|A Comparison of the Fascinating Properties and Applications of Silk Produced by Spiders and Insects
By Marisa Aslanian
University of Washington
The study of silk has been critical to our understanding in a variety of subject matter, from protein evolution to relationships in fibrous polymers and the construction of synthetic biomaterials. All spiders and many insects produce silk that is used for various purposes including: shelter, protection of eggs, and prey capture. Silks from different insects are very diverse and differ in their properties and applications. We will discuss and compare the major differences in spider and insect silk through silk analysis, cost estimates, and comparative analysis. Finally, we will discuss recent studies in the designing of synthetic silk proteins.
Silks are proteins produced by all spiders and many insects (Craig et al., 1999). Silk proteins are usually produced within specialized glands of the insect, which are then followed by secretion where the proteins are stored prior to spinning into fibers (Altman et al., 2002).
Silk has been around since 4000 BCE and the Chinese were recorded as the first people to raise and manufacture it (Baird 2000). For centuries, the Chinese used silkworm silk for fabric. (Gullen and Cranston 2005). Silk manufacturing turned into an industry known as sericulture.
Silk is an extremely unique combination of material due to its remarkably strong and highly elastic properties. The most extensively characterized silks are from the domesticated silkworm, Bombyx mori, and from spiders (Nephila clavipes and Araneus diadematus). In fact, Paul Hillyard states in The Book of the Spider 1994, "For an equal diameter, spider silk is stronger than steel, but, much more resilient and can stretch several times before breaking - it is twice as elastic as nylon and more difficult to break than rubber. The energy required to break spider silk (its 'toughness') is about ten times that of other natural materials such as cellulose, collagen and chitin. Dragline silk is especially strong - approximately twice that of silk from silkworms," (Earth-Life 2005).
Insects produce silk in a variety of ways. Silk is produced by at least one representative in all the arthropod orders except the Crustacea (Oup 2005). Systematic analyses of silk producers across the Arthropoda suggest that silks and proteins evolved independently and sometimes more than once (Oup 2005). Silks were first used for reproductive purposes and now serve a function as protection, an adaptive value to the insect. Embioptera (webspinners) contain silk glands that produce silk on the tarsi of their front legs. Interestingly enough, no other group of insects, fossil or modern, has silk-producing glands in the legs (Meyer 2005). Embioptera produce silk to construct nests and tunnels in which they reside and lay their eggs in. In contrast to Embioptera, Lepidoptera produce silk by the labial glands in the head of larvae and is spun from spinnerets around the mouth (Bland 1978). The silk is used to construct shelters and to form the cocoon in which the pupa will reside. The Hawaiian caterpillar, a newly described species, uses silk in a spider like fashion to restrain their live prey, snails (Rubinoff and Haines 2005). The Psychidae (Lepidoptera) larvae uses silk as a defensive role by using a silk case to protect themselves (Gullen and Cranston 2005). Lastly, spiders produce silk through a diverse range of glands and spigots producing different kinds of silk for different purposes such as: housing, web construction, defense, prey, mobility, and food (Lipkin 1996). The silk glands are located on the ventral side of the abdomen towards the rear, which are termed spinnerets. Spiders can have up to six spinnerets, with some having four or even two. Many other insects produce protective silk as larvae with their malpighian tubules or with modified salivary or labial glands (Meyer 2005).
Insects and spiders can be collected for our own use due to their chemical compounds, silk (Gullen and Cranston 2005). Much research has been done on the composition and properties of insect and spider silk. Silk comparative analysis has exposed researches to the different amino acids that make up silk as well as different amounts of ATP involved with silks of different insects. Other research has focused on the unique properties in the designing of new biochemical materials made from silk. Scientists have suggested applications ranging from surgical sutures and ligament repair to parachute straps and high-performance, flexible body armor (Lipkin 1996).
Properties of Silk
Silk is environmentally stable due to its extensive hydrogen bonding, the hydrophobic nature, and significant crystallinity (Altman et al., 2002). Silk protein is organized into β-sheet crystals. The origin of the novel mechanical properties exhibited by silk fibers are due to nanoscale features such as: precise orientation and numerous β-sheet crystals, and the shear alignment of the chains (Altman et al., 2002). The striking mechanical properties provide silk with a combination of strength, toughness, and elasticity (Altman et al., 2002).
Silk Comparative Analysis
Silk was compared across taxa by the amount of ATP invested in production of the amino acids (Craig et al., 1999). The data was compiled from Lepidoptera, Hymenoptera, Coleoptera, Neuroptera, Diptera, Trichoptera, Embioptera, and 17 species of spiders. Results show that Embioptera and Lepidoptera silk are composed largely of alanine, glycine, and serine. Spider silk contains either proline or glutamine. On the other hand, Mantodea, Coleoptera, and Trichoptera silks are more varied in their amino acid composition in that they have a more diverse range. This is because the silk is derived from different glands, that are produced during dissimilar stages of development and that are used for different purposes.
