by Merry Youle — Trou­bled by pesky flies in your home or of­fice? Tempted to reach for the toxic fly­pa­per? Here's a green al­ter­na­tive long known by the In­di­ans of the Cen­tral Mex­ico high­lands. Step out­side and look around for a tree with en­tire branches cov­ered by a large spi­der­web. Clip out a sec­tion…