Web architecture alteration of the orb web weaving spider Metellina merianae (Araneae, Tetragnathidae) induced by the parasitoid Megaetaira madida

. The polysphinctine wasp Megaetaira madida (Haliday, 1838) is a koinobiont ecto-parasitoid of spiders of the genus Metellina. Under the influence of the parasitoid's final instar larva, the spider host M. merianae (Scopoli, 1 763) built a three-dimensional web architecture, which differed considerably from the capturing orb web. The alteration of spider web behaviour induced by a parasitoid larva in this host-parasitoid pair is described for the first time.

submitted 10.5.2016, accepted 21 .7.2016, online 14.8.2016 fication induced by the parasitoid larva.Nielsen observed that wasp cocoons were surrounded by a tangle of threads which were present in all parts of the tubes.Further, Nielsen collec- ted one cocoon attached to a spruce twig in the field with no silk structure surrounding the cocoon.This resembles the Si- tuation in which the parasitized spider falls off the web in the period when the parasitoid larva reaches its final stage and is diverted from building any silk structure.Here, I present new host records and the first note on behavioural manipulation of a spider host by M. madia from Italy.

Material and methods
I collected parasitized spiders of the genus Metellina from the edge of a deciduous forest in Monterosso Grana (Province of Cuneo, Italy, 44°24'20'N, 7°19T7'E) over a two-day period at the end of October 2014.Spiders were collected by bea- ting tree canopies and undergrowth (30 to 200 cm above the ground) with a square-shaped beating net (1-m 2 area) placed underneath.Each spider was visually inspected for the pre- sence of a parasitoid larva.
The spider hosts were reared in plexiglass experimental arenas (frame 220 x 220 mm, depth 20 mm) with paper tape on four sides of the frame so that the spiders could build webs.The spiders were fed with a surplus of prey (small crickets and Drosophila flies).The web building activity of parasitized spiders was observed until the larva killed and consumed the spider and pupated.I used a Canon EOS 500D digital Camera with an EF-S 18-55 mm lens to record the architecture of the cocoon web.

Results
Two parasitized Metellina spiders, one M. segmentata and one M. merianae , were collected on 29th October.The parasitoid larva on M. segmentata died after ten days in the laboratory and when the spider host moulted the shrivelled dead larva feil out with its exuvia.The parasitoid larva on M. merianae pupated on 28th December 2014 and the male wasp emerged on 12th January 2015.In the latter spider host, the architec- ture of the normal web and the modified cocoon web induced by the final stage larva of M. madida were observed.The parasitized host M. merianae rebuilt its orb web several times until the larva reached its final stage, when the spider, under the influence of the parasitoid, built a unique three- dimensional (3D) structure in the upper part of the arena (Fig. la).This 3D cocoon web had no clear counterpart in the webs of unparasitized spiders.The wasp cocoon was placed in an upper corner of the experimental arena with a high density of threads (Figs la, b).

Discussion
Several studies have been devoted to the behavioural manipulation of orb web building spiders from the family Tetragnathidae by polysphinctine wasps.Wasps of the Neotropical genus Hymenoepimecis associated with spiders of the genus Leucauge were observed to induce the construction of a twodimensional (2D) cocoon web which consisted of a reduced number of radial threads radiating in a plane from a central hub; the cocoon was suspended from this central hub.This type of cocoon web is documented in Hymenoepimecis argyraphaga Gauld, 2000 associated with Leucauge argyra (Walckenaer, 1841) (Eberhard 2000a, 2000b, 2001) and Hymenoepimecis japi Sobczak, Loffredo, Penteado-Dias & Gonzaga, 2009 associated with Leucauge roseosignata Mello-Leitäo, 1943(Sobczak et al. 2009).A cocoon web with a similar 2D architecture, but protected by the 3D structure of a tangle po- sitioned below the hub, was documented in the interaction between Hymenoepimecis tedfordi Gauld, 1991 Gauld, 1991 induced its Leucauge host (L.mariana ) to build a 3D cocoon web (Eberhard 2013).The cocoon web of the orb web weaving spider Tetragnatha montana Simon, 1874 induced by the final stage larva of the Palearctic wasp Acrodactyla carinator (Aubert, 1965) consisted of one highly reinforced main thread, tensioned mostly by a reinforced side thread (Korenko et al. 2015).Acrodactyla carinator was misidentified as Holarctic Acrodactyla quadrisculpta (Graven- The cocoon web induced by M. madida resembled the co- coon web of E. gutfreundi from Costa Rica in the sense that both were 3D structures and the cocoons were oriented horizontally; however, the morphologies of the cocoons were con- siderably different (cocoon circular in cross-section covered by curled structure of M. madida vs. square in cross section with paper smooth surface in E. gutfreundi).The cocoon of M. madida had a densely-woven cocoon wall covered by curled fibres of various lengths and was circular in cross section (Fig. lb), whereas the cocoon of E. gutfreundi had a paper-like smooth surface and was square in cross section (Eberhard 2013).
The wasp M. madida was formerly included in the genus Acrodactyla , also known as the the madida species-complex, but Gauld & Dubois (2006) excluded it because it lacked several features characteristic of the remaining Acrodactyla species.Megaetaira madida was the only valid species within the genus Megaetaira , but two other Acrodactyla species have recently been transferred to this genus (Matsumoto in press).
Their relatedness to M. madida is also supported by the host ränge of M. varicarinata (Uchida & Momoi, 1958) associated with Meta reticuloides Yaginuma, 1958, which is related to the genus Metellina , the exclusive host of M. madida (Takasuka pers.comm.).Although wasps of both Acrodactyla and Megaetaira are associated with orb web building spiders from the family Tetragnathidae, their manipulation of the spider web architecture difters considerably.Acrodactyla wasps induce the production of a strong 2D cocoon web where the wasp co- coon is attached to the strongest main silk line (Korenko et   al. 2015).In contrast, the cocoon web induced by M. madida is a densely woven 3D structure.However, both strategies are effective for protecting the parasitoid during the pupal stage and both are also used by other polysphinctine parasitoids.
More observations of host parasitoid interactions are necessary to reveal further details of the host utilisation of this sparsely occurring spider parasitoid.

Fig. 1
Fig. 1: a. Cocoon web built by a Metellina merianae spider host induced by the final stage larva of the parasitoid Megaetaira madida; b. detail of co- coon.Scale = 20 mm (a) and 2 mm (b) horst, 1820) in studies by Korenko et al. (2015) and Belgers et al. (2013) (material revised by K. Holy and K. Zwakhals).Additional investigation revealed that the cocoon web of T. montana induced by the larva of A. quadrisculpta has the same architecture as that induced A. carinator (Korenko unpubl.data).