Pheidole symbiotica Wasmann , 1909 , an enigmatic supposed social parasite , is a nematode-infested form of Pheidole pallidula ( Nylander , 1849 ) ( Hymenoptera : Formicidae : Myrmicinae )

Pheidole symbiotica Wasmann, 1909 hitherto known only from type series collected in Portugal is recorded from Italy for the first time. Dissection of abdomen of this supposed social parasite of Pheidole pallidula (Nylander, 1849) showed that it is infested by a large mermithid nematode. That allows us to conclude that Pheidole symbiotica is only a teratological form of Pheidole pallidula . In consequence, we propose synonymy of Pheidole symbiotica Wasmann, 1909 under Pheidole pallidula (Nylander, 1849). High quality photographs of this form are presented for the first time and an illustration of its parasite is also given.


Introduction
Pheidole symbiotica Wasmann, 1909 is one of the most enigmatic European ants.It was described from material collected by P.W. Deckelmeyer in a Pheidole pallidula (Nylander, 1849) nest in Barrô [Portugal,Águeda county,40° 32' 11" N 8° 28' 12" E].In original description Wasmann (1909) noted only morphology of a specimen (no information about the number of examined specimens), presented sketch figures of dorsal view and head and discussed biological status of the species.He suggested that the described specimen may be a worker or ergatoid gyne.In subsequent supplementary paper (Wasmann, 1910) he provided information about four ergatoid gynes (two of them not fully sclerotized), one pupa P. symbiotica is distinguished from male of P. pallidula only by the structure of antennae (basal segment slightly wider than subsequent segments and last segment twice longer than penultimate segment at P. symbiotica whereas male of P. pallidula has globular basal segment, twice wider than subsequent segments and last segment only 1.5 times as long as penultimate segment).According to these differences in male's body structure Wasmann (1910) definitely stated that P. symbiotica represents a distinct species, a social parasite of P. pallidula.In 1913 Forel published a note about a nest of P. pallidula (found by M. Sahlberg on the Corfu, Greece) in which apart from the typical major and minor workers occurred a specimen of intermediate size.He suggested that it represents P. symbiotica and concluded that this form is only a simple intermediate worker of P. pallidula.However, there is no certainty that the specimen studied by Forel (1913) represents the form described by Wasmann (1909) as a P. symbiotica.Emery (1915), in list of synonyms of P. pallidula, noted P. symbiotica as "var.?symbiotica Wasm." with a short comment "an sp.distinguenda?".Collingwood (1978) in the checklist of Iberian ants noted P. symbiotica as a good species but missed this taxon in the key to Iberian species.In recent catalogues (Bolton, 1995;Bolton et al., 2006;Borowiec, 2014) P. symbiotica is treated as a good species.
Although Pheidole pallidula, a supposed host of P. symbiotica, is widely distributed and common species in the Mediterranean area (Borowiec, 2014) it is surprising that the form described by Wasmann (1909) under the name P. symbiotica has never been found again.P. pallidula is a pioneering species, one of the first to inhabit areas transformed by man, such as roadsides of new roads, mines, gravels and excavations of sand, but also it is common on sun-exposed rock walls, in grasslands and meadows.During our entomological travels to several Mediterranean countries we found several hundred nests of P. pallidula but we have never collected specimens distinguished from typical major and minor workers.
Such rarity of this enigmatic taxon suggested that perhaps Pheidole symbiotica is not a good species, but only a form of P. palidula, teratological due to parasitic infections.Some recent papers suggested that various ant parasites can affect the morphological changes of all castes.Trabalon et al. (2000) described modification of morphological characters of Leptothorax nylanderi (Förster) induced by cestodes.Csösz (2012), using noninvasive X-ray microtomography, proved that two Myrmica species: M. myrmecophila Wasmann, 1910 andM. symbiotica (Menozzi, 1925) are teratological forms of Myrmica sulcinodis Nylander, 1846 and Myrmica scabrinodis Nylander, 1846 developing as a result of infection by mermithid nematodes.Poinar (2012) gives an overview of examples of parasitic Nematoda infecting ants and affecting the morphology of workers and queens.The paper presents a photo of the nest of Pheidole pallidula with two atypical major workers infected by Nematoda.Unfortunately, lack of access to type material of P. symbiotica precluded verification of the hypothesis that this taxon is a teratological form of supposed host P. pallidula.
Unexpectedly, during senior author's trip to southern Italy (Campania and Calabria, August-September 2014) nine specimens with characters described for P. symbiotica were collected in two nests of P. pallidula.We dissected abdomen of two specimens and discovered that they are infested with large mermithid nematodes filling the entire abdomen (Fig 6).
Below we present a detailed redescription of "symbiotica" form with high-quality photographs and propose synonymization of Pheidole symbiotica Wasmann with Pheidole pallidula (Nylander).

