Embioptera

(Webspinners)

Class Insecta

Order Embioptera

Number of families 8

Evolution and systematics

Fossil embiids date from the Oligocene, with controversial records from the Lower Permian of the Urals in Russia. The exact phylogenetic position of webspinners is uncertain, but they are considered orthopteroid insects, sharing some characters with earwigs (Dermaptera), stoneflies (Plecoptera), stick insects (Phasmatodea), zorapterans (Zoraptera), and termites (Isoptera). The order Embioptera (or Embiidina) includes eight living families with only 300 described species, although it is estimated that the true number of species is around 2,000.

Physical characteristics

Webspinners are small-to-medium-size insects, with lengths ranging from 0.06 to 0.78 in (1.5 to 20 mm). They have narrow, elongated bodies, which are usually brown or black, an anteriorly directed head with long filiform antennae, and short legs. Both larvae and adults can be easily recognized by their greatly swollen foretarsi. This enlarged foretarsi houses about 100 silk glands, which are used to spin silk galleries, inside which webspinners spent almost their entire life.

The ten-segmented embiid abdomen ends in two-articulated cerci, which are surrounded with sensory hairs and provide tactile orientation against the walls of the galleries, especially when webspinners run backward. Males have large compound eyes and asymmetric external genitalia, and lack wings in some species. When present, the wings have only a few veins, are flexible, and can be folded forward over the body at any point, allowing for reverse movement in the narrow galleries. Some veins are transformed into blood sinuses, which provide temporary stiffness to the wings when used for flight. Females always lack wings, have smaller compound eyes, and are usually larger than males. Larvae look like the adult female, and only male larvae that will develop into winged adult males have external wing buds. Eggs are elongated, with a rimmed, circular operculum.

Distribution

The bulk of the embiid diversity is found in the tropics worldwide, although some species extend into the southern United States and temperate regions of Europe. Embiids are usually absent or poorly represented on islands. Some species have spread through commerce and as a result are widely distributed.

Habitat

Embiids spin their silk galleries on exposed bark or rock surfaces in humid areas; hidden under bark flakes, stones, or leaf litter; and in crevices or cracks in the soil, rocks, or termite mounds. Their galleries can also be found in hanging moss in mountain rainforests.

Behavior

With the exception of occasional dispersal, all embiid activity takes place inside the silk galleries. Webspinners are gregarious, with one or more adult females and their broods sharing a branched system of galleries, which they all spin and extend continuously. The galleries are just slightly wider than the body of the embiids, allowing their sensory body hairs to be in constant contact with the walls. Some species add vegetable detritus and frass to the galleries, providing additional cover and camouflage. Galleries are used as protected routes to food sources, to avoid desiccation, and as escape routes from predators. When an embiid is threatened, it rapidly retreats backward inside the labyrinth of silken tubes.

Feeding ecology and diet

Larvae and adult female embiids feed on vegetable matter, from live moss and lichens to dead leaves and bark. Adult males do not feed.

Reproductive biology

Males use their modified mandibles to hold the female's head during copulation. After mating, males soon die, and females lay a layer of eggs on a silken surface, which is used as a starting point for a new colony or an extension of an old one. Females show parental care, guarding their eggs and young larvae. In some species, females coat the eggs with feces, masticated vegetable matter, or leaf fragments; those in other species move the eggs about in the galleries. There are some known cases of parthenogenetic females, in which the young develop from unfertilized eggs. Development is through gradual metamorphosis.

Conservation status

No webspinner is listed by the IUCN.

Significance to humans

Webspinners are scarcely noticed by humans because of their secluded habits in galleries. They are of no economic importance, as they feed on dead vegetable matter in uncultivated areas.

