OGDEN NASH taught us most of what we need to know about centipedes when he wrote about the one that was happy until a toad asked, "Which leg goes after which when you run?" Whereupon the centipede pitched into a ditch, paralyzed by confusion. Recently, thanks to J. L. Cloudsley-Thompson's review of Secret Weapons, by Thomas Eisner, Maria Eisner and Melody Siegler (Times Literary Supplement, April 28), more light has been shed on the "defenses of insects, spiders, scorpions, and other ...