Oviposition behaviour by a spider‐ectoparasitoid, Zatypota maculata, exploits the specialized prey capture technique of its spider host

Oviposition on or into a living animal is an arduous undertaking for parasitoids, including in terms of counterattack or evasion by the host. Parasitoids of active spiders protected by their own webs (polysphinctine spider‐ectoparasitoids in the family Ichneumonidae) need to avoid entanglement with the spider's web to gain access to the spider host for oviposition. These circumstances have driven the evolution of offensive behaviours highly adapted to the web architecture of each spider host. We discovered offensive oviposition behaviour in a member of the Polysphincta‐group, Zatypota maculata, parasitizing a theridiid spider, Nihonhimea japonica, which constructs a ‘knockdown 3D web’. It consists of a non‐viscid 3D structure above, with a retreat (a dead leaf) hung at the centre, and a non‐viscid sheet below as a capturing device. When a prey insect is knocked down and then trapped by the sheet, the spider immediately drops onto the underside of the sheet by penetrating it to bite and wrap the prey. Zatypota maculata exhibits two behaviours to deal with this type of web: (1) the wasp climbs the 3D cobweb, creeps up slowly onto the spider's retreat, taking a long time so that the spider does not escape, and finally enters the retreat to sting the spider (creeping‐style); (2) the wasp dives from outside of the web onto the sheet, as a knocked‐down prey item would, to lure the spider out, before stinging it at the moment of contact (diving‐style). We discuss these unique offensive behaviours and compare them with behaviours of other polysphinctines and spider‐hunting aculeates. Ovicide by means of removal of pre‐existing eggs was also observed under experimental conditions. We discovered offensive oviposition behaviour in Zatypota maculata, parasitizing a theridiid spider, Nihonhimea japonica, that constructs a ‘knockdown 3D web’. It consists of a non‐viscid 3D structure above, with a retreat (a dead leaf) hung at the centre, and a non‐viscid sheet below as a capturing device. When a prey insect is knocked down and then trapped by the sheet, the spider immediately drops onto the underside of the sheet by penetrating it to bite and wrap the prey. Zatypota maculata exhibits two behaviours to deal with this type of web: (1) the wasp climbs the 3D cobweb, creeps up slowly onto the spider's retreat, taking a long time so that the spider does not escape, and finally enters the retreat to sting the spider (creeping‐style); (2) the wasp dives from outside of the web