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National Geographic : 1964 Jun
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Cloud of free-tailed bats flecks the Texas sky above Bracken Cave, home to millions of the furry mammals. Forays for insects usually begin in late afternoon, giving the vast population time to clear the cave for the night's hunt. Although bats can see, they do not depend on sight to snare food on the wing. Emitting high-pitched beeps similar to sonar's, they evaluate the echoes and swoop upon their prey. The same sound equipment enables them to avoid colli sions. Tadaridabrasiliensisuses a 12-inch wingspread to carry half an ounce of weight. Gentle squeeze with needle-nose pliers bands the delicate forearm of a western pipistrel. Tiny iden tification bracelets enable mammalogists to study the migrations, homing ability, and life span of bats. Thousands of the creatures have worn the telltale rings since 1932, when author Mohr became the first United States scientist to band a cave bat. I had first visited it in June, 1938, was a solid gray-brown color-a tapestry of nine million bats. By the time of our 1961-62 visits, the bat population in Carlsbad was estimated at less than a twentieth of what it was before World War II. The decline may be due to drought and to wide use of poisons that have thinned the insect population on which the bats feed. Rabies also has played a part. However, millions of people have visited Carlsbad without encountering a live bat. Bat droppings, or guano, in the Bat Cave section of Carlsbad were mined for many years. Starting in 1903, exploiters dug de posits, some as deep as 50 feet. A windlass hauled some 100,000 tons of guano 180 feet to the surface. No one has calculated how long it took for this highly nitrogenous bat excrement to accumulate, but its use as fertilizer for citrus groves earned fortunes for the enterprising operators in a relatively few years. We came upon abed of fresh guano marked off with square wire frames. Each nine-inch square was tagged with a number. On hands and knees, Ken Baker took a forceps from his pack and delicately probed the accumula tion within one of the frames. Masses of guano were interwoven with silken strands. "Moths," he said. "Each year by the thou sands they lay their eggs in bat guano and die. Nowhere above ground has anyone seen such numbers of moths." The larvae, or caterpillars, spin silken tubes. 831
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