposure elevates rates through metathetelous malformations in pupae and late larvae. by ichneumonid wasps (Nemeritis spp.) affects 5–15% of larvae in studied populations, with rates up to 50% in dense aggregations, primarily targeting immature stages.

Ecology

Predatory behavior and diet

Snakefly larvae are primarily ambush predators that inhabit concealed microhabitats such as under tree bark or in leaf litter, where they actively probe crevices with their elongated bodies to detect and capture prey. Their diet consists mainly of soft-bodied arthropods, including (Sternorrhyncha), (Lepidoptera larvae), small beetle larvae (Coleoptera), (Psocoptera), springtails (Collembola), mites, spiders, and eggs or larvae of various like Hymenoptera and . These larvae use their strong, toothed mandibles to grasp and immobilize prey for consumption. In contrast, adult snakeflies exhibit more opportunistic feeding habits and are less intensely predatory than their larvae, often targeting weakened or slow-moving while also supplementing their diet with , , or . Preferred prey includes and other , though adults in captivity readily consume injured arthropods; some species, particularly in the Inocelliidae family, show limited evidence of insect predation in the wild, with occasionally found in their guts. Adults employ short-distance flights to approach and seize prey, relying on their forelegs and mandibles for capture rather than prolonged pursuits. Snakeflies occupy a generalist trophic position as predators, with a strong preference for soft-bodied that are abundant in their arboreal or habitats, contributing to natural regulation in temperate forests. This prey specificity aligns with their morphological adaptations, such as elongated prothoraces in larvae for navigating tight spaces. The high consumption rates during the larval stage, which can last 1–3 years depending on and , support substantial energy accumulation necessary for pupation and emergence, underscoring the predatory phase's role in their prolonged development.

Reproduction and interactions

Snakeflies exhibit distinct reproductive behaviors primarily among adults, which are active during late spring and summer in temperate regions. typically involves elaborate rituals characterized by mutual grooming using the legs and antennae to clean each other, facilitating pair bonding. In the family Raphidiidae, copulation occurs in a "dragging position" where the male hangs head-first from the female, lasting from minutes to about 1.5 hours; in contrast, Inocelliidae employ a "tandem position" with the male positioned beneath the female, attached to her fifth sternite, and lasting up to three hours. Following mating, females use their elongated to insert eggs into bark crevices, , or other concealed locations under bark, often in small clutches of 1–7 eggs strung together and deposited in batches over a short period such as a week in species like Agulla. The eggs hatch in a few days to three weeks depending on environmental conditions. Snakeflies are solitary insects with no social structure or parental care; females deposit eggs without guarding them, and larvae develop independently, sometimes exhibiting cannibalism if confined together. Adult emergence follows larval diapause, typically peaking in temperate summers after overwintering. Ecologically, snakeflies engage in various interactions beyond predation. Adults and larvae are preyed upon by birds such as woodpeckers and by spiders, which target them in their bark habitats. Larvae are frequently parasitized by hymenopteran wasps, particularly ichneumonids like Nemeritis species, which account for 70-95% of recorded parasitoids and can infect 5-50% of populations depending on location. Snakefly larvae often share microhabitats with bark beetles under tree bark, utilizing galleries created by these beetles and preying on eggs and larvae of bark beetles and other insects found there.

Evolutionary history

Fossil record

The fossil record of snakeflies (order Raphidioptera) extends back to the Late Permian period, approximately 260 million years ago, marking the earliest known occurrences of the group. These initial fossils, primarily from deposits in , are attributed to stem-lineages such as Priscaenigmatomorpha and early representatives of Raphidiomorpha, indicating an ancient origin no earlier than this epoch. While sparse in the Permian and , snakefly diversity expanded notably from the around 200 million years ago, with diversification of higher taxa evident in Eurasian deposits, suggesting a prolonged evolutionary history prior to this radiation. Snakeflies attained their zenith of abundance and morphological variety during the era, particularly in the period (145–66 million years ago), when they were widespread across Laurasian continents. Exceptional preservation in has revealed numerous specimens, including larvae and pupae from mid- Burmese () amber dated to about 100 million years ago, showcasing active developmental stages trapped in resin. The extinct family Mesoraphidiidae dominated this interval, comprising a significant portion of the known and exhibiting greater ecological and morphological disparity than modern forms. The transition to the era witnessed a marked decline in snakefly diversity, likely influenced by the Cretaceous-Paleogene mass extinction and subsequent climatic shifts. Modern families, including Inocelliidae and Raphidiidae, emerged in the Eocene epoch (56–33.9 million years ago), with fossils from North American lagerstätten such as the Highlands and Formation documenting their early radiation. These Eocene records also reveal a historically broader distribution, extending into tropical and subtropical regions during the greenhouse conditions of the early , in contrast to the group's current restriction to temperate zones. Notable recent paleontological contributions include the 2024 description of new snakefly from the Laiyang Formation in , which underscore the extensive morphological variation within Mesoraphidiidae and its adaptive responses during the Terrestrial Revolution. Additionally, a 2025 discovery of pupae in highlights unusual antennal structures, providing insights into developmental anomalies in extinct lineages. To date, over 100 have been described across more than 40 genera, exceeding the approximately 250 extant and emphasizing snakeflies as a group with diminished modern diversity.

