Parasitic organisms have often evolved the ability to manipulate the host phenotype, including its morphology, physiology and behaviour, for their own benefit (Moore 2002). Polysphinctine wasps (the Polysphincta genus-group sensu Gauld and Dubois 2006), which are all external parasitoids of spiders, exhibit a unique trait within the Ichneumonidae in terms of development (Fitton et al. 1987). Their larva is attached to the dorsal side of the spider’s opisthosoma/prosoma, where it develops while the spider continues foraging. Shortly before pupation, some of the parasitoids (final instar larvae) manipulate the web-spinning activity of the host in order to establish effective protection against enemies and the environment (e.g. Eberhard 2000a, 2013; Matsumoto 2009; Korenko and Pekár 2011; Korenko et al. 2014). These effects of the larva are apparently due to chemical products that are introduced into the spider (Eberhard 2010).
A few studies have been devoted to the behavioural manipulation of orb web building spiders. Eberhard (2000a, b, 2001, 2013) and Sobczak et al. (2009) studied parasitoids associated with orb web building spiders from the family Tetragnathidae; Gonzaga et al. (2010) described the manipulation of spiders from the family Nephilidae; and Gonzaga and Sobczak (2007, 2011), Eberhard (2013) and Korenko et al. (2014) studied the manipulation of spiders from the family Araneidae. The studies revealed that the manipulated spider modifies the architecture of its web in various ways. The orb web is modified to the ‘cocoon web’ (termed by Eberhard (2000a, b) for the first time) when some of its components are reduced (e.g. web spiral, radii) and others are reinforced (e.g. radii, central hub, frame) or multiplied (e.g. threads). The cocoon web is stronger and effectively designed to provide more durable support for the wasp’s cocoon than the normal web (e.g. Eberhard 2000a, b). The tetragnathid spider Leucauge argyra (Walckenaer, 1841) is manipulated by the larva of Hymenoepimecis argyraphaga Gauld, 2000 to build a web which consists of a low number of radial threads radiating in a plane from a central hub; the architecture of the cocoon web remains two-dimensional (hereafter 2D) (Eberhard 2000a, b, 2001). A similar 2D cocoon web is built by the related species Leucauge roseosignata Mello-Leitão, 1943 manipulated by Hymenoepimecis japi Sobczak, Loffredo, Penteado-Dias and Gonzaga, 2009 (Sobczak et al. 2009). A similar 2D architecture of the cocoon web, but protected by the 3D structure of the tangle positioned below the hub, was recently described in the spider hosts Leucauge mariana (Keyserling, 1881) manipulated by Hymenoepimecis tedfordi Gauld, 1991 (Eberhard 2013) and Leucauge volupis (Keyserling, 1893) manipulated by Hymenoepimecis jordanensis Loffredo & Penteado-Dias, 2009 (Gonzaga et al. 2014). In contrast, the larva of Eruga gutfreundi Gauld, 1991 induced the same host (L. mariana) to build a completely different three-dimensional cocoon web (hereafter 3D) (Eberhard 2013).
Cocoon webs of spiders from the families Araneidae and Nephilidae are mostly 3D. Three-dimensional cocoon webs are induced in araneid spider hosts in which the webs of unparasitised individuals are only 2D (Gonzaga and Sobczak 2011; Korenko et al. 2014). The cocoon web for the Acrotaphus wasp built by araneid hosts Argiope argentata (Fabricius, 1775) is 3D composed of non-sticky threads (Gonzaga and Sobczak 2011). Further, the wasps Sinarachna pallipes (Holmgren, 1860), Polysphincta tuberosa (Gravenhorst, 1829) and P. boops Tschek, 1868 manipulate spiders of the genus Araniella in a similar way (Korenko et al. 2014). All three species induced the production of a 3D structure instead of a 2D web, but thread density, thread concentration and the location of pupa on the cocoon web differed among species. The normal web of nephilid spiders consists of a 2D orb web and a 3D tangle of barrier threads at the side and its resting web is only 3D. Both the orb and the 3D tangle of the normal web are rebuilt by the manipulated spider to form the cocoon web, whose architecture is similar to the 3D resting web (Gonzaga et al. 2010; Korenko, unpublished data). The nephilid spider Nephila clavipes (Linnaeus, 1767) was manipulated by Hymenoepimecis robertsae Gauld, 1991 and H. bicolor (Brulle, 1846) to build a cocoon web which consisted of a hub-like platform (part of the rebuilt orb web), from which the cocoon was suspended, and a 3D structure of non-adhesive threads of variable density. The radii and the spiral of the orb web were mostly reduced, and the wasp’s cocoon was attached to the reduced orb and the barrier threads on the side (Gonzaga et al. 2010). An interesting modification of the web architecture of orb web weavers of the genera Cyclosa (Araneidae), in which the 2D orb web retains its 2D structure after modification (the suppression of adhesive components and a change in the radii structure), was documented in wasps of the genus Reclinervellus, which used the web stabilimentum (the structure built by the spider serving as camouflage) as the same camouflage for its cocoon (Matsumoto and Konishi 2007). The number of descriptions of web architecture alterations induced by polysphinctine final instar larvae has increased in the last few decades, but no detailed study of the web alteration induced by the Acrodactyla wasp has been performed.
We studied the interaction between the parasitoid wasp Acrodactyla quadrisculpta (Gravenhorst, 1820) and its spider host Tetragnatha montana Simon 1874 and described in detail the manipulation of web architecture induced by the parasitoid larva. A. quadrisculpta is reported from most of the Holarctic, and the species is known to be associated with the following tetragnathid spiders: T. montana, T. obtusa Koch, 1837 and T. extensa (Linnaeus, 1785) (Nielsen 1937; Fitton et al. 1988). However, very little is known about its biology and its interaction with the spider host (only Nielsen 1937 and Belgers et al. 2013).