In the context of the passage, the main function of the first paragraph (lines 1–8) is to:
","rationale_html":null,"correct_answer":["B"],"equiv_key":"b9f2cd73412d853fbb5f4538","has_media":false,"has_mathml":false,"has_table":false,"section_label":"Reading","options":[{"label":"A","content_html":"provide an overview of the internal process that enables spiders to produce different types of silk.
","ord":0},{"label":"B","content_html":"illustrate the strength and versatility of spider silks by describing how one particular spider uses its silks to create a web.
","ord":1},{"label":"C","content_html":"introduce the idea that spiders are resourceful by describing the obstacles they encounter when producing silks for their webs.
","ord":2},{"label":"D","content_html":"point out that the properties of silks made by spiders are similar to those of silks made by other animals.
","ord":3}],"passage":{"id":"act_practice_pdf:pt2:reading:p3","source_id":"act_practice_pdf","form_id":"act_practice_pdf:pt2","section":"reading","number":"III","kind":"INFORMATIONAL","title":null,"content_html":"This passage is adapted from the article “Spiders: Web of Intrigue” by Katherine Bourzac (©2015 by Springer Nature). The graphic is adapted from the article “Spider Silk–Inspired Artificial Fibers” by Jiatian Li et al. (©2021 The Authors, Wiley-VCG GmbH). A Madagascan bark spider releases a silk dragline into the air. The wind carries the thin threads to the other side of a river, where they land on foliage on the opposite bank 25 metres away. The bark spider (Caero5 stris darwini) then stretches the bridgeline to establish tension, reinforces it, and draws on a palette of other silks, stretchier or stickier as needed, to fashion a web to capture the bugs flying over the water. C. darwini’s bridging silk is the world’s toughest 10 known biomaterial—it is even tougher than steel fibre. But C. darwini’s versatility in producing different kinds of silk is not unique. Many spiders can spin several silks: stiff, structural strands to stabilize their webs; gooey, stretchy spirals to capture flying insects; adhe15 sive pads to anchor their homes in place; and extraordinarily robust draglines from which to hang. The remarkable mechanical properties of these natural fibres have attracted the attention of materials scientists. Researchers are looking to arachnids and 20 other silk makers for ideas about how to make new structural materials for bridges and vehicles, dirtresistant adhesives for climbing robots and sturdy polymers for biomedical devices. Many silks bring together properties that are not readily present in man-made 25 materials—the extreme toughness and elasticity seen in spider threads is one example. Silk proteins can be moulded like plastic or perform optical functions like silicon. Yet because they’re organic, biological materials, silks are environmentally friendly and biocompati30 ble. Silk proteins can be fashioned into films that can be implanted in the body, releasing drugs as they dissolve. This combination of features is unavailable in polyester or collagen or anything else, says David Kaplan, an early proponent of high-tech biomedical 35 silk at Tufts University in Medford, Massachusetts. “There’s clearly a need for new biomaterials,” he says. For Kaplan and others, silk is the best way to meet that need. Silk evolved independently in many invertebrates, 40 including spiders, honeybees and silkworms. Individual spiders can make as many as six different kinds of silk proteins (and two glue proteins), each of which has evolved over the creatures’ 400 million years of natural history. Each spider species uses its own variations of 45 these proteins to make many different types of thread. “We think that a primordial spider had one kind of silk, and then there were multiple events when the gene duplicated and evolved,” says Cheryl Hayashi, a spider specialist at the University of California, Riverside. 50 The species that are more closely related to these ancestors, such as tarantulas and trapdoor spiders, make silks of simple designs—messy tangles to trap walking insects, for example, using fewer types of silk. Other spiders evolved to make more complex spiralling orb 55 webs, in which different regions are composed of different kinds of silk—some optimized for capturing prey, others for structural support of large web designs. This evolutionary bounty has happy implications for engineers looking to put spider silk into human ser60 vice. If a design calls for a fibre with a particular ratio of strength to stretchiness, “it’s probably already been invented” by one of the tens of thousands of types of spider, says Hayashi. Most research has centred on taking advantage of 65 the toughness of spider silk—in materials science, toughness is a measure of how much energy it takes to break something. Materials such as spider silk are both strong and elastic. A large insect that flies into a spiderweb at top speed stretches the superfine fibres in the 70 web but does not break them. The toughest silks are found in spider draglines, which researchers are studying intensely. Spiders use draglines to dangle safely, to make the frames of their webs, and for situations in which resistance to breakage 75 is paramount. In a scene from the 2004 movie SpiderMan 2, the eponymous superhero stops a runaway New York City subway train with his webbing, which is not too far of a stretch from reality. Mechanical Properties of Natural and Synthetic Fibers Strength Elasticity Toughness ‡ 3§ Material (GPa ) (%) (MJ/m ) Bark spider 1.6 52 354 MA silk* Silver garden † 0.095 465 75 spider flag silk Domestic 0.6 18 70 silkworm silk Nylon fiber 0.95 18 80 Kevlar 49 fiber 3.6 2.7 50 Carbon fiber 4 1.3 25 High-tensile 1.5 0.8 6 steel fiber *MA silk: non-sticky; used to make draglines and bridgelines and to anchor webs † flag silk: sticky; used to capture prey in webs ‡ gigapascal § megajoule per cubic meter
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