On the evening that it will kill its orb-weaving spider host, the larva of the ichneumonid wasp Hymenoepimecis sp. induces the spider to build an otherwise unique "cocoon web" to serve as a durable support for the wasp larva's cocoon. The construction of this cocoon web is highly stereotyped, consisting of many repetitions that are almost identical to the early stages of one subroutine of normal orb weaving, the other components of which are repressed. Here I investigate this activity and show that the mechanism employed by the larva to manipulate the spider's behaviour is fast-acting, apparently chemical, and has long-term effects. none