Ask Audubon

Why don't spiders get entangled in their webs?

Linnea Saukko, Hilliard, Ohio

Arachnophobes may derive a modicum of satisfaction from learning that spiders can indeed get caught in their own webs. But these arachnids also have Houdini-like powers to extricate themselves, unlike their hapless victims. While all spiders produce silk, a material that is, ounce for ounce, stronger than steel, not all silk is adhesive. Depending on its modus operandi, a species may secrete up to seven different types of filament for various purposes, such as wrapping quarry, reeling out draglines, building nests, or encasing eggs. Of the 35,000 known species, only about half build prey-trapping webs, including orb weavers. These master spinners employ both glue-beaded strands, for snaring, and nonstick anchor lines, and they instinctively know which ones to avoid, says University of Cincinnati entomologist George Uetz. As an added measure, they have specially modified tarsi, or feet, with hooked claws and opposing serrated hairs, between which a strand is grasped. A spider uses these claws to tiptoe from fiber to fiber without touching the tacky parts, enabling it to move over its domicile of doom without getting stuck. If accidentally ensnared, some wily spiders will join clingy threads together to break free. This way, they can slip from their bindings in time to pounce on the next web-foiled meal.

Do conifers become dormant over winter, since they do not drop their needles or lose their green color?

Mark Glaser, Dickinson, North Dakota

No plants retain all their chlorophyll-laden leaves forever, and none can sustain normal growth during the killing frosts, shorter days, and aridity of a northern winter. Evergreens--the term typically applied to temperate-zone conifers but also some broad-leaved species--are not exempt, despite what their name connotes. Like deciduous trees preparing themselves for the wintry onslaught, evergreens lose their leaves, just not all at once. Instead, cone-bearing trees continuously shed and replace their needles, which are more watertight than broad leaves and can carry out photosynthesis year-round, albeit at a diminished rate in cold seasons. To deal with winter's adversities, all trees undergo a slowdown in cellular activity. How long needles stay on branches varies with the species. Hemlock needles are relatively short-lived, hanging on for only a couple of years before dropping. Bristlecone pine needles, on the other hand, can persist as long as three decades. Some of these hardy trees have weathered more than 4,700 growing seasons, making them the planet's oldest living things. Because of their survival strategies, evergreens are among the most tenacious of organisms; many of the 630 conifer species thrive in the coldest, droughtiest, most inhospitable habitats on earth.

How can a platypus be a mammal if it lays eggs?

Eric McClanahan, St. Louis, Missouri

Victorian scientists were so mystified by the first platypus specimen sent home by British explorers that they suspected it to be the handiwork of a mischievous taxidermist. With its furry coat, beaverlike tail, duck-shaped bill, and webbed feet, it defied not only belief but also easy classification ("partly birdly, partly mammaly," wrote the poetic humorist Ogden Nash). The animal laid eggs but had neither feathers nor wings. Parts of its body were clearly reptilian, but it was warm-blooded. Was this anatomical hodgepodge some misbegotten hybrid, a living fossil, or an entirely new kind of vertebrate? Only one thing was irrefutable: The platypus crossed too many boundaries to fit into the existing scheme. To the hard-pressed taxonomic community, the curious creature from Down Under posed intellectual, religious, and political challenges that lasted throughout much of the 19th century. In the meantime, the inscrutable species--once named Ornithorhynchus paradoxus, or, literally, "paradoxical bird snout"-- was shifted from taxon to taxon. Eventually, it was resolved that oviparous (egg-laying) animals could, in fact, be mammals if they had the right stuff--namely hair and mammary glands--and a separate order, Monotremata, was created just for them. Platypuses and their relatives, the short- and long-nosed echidnas, are thought to be the sole surviving members of a primitive group of egg-laying mammals that was once more diverse and widespread, says Dwight G. Smith, chairman of Southern Connecticut State University's biology department. In short, he says, "they show the close evolutionary relationships between mammals and their reptilian ancestors." And they also show that nature's unlikeliest creations are sometimes beyond the grasp of the mammals that categorize them.


 

© 2001  NASI

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