Choreography Of Silk Spinning By Webspinners
All webspinners have a remarkably similar body form, although they do vary in coloration and size. The majority are brown or black, ranging to pink or reddish shades in some species, and range in length from 15 to 20 mm (0.6 to 0.8 in). The body form of these insects is completely specialised for the silk tunnels and chambers in which they reside, being cylindrical, long, narrow and highly flexible.[16] 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.[17][18] 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.[6]
Choreography of silk spinning by webspinners
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.[27]
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.[4] These glands are linked to a bristle-like cuticular process known as a silk ejector,[28] 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,[21] and requires modest energy output.[29]
Adult webspinners are vulnerable when they emerge from their galleries, and are preyed on by birds, geckos, ants and spiders. They have been observed being attacked by owlfly larvae. 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.[6]
I investigate questions about the behavior and ecology of insects. I have studied treehole mosquitoes, embiids (webspinning insects) and tent caterpillars. Presently, my students and I are concentrating our attention on the little known group of insects, the embiids (Order Embiidina or Embioptera). I'm working with colleagues at the University of New Mexico and the University of California Riverside on the evolution of silk spinning in embiids.
I studied webspinners for my Ph.D. work at Cornell University and continued to study them off and on over the years. Mostly my work focuses on basic behavior and ecology of these insects. They remain one of the least known of all the insect orders, perhaps because they are mostly tropical and quite secretive. They live under silken covers and within tunnels that they spin with silk produced by glands in their front feet. My research seeks to determine the level of social organization in these colonial insects, and to discover which factors influence their behavior including their use of silk. I also wish to describe their basic behavior in a detailed manner,because until now most reports are anecdotal. Below I list some of the research problems I am investigating with colleagues and my students.
Embiids vary in their tendency to spin silk although the motions of spinning are probably very similar. Some spin copious amounts whereas others are reluctant to spin at all when removed from their silken domiciles. The latter seems to rely on substrate materials to construct their homes and use silk as an adhesive. The former creates elaborate silk domiciles using the substrate as foundation only. Questions arise: does their variable reliance on silk relate to their tendency to share silk? Are colonial embiids more likely to need and use more silk, and hence can they benefit from sharing? Is it costly to spin? Recent work in my lab suggests that silk spinning is about half as metabolically costly as locomotion. As yet we have not been able to demonstrate a reproductive consequence of silk spinning for three species tested thus far. Embiid silk is constructed of the simplest of all amino acids, those that are readily available to an animal producing such a protein. Perhaps silk is not costly to produce because the amino acids are so ubiquitous in their food. We are still investigating this question. Because silk dominates the lifestyle of webspinners the question deserves to be asked and hopefully answered.
Silk spinning defines the morphologically constrained embiopterans. All individuals spin for protection, including immatures, adult males and the wingless females. Enlarged front tarsi are packed with silk glands and clothed with ejectors. They spin by stepping with their front feet and releasing silk against substrates and onto preexisting silk, often cloth-like. Spinning is stereotypical and appears to differ between species in frequency and probability of transition between two spin-step positions. This spinning choreography was assessed using thousands of spin-steps scored in the laboratory for 22 species to test: (1) the body size hypothesis predicting that spinning would be more complex for larger species; and (2) the phylogeny hypothesis which predicted that spinning would display phylogenetic signal. Tests relied on published phylogenies for the order Embioptera. Independent contrast analysis revealed relationships between five spin characteristics and body size, whereby, for example, larger webspinners invested in relatively larger prothoracic tarsi used for spinning and in spin-steps that would yield expansive silk coverings. Spin-step dynamics displayed a phylogenetic signal for the frequency of six spin-steps and for 16 spin-step transitions. Discussion focuses on patterns revealed by analysis of phylogenetic signal and the relationship to life style and to recently discovered chemical characteristics of silk.
Figure 2. Variation in silk domiciles and habitat for webspinners and potential predators of Antipaluria urichi of Trinidad. (A) Haploembia tarsalis and (B) H. solieri silk in California. (C) Pararhagadochir trinitatis silk in Trinidad. (D) Notoligotoma hardyi silk on granite outcrop on Magnetic Island. (E)Metoligotoma incompta silk on Magnetic Island, Queensland. (F) H. tarsalis adult female (1.0 cm in length). (G) A. urichi silk in Trinidad showing thick silk typical of an egg-guarding female. (H) Bay-headed Tanager and Woodcreeper, two birds seen tearing into Antipaluria urichi silk. (I) Gecko preying upon adult female of A. urichi caught outside of her silk. For scale, the webspinner is approximately 1.7 cm in length. Photographs by the author; painting of bird predation by Edward C. Rooks. 041b061a72