from the book, "Our Fascinating Earth"
Some insects have taken on a strange relationship with spiders: they steal the food stored by spiders in their webs.
One species of dance fly hovers near the edge of orb spider webs and lands carefully on tiny insects that have been entangle in the web but are too small for the spider to notice.
Another type of insect, the scorpion fly, also lands on spider webs to steal the entrapped prey. But the scorpion fly plays a dangerous game. It is much too large to go unnoticed by the resident spider, so it employs an unusual weapon. The invader's tactic is to regurgitate a liquid that prevents the fly from becoming entangled in the sticky glue on the web's thread while it repels the attacking spider. This technique is, of course, risky, and if the spider attack succeeds, the robber fly becomes an additional victim. One noted entomologist estimates that the major cause of death for scorpion flies is predation by web—building spiders.
A species of flies of the family Chloropidae has evolved a more effective stratagem. It spends much time resting near an orb weaver that hides its web in a curled leaf. Any insect landing on the web can expect to be killed by the spider, wrapped in silk, and stored for a later meal. The fly stealthily closes in on the bundle of silk, avoiding entanglement in the web, and, when it is close enough, unfolds a long, sucking proboscis and gently sticks it into the prey. A minute or two later the fly flutters away with its abdomen bulging, leaving the spider with an empty shell of a meal.
Another species of robber fly actually sits right on the dorsal part of the spider's body, there the spider cannot brush it off. When the spider is feeding, the fly rushes in, sucks up a quick meal, and hastily returns to its position on the spider's body.
An Australian species belonging to the same family has added a symbiotic twist to its relationship with spiders. It has apparently reached an understanding with the spider: in payment for a meal pilfered from its host, the fly cleans the spider after a meal. It carefully mops the spider's mouth and fangs and even throws in an extra service by cleaning waste matter from around its anal area.