With the exception of the crustacea, all insect taxa produce silks, which are derived from different glands including: colleterial, salivary, dermal glands, and malpighian tubules (Craig et al., 1999). Depending on their foraging ecology, silk is either spun to produce cocoons and egg cases or it is used as glues. The amino acid distributions of the silks were plotted, which determine the insects’ phylogenetic differences. Comparisons were made from silks derived from similar glands, used for similar purposes and produced during similar stages of arthropod development. Results illustrate that The Mantids, Coleoptera, and Neuroptera all use similar glands during similar stages of their lives. They use silk to protect their eggs (Craig et al., 1999). The holometabolous insects: Hymenoptera, Diptera, Siphonaptera, Trichoptera, and Lepidoptera produce silk with the labial gland (Craig et al., 1999). The function of the silk produced by Lepidoptera is for protective shelter. The function of the silk produced by Trichoptera is for shelter and foraging. In summary, results show that the amino acid composition of silks produced by arthropods is highly variable across orders, shows no over-arching trends suggesting phylogenetic constraint.
Comparison of Properties
The remainder of this paper will focus on silks produced by Lepidoptera, the silkworm (Bombyx mori) and spiders due to the substantial data available. By using phylogenies of the araneomorph spiders and Lepidoptera, estimation of the evolutionary relationships were constructed by examining the silk amino acid composition. A Comparison of the locations of the amino acid distribution of silks spun by Lepidoptera and dragline silks spun by spiders differ significantly (Craig et al., 1999). This may be due to the evolution of silk glands and silk synthesis in spiders being more complex than the evolution of silks in insects due to spiders producing silks in multiple glands throughout their lives (Craig et al., 1999). A Comparison of the mean costs, meaning, the amount of ATP to produce silk, shows that dragline silks spun by spiders are significantly more costly to synthesize than silks produced by Lepidoptera. This is because spider silk is mainly composed of proline, glutamine and asparagines, which is extremely important to overall silk cost. On the other hand, Lepidoptera silk is mainly composed of glycine, alanine, and serine; glycine lowers overall cost. These differences in silk composition of the spider and Lepidoptera may be partly due to the variation in their food resources. Lepidoptera have a diet rich in energy but protein poor. Spiders have a diet poor in energy but rich in protein. Therefore, Lepidoptera are able to produce silk at a lower cost due to the abundance of energy. On the other hand, it is difficult to meet silk production needs in a spider because their diet consists of a low energy diet and so, they have a high cost to produce silk. Consequently, Craig and team members propose that the diverse silk-producing systems of the spiders have evolved in response to diet, which enables them to produce a large volume of silk daily that is high in cost. In contrast, Lepidoptera produce silk that is low in cost just once. With the diversity of silk-like proteins from spiders and insects, a range of native or bioengineered variants can be expected for various applications (Altman et al., 2003).
Silk’s diverse and novel mechanical properties, ease of chemical modification, and genetically tailorable properties makes it the perfect candidate for biomedical applications (Hinman et al., 2000). However, for silk to be used in applications, it must be harvested from arthropods. Because of this, synthetic silk is still underdevelopment. Silkworm silk has been used commercially as a biomedical material because it is able to produce high yields of silk. On the other hand, spiders have a very low production of silk due to the predatory and territorial nature of spiders thus there has been no commercialization or biomedical applications thus far (Hinman et al., 2000). But, spider silk remains ideal because it is stronger and more flexible than silkworm silk. What gives spider silk its amazing properties? Helen Hansma and colleagues from the Department of Physics, University of California, Santa Barbara delved into this question. They used the atomic force microscopy (AFM) to analyze a protein solution that had been synthesized from spider dragline silk. Basically, they observed the protein self-assemble into nanofibers, each with a segmented substructure which produced nanocrystals (Gould 2002). Genetic engineering techniques are actively being explored to create synthetic spider silks. Research groups are struggling to spin the first artificial spider silk. Researchers are struggling to determine the fiber's molecular architecture, understand the genes that yield silk proteins, and learn how to spin the raw material into threads (Lipkin 1996).
Research has been very effective in comparing silk between arthropods, which in turn has shed light on information regarding insects and spiders. The observed diversity of silks and complex nature of the composite silk fibroins suggest that their evolution may also be
complex (Oup 2005). It is true that arthropods’ amino acid composition of silks are highly variable, apparently a result of a difference and availability in food resources. The high glycine, alanine, and serine composition of silks in Lepidoptera reflect an energy rich diet leading to a low cost in silk production in contrast to the spiders with a more diverse diet and high multi-gland silk production. Scientists know that alanine and glycine, with lesser amounts of glutamine, leucine, arginine, tyrosine, and serine serve as silk's primary components. However, their exact sequences and structural relationships remain obscure (Lipkin 1996). Perhaps, this could be an area of research interest to scientists.
Silk is everything one desires in a fiber; it’s stable, chemically inert, waterproof, and non-allergenic. This has provided us with invaluable information on new biomaterials such as: stronger non-allergenic sutures, artificial tendons, implantable medical devices, military supplies, light weight mountaineering gear and a whole spectrum of useful materials (Lipkin 1996). However, our complete understanding of silk remains incomplete. Scientists have been unable to synthesize silk. They are moving towards creating spider’s silk by using genetic engineering techniques. I feel this area of research should remain a primary focus. Dragline spider silk is every inch the 21st century super-material--no wonder the field is attracting interest from university researchers, and even military personnel (Gould 2002).
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