Webspinners, also commonly known as footspinners, are subsocial insects that constitute the order Embioptera. They produce threads of fine silk from specialized silk-producing glands on their forelegs, building extensive tunnels and chambers (galleries) on substrates such as rocks, tree barks, or in leaf litter. Each silk thread is around 90 to 100 nm in diameter, making it one of the thinnest of all animal silks.
These insects are hemimetabolous, undergoing incomplete metamorphosis, with three life cycle stages: egg, nymph, and adult.
They are typically 0.6 to 0.8 in (15 to 20 mm) long.
These insects are slender, soft-bodied, and well-adapted to life within silk tunnels. While most are black or brown, a few species, such as the pink webspinner (Haploembia tarsalis), have body shades of pink, pale orange, or red. Like other insects, their bodies are divided into three distinct regions: the head, thorax, and abdomen.
The head is prognathous, with the mouthparts directed forward. They have chewing-type mandibles adapted for feeding on plant matter. Additionally, the head bears a pair of long, thread-like antennae for sensory reception. Each antenna comprises up to 32 bead-like segments, depending on the species.
A pair of kidney-shaped compound eyes is located laterally on the sides of the head, just behind the base of the antennae. However, they lack simple eyes or ocelli.
The thoracic region is subdivided into pro-, meso, and metathorax. While the prothorax is small and narrow, the mesothoracic and metathoracic segments are increasingly larger and broader. Each of the three segments bears a pair of short, sturdy legs, with each leg having five basic segments: coxa, trochanter, femur, tibia, tarsus.
The tarsus is subdivided into three smaller subsegments called tarsomeres. In webspinners, the first segment of the tarsus (basal tarsomere) in the forelegs is greatly enlarged and bears silk-producing glands. Unlike members of both Lepidoptera and Hymenoptera, which have one pair of silk glands per individual, some webspinners, such as the black webspinner (Oligotoma nigra), possess up to 300 silk glands.[1]
The females and nymphs are wingless and hence, flightless. Depending on the species, the males may be winged or wingless. In winged species, such as the Saunders’ embiid (Oligotoma saundersii), the mesothorax and the metathorax bear the forewings and the hindwings.
The highly flexible abdominal region is composed of 10 visible segments, with the last segment terminating in a pair of sensory appendages called cerci.
In females, the abdominal tip is equipped with a short ovipositor for laying eggs, while in males it carries a pair of claspers used to grasp the female during copulation.
The common name, webspinner, derives from their unique ability to spin strands of silk using the glands on their forelegs. The order name, Embioptera, has historically had several synonyms, including Embidina, Embiida, Embiidina, and Embiodea.
Over 400 species of webspinners are classified into 11 families.[2]
The oldest known webspinners are represented by Sinembia rossi and Juraembia ningchengensis, two Middle Jurassic fossils (around 165 million years ago) discovered from the Jiulongshan Formation of Inner Mongolia, China.[3]
These insects are found worldwide except in Antarctica, with notably high diversity in the tropics. However, webspinners are also found in temperate areas such as the Mediterranean, the southern United States, and parts of East Asia.[4]
They are highly prone to desiccation and prefer building their silk galleries in shaded, hidden areas to avoid exposure to the sun. For instance, they are found living under bark flakes, stones, or leaf litter, as well as in soil and rock crevices. Some species, such as Antipaluria urichi, are arboral, living on vertical surfaces like tree trunks.
The larvae and adult females are generalist herbivores, feeding on moss, lichens, bark, algae, and plant debris. The males of most species, on the other hand, do not feed (despite having functional mouthparts) and die soon after mating.
While adult females typically live for 3 to 6 months, the males survive only 1 to 2 weeks.
Most species reproduce sexually, but some, such as Rhagadochir virgo, are exclusively parthenogenetic and can reproduce asexually without the need for fertilization. Although many webspinners breed once a year, or even once in two years, some species, such as Aposthonia ceylonica, produce four to five batches of eggs annually.
Males leave their galleries to search for females, sensing their presence with their antennae. During copulation, the male grasps the female with his claspers and transfers his sperm to the female’s reproductive tract. After fertilization, the female lays small, ovoid eggs (either in clusters or singly) inside silk galleries or protected crevices beneath bark, leaves, or stones. Once the eggs are laid, the mother becomes territorial and aggressively guards them, even toward individuals with whom she previously lived peacefully.
The eggs hatch into nymphs that resemble small, wingless adults. These nymphs undergo a total of 4 molts before becoming adults.
Adult webspinners are vulnerable, especially right after their emergence, to birds, ants, harvestmen, and spiders. They have also been found to be attacked by larvae of owlflies.[5]
Several parasitoids affect these insects at different stages of their life cycle. For instance, wasps of the family Sclerogibbidae lay eggs on the abdomen of webspinner nymphs. Similarly, a few wasps of the family Scelionidae are egg parasitoids.[6]
A tachinid fly, Perumyia embiaphaga, and a wasp in the genus Sericobracon parasitize adults.[7]
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