... ) The Asiatic freshwater clam has spread rapidly across North America since its accidental introduction about 50 years ago. In addition to its natural dispersal via its more mobile larvae, the young adult clams have a surprising method of hitchhiking rides on the water currents. Through their siphons they deploy long mucous threads. Water currents pull on these threads just as air currents catch the silken threads of migrating spiders. Given a water current of 10-20 cm/sec, the small clams manufacture and deploy their threads, and off they go downstream. (Prezant, Robert S., and Chalermwat, Kashane; "Flotation of the Bivalve Corbicula Fluminea as a Means of Dispersal," Science, 225:1491, 1984.) Australia boasts many peculiar animals and plants. One of these is an orchid that grows underground. Obviously this orchid cannot employ photosynthesis. Rather, it grows in conjunction with a fungus that obtains nutrients from surrounding roots. The fungal threads penetrate both roots and orchids. The orchids make ends meet by systematically killing and digesting the nutrient-laden fungal threads. When the orchid flowers, it pushes toward the surface just enough to open some tiny cracks in the earth. In these cracks, still below the surface, appear the tiny burgundy red flowers. These flowers are pollinated by minute flies, but just how the orchid's seeds are dispersed is still a mystery. (Cooke, John; "Hidden Assets," Natural History, 93:75, October 1984.) Bats navigate by somehow constructing an image of the external world from the echoes of ...