Webspinners

Webspinners or footspinners belong to the Phylum Arthropoda, Class Insecta, Subclass Pterygota, and Order Embioptera. They are a small group of mostly tropical and subtropical insects. More than 400 described species in eleven (eight extant) families are known, but it has been estimated that there might be around 2,000 extant species. Only three families (nine species) are present in North America: Anisembiidae (two species), Oligotomidae (three species), and Teratembiidae (four species). All of these occur in the southern United States, and most have fairly restricted ranges. At least two species occur in Arkansas, although very little is known about the embiopterans of the state.

Embiopterans are found on every continent except Antarctica, with the highest density and diversity of species being in the tropics; some also extend into the temperate regions of Europe. Interestingly, they are usually absent or poorly represented on islands. Webspinners are mostly restricted to warmer locations and are found as far north as Virginia at 38°N latitude and at an altitude as high as 3,500 m (11,500 ft.) in Ecuador in South America. Some common species have been accidentally introduced through commerce to other regions of the planet, while many native species are inconspicuous and are yet to be discovered or described. For example, webspinners were thought to be absent from the United Kingdom until 2019, when a southeastern Asian species (Aposthonia ceylonica) was found in a glasshouse at the Royal Horticultural Society Garden in London.

Entomologists of the past considered the webspinners as a group within the termites (Isoptera) or the neuropterans (Neuroptera), and a variety of taxonomic group names were suggested, including Adenopoda, Aetioptera, Embidaria, and Embiaria. In 1909, German zoologist and entomologist Günther Enderlein (1872‒1968) introduced the name Embiidina, which was widely in use for a while. In 2007, Curator Emeritus at the California Academy of Sciences in San Francisco and world authority on webspinners, Edward S. Ross (1915–2016), suggested a new name, Embiomorpha. Today, the most widely accepted ordinal name is Embioptera, suggested in 1904 by English zoologist Arthur Shipley (1861‒1927).

Fossils of webspinners are rare. The oldest known fossils, described in 2009, were Juraembia ningchengensis and Sinembia rossi (family Sinembiidae) from the Middle Jurassic (174 to 163 million years ago) of Inner Mongolia. The female of J. ningchengensis had wings, supporting a proposal that both sexes of ancestral Embioptera had wings. The largest number of fossil embiids has been found in mid-Eocene Baltic amber (forty-four million years ago) and early Miocene Dominican amber (fifteen to twenty million years ago). In the United States, flattened compression fossils that have been understood as being webspinners have been found from the thirty-four-million-year-old Eocene/Oligocene shales of Florissant (Teller County) in Colorado.

The exact phylogenetic position of webspinners is unclear, but they are considered by some to be included within the orthopteroid complex of insects, sharing some characters with earwigs (Dermaptera), stoneflies (Plecoptera), stick insects (Phasmatodea), zorapterans (Zoraptera), and termites (Isoptera). However, a good number of insect taxonomists believe webspinners represent another evolutionary “dead end” that diverged about the same time as Plecoptera.

All webspinners have similar body forms and range in length from 15 to 20 mm (0.6 to 0.8 in.). Coloration varies in specimens and ranges from brown to black, or from pink to red. Species are alike in appearance, having long, narrow, and highly flexible bodies, along with short legs. The first segment of the thorax is small and narrow, and the second and third are larger and broader, especially in the males, where they include the flight muscles. The head has forward-facing (prognathous) mouthparts projecting with chewing mandibles and they possess kidney-shaped compound eyes with no ocelli (simple eyes). Their thread-like, flexible antennae are long, with up to thirty-two segments. Most adult males (depending on species) possess wings (but some are without wings or full development of other normal adult features); nymphs and female specimens are notoriously difficult to identify. Therefore, identification keys are based entirely on male characteristics. Because morphology is so similar between taxa, the main form of taxonomic identification used in the past has been close observation of distinctive copulatory structures of males (although this method is now thought by some entomologists and taxonomists as providing insufficient detail).

Two pairs of wings occur in webspinners, similar in size and shape and having few cross veins, with long and narrow simple venation. This morphology allows them to fold forward and enables the male to dart backward, permitting easy movement through their narrow silk galleries, either forward or backward, without resulting in damage.