Species accounts

List of Species

No common name

Australembia rileyi

family

Australembiidae

taxonomy

Metoligotoma rileyi Davis, 1940, Queensland, Australia.

other common names

None known.

physical characteristics

Both sexes apterous; males with large maxillary palp, elongate submentum, and one-segmented left cercus.

distribution

Restricted to savanna zones of North Queensland, Australia.

habitat

Under leaf litter.

behavior

Nothing is known.

feeding ecology and diet

Feeds on vegetable detritus.

reproductive biology

Nothing is known.

conservation status

Not threatened.

significance to humans

None known.

No common name

Antipaluria urichi

family

Clothodidae

taxonomy

Clothoda urichi Saussure, 1896, Trinidad.

other common names

None known.

physical characteristics

Exceptionally large body, at least 0.6 in (16 mm) long. Hind basitarsi with two conspicuous ventral papillae. Strong wing venation. Male caudal tergal processes short and of almost equal size.

distribution

Trinidad.

habitat

Conspicuous colonies on trunks of mountain rainforest trees.

behavior

Silk galleries are bright and visible against the substrate. They have retreat areas with thickened reinforced silk coverings, foraging zones at edges of silk, and silken tunnels built up as embiids travel back and forth between foraging sites and retreats, where they remain during the day. Three styles of spinning behavior: construction of silk scaffold around body and over back; mending of holes by repeatedly attaching silk to existing silk around the hole and stretching new silk across opening; and reinforcement of silk covering by facing it and stepping up and down while releasing many strands of silk.

feeding ecology and diet

Feeds on lichens and algae growing on bark.

reproductive biology

Forms colonies with maternal care of offspring.

conservation status

Not threatened.

significance to humans

None known.

Saunders embiid

Oligotoma saundersii

family

Oligotomidae

taxonomy

Embia saundersii Westwood, 1837, Australia and Brazil.

other common names

None known.

physical characteristics

Adult males have dentate mandibles and unbranched anterior medial wing vein; reddish brown body.

distribution

Native to north central India; widespread by means of commerce throughout warm and tropical regions, even to North America and Southeast Asia.

habitat

Common on trunks of rainforest trees and royal palms (Roystonea spp.) in landscaped areas.

behavior

Tends to live in colonies of mothers and offspring. Area of silk reflects number of gallery occupants, although many silk coverings can be empty, suggesting embiids failed to establish successful colony, have been killed, or dispersed. Gallery surfaces almost completely camouflaged with fecal pellets or finely masticated chips of wood and bark from substrate spun into surface of silk. Males frequently fly to lights.

feeding ecology and diet

Feeds on lichens and algae on bark of trunk of host tree.

reproductive biology

Exhibits maternal care.

conservation status

Not threatened.

significance to humans

None known.

Resources

Books

Ross, E. S. "Embiidina (Embioptera, Webspinners)" In Encyclopedia of Insects, edited by V. H. Resh and R. T. Cardé. San Diego: Academic Press/Elsevier Science, 2003.

——. "Embioptera: Embiidina (Embiids, Web-Spinners, Foot-Spinners)." In The Insects of Australia: A Textbook for Students and Research Workers. Vol. 1, 2nd edition, edited by CSIRO. Carlton, Australia: Melbourne University Press, 1991.

——. "Order Embiidina (Embioptera)." In Immature Insects. Vol. 1, edited by F. W. Stehr. Dubuque, IA: Kendall/Hunt Publishing, 1987.

——. "Web Spinners of Panama (Embiidina)." In Insects of Panama and Mesoamerica, edited by D. Quintero and A. Aiello. New York: Oxford University Press, 1992.

Periodicals

Edgerly, J. S. "Maternal Behavior of a Webspinner (Order Embiidina)." Ecological Entomology 12 (1987): 1–11.

Ross, E. S. "Embia. 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.

——. "Embia. Contributions to the Biosystematics of the Insect Order Embiidina." Part 2, "A Review of the Biology of Embiidina." Occasional Papers of the California Academy of Sciences 149 (2000): 1–36.

Valentine, B. D. "Grooming Behavior in Embioptera and Zoraptera (Insecta)." Ohio Journal of Science 86 (1986): 150–152.

Natalia von Ellenrieder, PhD