Materials and Methods
Ants were collected in Italy, Campania, Salerno Prov., Cilento National Park, Monte Bulgheria n.Poderia.Description, measurements and photos were made under Nikon SMZ 1500 stereomicroscope, Nikon D5200 photo camera and Helicon Focus software.Examined specimens are deposited in the collection of senior author.
Standard measurements and indices are as defined in Bolton (1975): Measurements HL -head length; measured in straight line from midpoint of anterior clypeal margin to mid-point of occipital margin; in full face view; HW -head width; width of head at anterior margin of eyes in full-face view; EL -eye length; measured along the maximum diameter of eye; EW -eye width; measured along the maximum width of eye perpendicular to Pheidole pallidula var.tristis Forel, 1907: 204 (terra typica: Tunisia), not Pheidole tristis (Smith, 1858: 132).
In darker form the head is mostly black with dark brown clypeus and yellowish brown underside, antennae and legs yellowish brown to partly brown, pronotum black, mesonotum brown, propodeum from brown to yellowish brown, petiole and postpetiole yellowish brown, abdomen brown, apical margins of abdominal segments yellowish (Figs 3,4).
In full-face view head rectangular, almost as wide as long (Fig 5).Posterior margin shallowly concave, posterior corners rounded.Mandibles elongately triangular, their surface with setose, partly elongate punctures but without with sparse setose punctures.Antennal scrobes deep, with semicircular striation surrounding scrobe dorsally and laterally.Gena with longitudinal ridges, prolongated along inner margin of eyes up to 1/2-2/3 length of head, postocular area, top and venter of head without ridges or striation, with fine setose punctures.Eyes small, almost round, approximately 0.4 times as long as tempora and 0.5 times as long as gena.Whole surface of head shiny, covered with semi-erect and erect setae, the longest on clypeus, along frontal lateral ridges, on top of head and partly on ventral surface of head.The longest setae twice longer than eye length.Antennae moderately elongate, scapes slightly shorter than head width, funicles approximately 1.4 times as long as scapes, with large 3-segmented club 1.3 times as long as eight basal funicle segments joined.Scapus and funicles covered with long semierect to erect setae.
Mesosoma 1.9 times as long as wide, promesonotum subangulate in profile, pronotum separated from mesonotum by sharp margin.Dorsal surface of mesonotum almost flat with small tubercle laterally, distinctly separated from propodeum by moderately deep transverse sulcus.Dorsal surface of propodeum flat in profile, with small, triangular propodeal spines then with oblique, concave posterior surface (Figs 2, 4).Surface of pronotum shiny, with sparse, small setose punctures, top of mesonotum shiny, sides with distinct microreticulate sculpture, propodeum dorsally with transverse ridges and microreticulate sculpture, laterally with microreticulate sculpture and fine oblique to semicircular striation.Petiole elongate with long peduncle, its anterior face straight to shallowly concave, node triangular to slightly obtuse in profile.Posterior face of petiole straight, ventral margin straight, without spine or angulation.In dorsal view, petiole with concave sides and more or less angulate anterior and posterior corners.Postpetiole in profile rounded to subangulate.In dorsal view postpetiole 1.7 times as long as wide, regularly widened posteriorly, apical half with gently rounded sides.
Comments: At first glance the "symbiotica" form looks intermediate between minor and major workers of P. pallidula.The most striking feature is the very large abdomen in this form, which is longer than the summed total length of the head and thorax, while in typical workers of Pheidole  pallidula the abdomen is clearly shorter.The development of such a large abdomen is probably stimulated by the parasite, which is large and tightly fills the whole volume of the abdomen.Form "symbiotica" does not have any features of the gyne, even ergatoid form, there are no wing buds and body proportions and construction of trunk are similar to the body shape of a big major worker.The parasite is believed to stimulate the development of workers to such a large size, so that it could accommodate in abdominal cavity such a large nematode specimen (Csösz & Majoros, 2009;Csösz, 2012).

Discussion
True social parasitism was observed in the genus Pheidole Westwood (Buschinger, 2009).Although Wilson (1984) noted that this phenomenon is rare in tropical ants but pointed that Neotropical Pheidole microgyna Wheeler, 1928 is a temporary social parasite of Pheidole minutula Mayr, 1878 and described two species from the Oriental region: Pheidole languinosa and Pheidole parasitica, both parasitizing in common and widespread Pheidole indica Mayr, 1879.He listed another six Pheidole species as potential social parasites including Pheidole symbiotica (sic!) (Kusnezov, 1951) described from Argentina.This homonymic name was later replaced by Pheidole kusnezovi by Wilson (2003).
Our observations of the "symbiotica" form show that for the description of a new species of parasitic ant from a single specimen of caste is not enough.We confirm Csösz's (2012) conclusion that especially taxa reported only once or only few times should be verified by allowing the possibility of infection by internal parasites.Thus far the most drastic changes in the morphology and development of ants was observed in the specimens infected by nematodes (Csösz & Majoros, 2009;Poinar, 2012), but there are also examples of altered host morphology resulting from infections by other internal parasites (Heinze et al., 1998;Trabalon et al., 2000;Miura et al., 2006).