In both sexes the legs are short and sturdy, with an enlarged basal tarsomere on the front pair, containing the silk-producing glands; the mid and hind legs also have three tarsal segments, with the hind femur enlarged to house the strong tibial depressor muscles that enable rapid reverse movement. The abdomen has ten segments, with a pair of cerci on the last segment. These cerci, made up of two segments and asymmetric in length (especially in the males), are highly sensitive to touch and allow the insect to navigate while moving backward through the gallery tunnels, which are too narrow to allow the individual to turn round. Although males have mouthparts similar to those of the females, they never eat during their adult stage. Their mouthparts are used to hold on to the female during copulation.

Silk galleries (with spherical secretory glands in the swollen tarsi located in the forelegs) are produced by webspinners, and they are composed of spun tunnels and chambers woven from the silk they produce. Not one other taxon of insect, either extinct or modern, possesses silk-producing glands in its legs. With the exception of occasional dispersal, all embiid activity takes place inside these galleries. One or more adult females and their broods share a branched system of galleries, which they all continuously spin and lengthen. Some species conceal their galleries by adorning the outer layers with bits of leaf litter or other materials to match their surroundings. The galleries are essential to their life cycle and are used as protected routes to food sources, in maintaining moisture in their environment to avoid desiccation, and also offering protection and escape routes from predators and the elements while foraging, breeding, and simply existing. When an embiid is threatened, it rapidly retreats backward inside the labyrinth of silken tubes.

Embiopterans only leave the gallery complex in search of a mate, or when females explore the immediate area in search of a new food source. Webspinners continually extend their galleries to reach new food sources, and they expand their current galleries as they grow in size. The insects spin silk by moving their forelegs back and forth over the substrate, rotating their bodies to create a cylindroid, silk-lined tunnel. Older galleries are differentiated by having multiple laminate layers of silk. Each gallery complex may contains several individuals, often stemming from a single female, and forms a maze-like structure, extending from a secure retreat into whatever vegetable food matter is available nearby. The size and complexity of the colony varies among species, and they can be very extensive in those species that live in climates that are hot and humid.

For reproduction, adult males, which never eat, exit the home colony almost immediately to disperse and search for a female to mate with. However, those that cannot fly often mate with females in neighboring colonies, causing their chosen mates to often be siblings or close kin. Males use their modified mandibles to hold the female’s head during copulation. In some species, the female ingests the male after mating, but in any event, the male soon dies from starvation. After molting and mating, the females lay a single lot of eggs on a silken surface, which is used as a foundational point for a new colony or an extension of a former one, or simply stray away elsewhere to start a new colony. Most females exhibit parental care by guarding their eggs and young larvae, the main exception being species such as Saussurembia calypso, which scatter their eggs widely. In some species, females coat their eggs with fecal matter, masticated vegetable matter, or leaf fragments; those in other species move the eggs about in the galleries. After a short period of parental care, the nymphs (resembling small, wingless adults) undergo hemimetabolosis (incomplete metamorphosis), molting a total of four times before reaching adult stage. There are some known cases of parthenogenetic females (as in Rhagadochir virgo), in which the young develop from unfertilized eggs. This phenomenon occurs when a female is unable to find a male to mate with, thus giving her and her species reproductive security at all times. In addition, although some species breed once per year, or even once every two years, others breed more frequently. For example, A. ceylonica produces four or five batches of eggs in a year.

In terms of microhabitat, some webspinner species live underground or conceal themselves under rocks or behind sections of loose tree bark. Others may be found out in the open, either enveloped in sheets of blue or white silk, or hidden in less-conspicuous silken tubes, on the ground or within the trunks of trees or on the surface of granite rocks. Galleries can be found on substrates such as rock surfaces in humid areas; under the bark of trees, under stones, or in leaf litter; and in crevices or cracks in soil, rocks, or termite mounds. Also, their galleries can be found in hanging moss in mountainous rainforests.

Most, if not all, embiopteran species are gregarious but subsocial. Typically, adult females inhabit large colonies with other adult females, creating and sharing the webbing cover that assists in protecting them against predation. The evolutionary advantages of living in these colonies outweigh the disadvantage that results from the increased parasite load that this lifestyle entails.

The diet of embiopterans fluctuates among species, with available food sources changing with varying habitat. The nymphs and adult females feed on algae, bark, dead leaves, grass, lichens, moss, and plant litter. They are generalist herbivores, and those sustained in the laboratory have survived on a diet of lettuce and dry oak (Quercus) leaves.

When they emerge from their galleries, adult webspinners are helpless and are preyed upon by ants, spiders, geckos, and birds. For example, birds may pull sheets of silk off the galleries to expose their prey, ants may cut holes to gain entry, and harvestmen may pierce the silk to feed on the webspinners inside. They have also been observed being attacked by owl fly (Neuroptera, Ascalaphidae) larvae.

