Flies do not need to evolve any adaptive behaviour towards avoiding spider webs, they already possess an advantage that keeps their gene pool large and varied – great numbers. Flies, herring, mice, etc counteract predation with prodigious reproduction rates. The primary limiting factor to population growth and adaptive change at the bottom of the food chain is available resources (food). Predation does not play a large role in evolutionary adaptive behaviour for these types of animals, IIRC.