The order Embioptera, commonly known as webspinners or footspinners, are a small group of mostly tropical and subtropical insects, classified under the subclass Pterygota. The order has also been called Embiodea or Embiidina. More than 400 species in 11 families have been described, the oldest known fossils of the group being from the mid-Jurassic. Species are very similar in appearance, having long, flexible bodies, short legs, and only males having wings.

Webspinners are gregarious, living subsocially in galleries of fine silk which they spin from glands on their forelegs. Members of these colonies are often related females and their offspring; adult males do not feed and die soon after mating. Males of some species have wings and are able to disperse, whereas the females remain near where they were hatched. Newly mated females may vacate the colony and establish a new one nearby. Others may emerge to search for a new food source to which the galleries can be extended, but in general, the insects rarely venture from their galleries.

Name and entomology

The name Embioptera ("lively wings") comes from Greek (), meaning "lively", and (), meaning "wing", a name that has not been considered to be particularly descriptive for this group of fliers, The common name webspinner comes from the insects' unique tarsi on their front legs, which produce multiple strands of silk. They use the silk to make web-like galleries in which they live.

Early entomologists considered the webspinners to be a group within the termites or the neuropterans and a variety of group names have been suggested including Adenopoda, Embidaria, Embiaria, and Aetioptera. In 1909 Günther Enderlein used the name Embiidina which was used widely for a while. Edward S. Ross suggested a new name, Embiomorpha in 2007. The currently most-widely accepted ordinal name is Embioptera, suggested by Arthur Shipley in 1904.

Evolution

Fossil history

Fossils of webspinners are rare.

Species such as Atmetoclothoda orthotenes, possibly the first fossil member of the Clothodidae to be discovered, sometimes thought to be a "primitive" family, have been found in mid-Cretaceous amber from northern Myanmar. Litoclostes delicatus (Oligotomidae) has been found in the same locality.

The largest number of fossils have been found in mid-Eocene Baltic amber and early-Miocene Dominican amber. It is estimated that there may be around 2000 species extant today.

The external phylogeny of Embioptera has been debated, with the polyneopteran order controversially classed in 2007 as a sister group to both Zoraptera (angel insects) and Phasmatodea (stick insects). The position of the Embioptera within the Polyneoptera suggested by a phylogenetic analysis carried out in 2012 by Miller et al., combining morphological and molecular evidence, is shown in the cladogram. The head has projecting mouthparts with chewing mandibles. The compound eyes are kidney-shaped, there are no ocelli, and the thread-like antennae are long, with up to 32 segments. The antennae are flexible, so they do not become entangled in the silk, and the wings have a crosswise crease, allowing them to fold forwards and enable the male to dart backwards without the wings snagging the fabric. It is these silk glands on the front tarsi that distinguish the embiopterans; other noteworthy characteristics of this group include three-jointed tarsi, simple wing venation with few cross veins, prognathous (head with forward-facing mouthparts), and absence of ocelli (simple eyes).

The abdomen has ten segments, with a pair of cerci on the final segment. These cerci, made up of two segments and asymmetric in length especially in the males are highly sensitive to touch, and allow the animal to navigate while moving backwards through the gallery tunnels, which are too narrow to allow the insect to turn round.

Life cycle

The eggs hatch into nymphs that resemble small, wingless adults. After a short period of parental care, the nymphs undergo hemimetabolosis (incomplete metamorphosis), moulting a total of four times before reaching adult form. Adult males never eat, and leave the home colony almost immediately to find a female and mate. Those males that cannot fly often mate with females in nearby colonies, meaning their chosen mates are often siblings or close relatives. In some species, the female eats the male after mating, but in any event, the male does not survive for long. A few species are parthenogenetic, meaning they can produce viable offspring without fertilisation of the eggs. This phenomenon occurs when a female is, for whatever reason, unable to find a male to mate with, thus giving her and her species reproductive security at all times. Although some species breed once a year, or even once in two years, others breed more frequently, with Aposthonia ceylonica producing four or five batches of eggs in a twelve-month period. The parthenogenetic Rhagadochir virgo incorporates scraps of lichen into the silk wrapping the eggs, and this may be eaten by newly hatched nymphs. Perhaps because individuals of this species are so closely related, the adults spin silk together and move around in coordinated groups. Even in species that provide no further parental care, the nymphs in the colony benefit from the greater silk-producing power of the adults and the extra protection that the more copious silk covering brings.

When webspinners clean their antennae, they may differ in their behavior from other insects which typically make use of the forelegs to either clean or bring the antennae toward the mouthparts for manipulation. Webspinners (as observed in the genus Oligembia) instead fold the antennae under the body and clean the antennae as they are held between the mouthparts and the substrate.

When constructing their silken galleries, webspinners use characteristic cyclic movements of their forelegs, alternating actions with the left and right legs while also moving. There are variations in the choreography of these movements across species.

Silk web production

Embiopterans produce a silk thread similar to that produced by the silkworm, Bombyx mori. The silk is produced in spherical secretory glands in the swollen tarsi (lower leg segments) of the forelimbs, and can be produced by both adults and larvae. Unlike Bombyx mori and other silk-producing (and spinning) members of both Lepidoptera and Hymenoptera, which only have one pair of silk glands per individual, some species of embiid are estimated to have up to 300 silk glands: 150 in each forelimb. and their exceedingly high numbers allow individuals to spin large amounts of silk very quickly, creating extensive galleries. The silk web is produced throughout all stages of the embiopteran lifespan,

Webspinner silk is among the thinnest of all animal silks, being in most species about 90 to 100 nanometres in diameter. The finest of any insect are those of the webspinner Aposthonia gurneyi, averaging about 65 nanometres in diameter. Each thread consists of a protein core folded into pleated beta-sheets, with a water-repellent coating rich in waxy alkanes. and a braconid wasp species in the genus Sericobracon, are known to be parasitoids of adult embiopterans. A few scelionid wasps in the tribe Embidobiini are egg parasitoids of the Embioptera. A protozoan parasite in Italy effectively sterilises males, forcing the remaining female population to become parthenogenetic. These parasites and agents of disease may put evolutionary pressure on embiopterans to live more socially. Some common species have been accidentally transported to other parts of the world, while many native species are unobtrusive and yet to be detected. Some species live underground, or concealed under rocks or behind sections of loose bark. Others live out in the open, either swathed in sheets of white or blue silk, or hidden in less-conspicuous silken tubes, on the ground, on the trunks of trees or on the surface of granite rocks.

References

Further reading

  • World list of extant and fossil Embiidina (California Academy of Sciences)
  • Insects and Human Society: Webspinners with video by The Bug Chicks