Phylogeny and recent research

Snakeflies (order Raphidioptera) occupy a basal position among holometabolous as part of the superorder , which also includes the orders Megaloptera and . Within , Raphidioptera forms the to the comprising Megaloptera and , a relationship supported by both morphological and genomic analyses. This placement underscores the evolutionary significance of snakeflies in elucidating the early diversification of endopterygote , with their retention of traits highlighting a lineage that has remained relatively unchanged since the . At the family level, the two extant families of Raphidioptera—Raphidiidae and Inocelliidae—exhibit a basal-derived relationship, with Raphidiidae positioned as the more primitive group and Inocelliidae as derived, distinguished by features such as the absence of ocelli in the latter. Molecular dating estimates place the divergence of these families in the , approximately 136 million years ago, marking a key event in the radiation of modern snakefly lineages. Recent genomic studies have advanced understanding of snakefly evolution, including the first chromosome-level assemblies of species such as Mongoloraphidia duomilia (Raphidiidae), which spans 653.56 Mb across 13 chromosomes, and Venustoraphidia nigricollis (Raphidiidae), at 669 Mb. These genomes reveal a pattern of slow , consistent with the "living fossil" status of Raphidioptera, as their morphology and genetic architecture show minimal divergence from ancestors. Additionally, the 2025 assembly of Xanthostigma xanthostigma (Raphidiidae), identifying 13,251 protein-coding genes, further enables comparative analyses across . Contemporary molecular research, including phylogenomic approaches, has illuminated the Cretaceous radiation of snakefly lineages, with studies in 2024 demonstrating that the morphological disparity and of the extinct family Mesoraphidiidae originated in the Lower , responding to broader changes during that period. These insights highlight a diversification event tied to the Cretaceous Terrestrial Revolution, though focused on taxa. Despite progress, gaps persist in snakefly phylogeny, particularly regarding links between extant temperate distributions and incomplete tropical fossil records from deposits like amber, which suggest undiscovered ancestral forms in warmer paleoenvironments. Ongoing cladistic analyses integrating morphological and molecular data aim to resolve these ambiguities and refine higher-level relationships within Raphidioptera.

Biological significance

Role in pest control

Snakefly larvae are voracious predators of soft-bodied pests such as aphids, psyllids, and scale insects, particularly in orchard settings where they reside under tree bark. In Washington state pear orchards, for instance, larvae contribute to controlling pear psylla (Cacopsylla pyricola) by feeding on nymphs and eggs during early developmental stages. This predatory behavior aligns with their general diet of small arthropods, making them valuable in natural ecosystems adjacent to agriculture. Adult snakeflies play a minor role in , primarily through occasional predation on weak prey like , supplemented by feeding that supports their survival without significant impact on populations. Unlike larvae, adults are less effective predators due to their shorter activity period and focus on and sources. In (IPM) programs for temperate fruit crops, snakeflies are encouraged through habitat preservation, such as maintaining bark cover and nearby woodlands, with natural colonization preferred over artificial releases due to challenges in rearing and establishment. Their efficacy is notable for early-season suppression of pests like pear psylla in cooler regions, but populations decline in hot climates where high temperatures exceed their tolerance, limiting their use in subtropical orchards. Long life cycles often constrain widespread adoption of snakeflies in pest control efforts.

Conservation status

Snakeflies (order Raphidioptera) are not assessed as globally threatened, with no species currently listed on the . However, many populations are locally rare or declining due to their restricted distributions, often confined to specific habitats that are increasingly fragmented. The primary threats to snakefly populations include habitat loss from and , which disrupts their reliance on mature woodlands for and prey availability. use in and further endangers them by reducing populations of small arthropods that serve as their primary food source, while exacerbates range shifts and habitat unsuitability, particularly for temperate species. In , broader declines—estimated at around 25% since 1990—mirror pressures on snakeflies, with Mediterranean populations showing heightened vulnerability due to postglacial refugia now facing intensified fragmentation. Particularly at risk are insular endemics, such as those restricted to Mediterranean islands, which face and habitat alteration. High-altitude species in mountainous regions, including some Inocelliidae taxa in , are also susceptible to warming temperatures that alter forest microclimates essential for their larval development. efforts emphasize the preservation of old-growth forests, which support higher predator diversity, including snakeflies, by maintaining structural complexity like dead wood and canopy layers. Strategies also include reducing agrochemical inputs through to protect prey bases and promoting habitat connectivity to counter fragmentation. Monitoring programs, often leveraging platforms, aid in tracking local abundances and informing targeted protections. Recent 2024 genomic research on snakefly , including a February genome assembly of the black-necked snakefly (Venustoraphidia nigricollis) and a chromosome-level assembly of another , underscores the link between habitat integrity and genetic stability, revealing adaptations that enable persistence in isolated patches but highlighting the risks of further fragmentation to long-term viability.