On the evening that it will kill its host, the orb-weaving spider Plesiometa argyra, the larva of the ichneumonid wasp Hymenoepimecis argyraphaga induces the spider to perform highly stereotyped construction behavior and build an otherwise unique “cocoon web” that is particularly well-designed to support the wasp larva's cocoon. Cocoon web construction behavior is nearly identical with the early steps in one subroutine of normal orb construction, and is repeated over and over. Usually all other normal orb construction behavior patterns are completely or nearly completely repressed. Experimental removal of the larva one or a few hours before cocoon construction would normally occur is sometimes followed by nearly normal cocoon web construction, and sometimes by construction of other highly altered web designs. The mechanism by which the larva induces these changes in the spider's behavior is thus apparently a fast-acting chemical, with effects that are manifested gradually. Partial recovery of orb designs sometimes occurred several days later.