WEBSPINNERS: Embioptera

PHYSICAL CHARACTERISTICS

Webspinners are small to medium insects, ranging from 0.06 to 0.78 inches (1.5 to 20 millimeters) in length. They have long, narrow bodies that are usually brown or black in color. Their distinctive head has chewing mouthparts that are directed forward. The antennae (an-TEH-nee), or sense organs, are long and threadlike. They have compound eyes, with each eye having multiple lenses, but no simple eyes, each with a single lens.

The legs are short and thick. Both adults and larvae (LAR-vee), or the developing young form of the animal, are easily distinguished from other insects by their enlarged front feet. These feet contain about one hundred silk glands. The glands are used to spin silk into a network of narrow, hollow tubes, or galleries, that make up the webspinner's home. Their legs are built in such a way that they can move forward and backward with equal agility and speed. Both winged and wingless males may occur in the same species, but the larvae and females are always wingless. All four of the male's wings are similar in size and shape. In flight the wings stiffen through increased blood pressure within special chambers inside each wing. At rest the chambers deflate and the wings lay flat over the body. They are capable of bending or crumpling at any point without damage and are easily bent forward to allow for easy backward movement through the narrow galleries.

The abdomen appears ten-segmented. The eleventh segment is small and difficult to see. The abdomen ends in a pair of short, fingerlike projections bristling with tiny hairs. These structures function like antennae and help to guide their backward movement within the galleries.

GEOGRAPHIC RANGE

There are about three hundred species of webspinners known worldwide, but it has been estimated that there may be as many as two thousand. There are thirteen species found across the southern United States. Webspinners are found on all continents, except Antarctica. Most species live in tropical or subtropical climates. There are usually very few or no species living on remote islands. Some species have spread to several continents through overseas trade.

HABITAT

Webspinners build their silk galleries on exposed bark or rock surfaces in humid habitats or underneath bark, stones, or leaf litter. Others live in crevices (KREH-vuh-ses) or cracks in bark, soil, rocks, or termite mounds. Galleries are also found on hanging moss in mountain rainforests.

DIET

Larval and adult female webspinners feed mostly on vegetable matter, including moss, lichens (LIE-kuhns), dead leaves, and old bark. Adult males do not feed.

BEHAVIOR AND REPRODUCTION

Webspinners spend most of their lives inside their silk galleries. The galleries maintain a moist environment, provide clear routes to food sources, and serve as shelters from predators (PREH-duh-ters), or animals that hunt other animals for food. The galleries are slightly wider than the webspinners to allow the sensory hairs covering their bodies to remain in constant contact with the walls. Some species add bits of vegetable materials and their own waste to the outside gallery walls to provide additional camouflage and protection. Both larvae and adults are capable of spinning silk, and the galleries are continuously expanded over or into new food supplies. Webspinners live in groups with one or more adult females and their young. When threatened, they will retreat backwards into the silken tubes or, on occasion, pretend to be dead.

On warm afternoons or after the first rains following the dry season, winged males take to the air in search of mates. Mating takes place within the safety of the gallery. Males use their mouthparts to hold the female's head while mating. Because of their lack of mobility wingless or reduced-winged males often mate with their sisters. Males die soon afterward, and the females lay a single layer of eggs in a batch in the gallery. In some species, the females are able to reproduce by parthenogenesis (PAR-thuh-no-JEH-nuh-sihs), a process where the young develop from unfertilized eggs. Females guard their eggs and young. In some species, females coat their eggs with their own waste and chewed up bits of vegetable material, while others move the eggs about inside the galleries. The young strongly resemble the adults and develop gradually through a series of molts, or shedding of their exoskeleton, or hard outer covering. Wing pads develop only in the male larvae of winged species.

WEBSPINNERS AND PEOPLE

Because most species are very secretive, spending most of their lives in silk galleries, webspinners are hardly noticed by humans. They are never considered pests because they feed on dead vegetable matter.

DR. WEBSPINNER

For more than 60 years, Dr. Edward S. Ross, Curator Emeritus at the California Academy of Sciences in San Francisco, has devoted his life to the study of webspinners. He has traveled thousands of miles on every continent that webspinners call home. Over the years he has collected more than 300,000 specimens, representing about 1,000 species. Of these, approximately 750 are new to science. Dr. Ross has maintained hundreds of living colonies of webspinners in his laboratory to study and photograph their behavior.

CONSERVATION STATUS

No webspinner is listed as endangered or threatened.

SAUNDERS EMBIID (Oligotoma saundersii): SPECIES ACCOUNT

Physical characteristics: Adults measure 0.35 to 0.5 inches (8.9 to 12.7 millimeters) in length. Males have toothed jaws and reddish brown bodies, while the females are chocolate brown.

Geographic range: This species is native to north central India but is now widespread throughout the tropical regions of the world, as well as parts of the southern United States and Southeast Asia.

Habitat: This species is commonly found on trunks of rainforest trees, as well as on royal palms in gardens and parks.

Diet: They eat lichens (LIE-kuhns) or mosses and algae (AL-jee), tiny plantlike organisms, growing on the bark of tree trunks.

Behavior and reproduction: This species lives in colonies made up primarily of mothers and their offspring. The females guard the eggs. Gallery surfaces are almost completely camouflaged with webspinner waste or finely chewed pieces of wood and bark. Winged males are commonly attracted to lights.

Saunders embiids and people: This species does not impact humans or their activities.

Conservation status: This species is not endangered or threatened ∎

FOR MORE INFORMATION

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.

Periodicals:

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

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

Web sites:

"Embioptera." Department of Entomology, North Carolina State University. http://www.cals.ncsu.edu/course/ent425/copendium/webspi~1.html (accessed on September 27, 2004).

"Embioptera." Ecowatch.http://www.ento.csiro.au/Ecowatch/Insects_Invertebrates/embioptera.htm (accessed on September 27, 2004).