There are several specialist parasites (parasitoids) of webspinners. A small family of aculeate wasps (Hymenoptera: Sclerogibbidae) includes individuals that lay an egg on the abdomen of a nymphal webspinner. The larval wasp emerges and attaches itself to the host’s body and, as it grows, consumes the host’s tissues and eventually forms a cocoon and falls off the carcass. Others include a Neotropical tachinid dipteran (Perumyia embiaphaga) and a braconid wasp species in the genus Sericobracon. A few scelionid wasps in the tribe Embidobiini are egg parasitoids of the Embioptera. A protist parasite of embiopterans in Italy efficiently sterilizes males, forcing the remaining female population to become parthenogenetic.

Some associates of webspinners live inside their galleries and include true bugs (Hemiptera) in the family Plokiophilidae. However, it is unclear whether they are feeding on eggs or larvae, are eating mites and other residents of the gallery, or are simply scavengers. The embiopteran A. ceylonica has been found living inside a colony of the Indian cooperative spider (Stegodyphus sarasinorum), probably feeding on algae growing on the spider sheetweb, and two webspinner species have been discovered living in the outer covering of termite (Isoptera) nests, where their silk galleries may protect them from attack.

Very little is known about the embiopterans of Arkansas. The first specimens were reported from the state in 1941. The black webspinner, Oligotoma nigra, an introduced species, was reported from Arkansas in 1988. It is thought that O. nigra was accidentally brought to the United States in the late 1800s on a shipment of date palms shipped from Egypt to the Gulf Coast region. Another species, Anisembia texana, is also known from the state.

For additional information:
Alberti, G., and V. Storch. “Ultrastructural Investigations on Silk Glands of Embioptera (Insecta).” Zoologischer Anzeiger 197 (1976): 179–186.

Arnaud, P. H. “Perumyia embiaphaga, a New Genus and Species of Neotropical Tachinidae (Diptera) Parasitic on Embioptera.” American Museum Novitates 2143 (1963): 1–9.

Carlton, C. E., and D. G. Bassi. “Oligotoma nigra (Hagen) (Embiidina: Oligotomidae), an Introduced Species New to Arkansas.” Journal of the Kansas Entomological Society 61 (1988): 352‒353.

Choe, J. C. “Communal Nesting and Subsociality in a Webspinner, Anisembia texana (Insecta, Embiidina, Anisembiidae).” Animal Behaviour 47 (1994): 971‒973.

Edgerly, Janice S., J. A. Davilla, and N. Schoenfeld. “Silk Spinning Behaviour and Domicile Construction in Webspinners.” Journal of Insect Behavior 15 (2002): 219–242.

Edgerly, Janice S., S. M. Shenoy, and V. G. Werner. “Relating the Cost of Spinning Silk to the Tendency to Share it for Three Embiids with Different Lifestyles (Order Embiidina: Clothodidea, Notoligotomidae, and Australembiidea).” Environmental Entomology 35 (2006): 448–457.

Engel, Michael S., and David Grimaldi. “The Earliest Webspinners (Insecta: Embiodea).” American Museum Novitates 3514 (2006): 1–22.

Ross, Edward S. “Biosystematics of the Embioptera.” Annual Review of Entomology 15 (1970): 157–172.

–——. “Contributions to the Biosystematics of the Insect Order Embiidina. Part 1. Origin, Relationships and Integumental Anatomy of the Insect Order Embiidina.” Occasional Papers of the California Academy of Sciences 149 (2000): 1–53.

———. “The Embioptera of California.” Bulletin of the California Insect Survey 6 (1957): 51‒58.

———. “A Revision of the Embioptera, or Web-Spinners, of the New World.” Proceedings of the United States National Museum 94 (1944): 401–504.

———. “A Synopsis of the Embiidina of the United States.” Proceedings of the Entomological Society of Washington 86 (1984): 82–93.

Sanderson, M. W. “The Order Embioptera New to Arkansas.” Journal of the Kansas Entomological Society 14 (1941): 60.

Shaw, S. R., and Janice S. Edgerly. “A New Braconid Genus (Hymenoptera) Parasitising Webspinners (Embiidina) in Trinidad.” Psyche 92 (1985): 505–511.

Valentine, Barry D. “Grooming Behavior in Embioptera and Soraptera (Insecta).” The Ohio Journal of Science 86 (1986): 150–152.

Chris T. McAllister
Eastern Oklahoma State College