Formicoxenus nitidulus is an inquiline species in the nests of several Formica rufa group species.

They are ignored, or at the very most only threatened 1, by Formica workers, and move free through the nest. The colonies are small, with about 100-500 workers2, but it's possible to find more than one colony into one Formica nest. The nests are located inside wood fragments, branches and stems inside the mound or in the earth mound floor. Males are ergatoid, and mating occurs on the surface of the mound 3, where the males come attracted by a pheromone released by queens4. If their hosts change their nest, F. nitidulus can migrate following them5.

There are three morfologies among the fertile female castes67 , covering all the morfologies between workers and queens: ergatomorphs, intermorphs and gynomorphs. All of them have spermatheque, can mate and can play the rol of queen of the colony.

The following image shows the moment of the first finding of F. nitidulus in the spanish Pirinees8, in Son (Lleida) in june of 2007. Xavier Espadaler, Xavier Roig and Kiko Gómez are in the image, and I'm inmortalizing the moment.

The nest of F. nitidulus was under the bark of a fallen trunk which was embeded in a Formica lugubris mound. The log was put in placeafter the search, so don't worry about the dismounted nest of Formica,  I have seen more damaged  mounds recovering their shape in some weeks.

Since most nests of F. lugubris in the area have a wood core (like this in the following image), and the only one that was checked looking for inquilines had them, possibly F. nitidulus could be more frequent than suspected.

1. 1. Donisthorpe HSJK .  1927.  The guests of British ants. Their habits and life-histories. :243.
2. 2. Hölldobler B, Wilson EO .  1990.  The ants. :733.
3. 3. Collingwood AC .  1979.  The Formicidae (Hymenoptera) of Fennoscandia and Denmark. Fauna Entomologica Scandinavica. 8:1-174.
4. 4. Passera L, Aron S .  2005.  Les fourmis: comportement, organisation sociale et evolution. :480.
5. 5. Donisthorpe HSJK .  1927.  The guests of British ants. Their habits and life-histories. :243.
6. 6. Francoeur A, Loiselle R, Buschinger A .  1985.  Biosystématique de la tribu Leptothoracini (Formicidae, Hymenoptera). 1. Le genre Formicoxenus dans la région holarctique. Naturaliste Canadien (Québec). 112:343-403.
7. 7. Passera L, Aron S .  2005.  Les fourmis: comportement, organisation sociale et evolution. :480.
8. 8. Roig X, Espadaler X, Cuscó R, García F, Gómez K, Serrano S, Vila R .  2008.  Hormigas en zonas gipsófilas. Primera cita para Cataluña (Península Ibérica) de Cardiocondyla batesii Forel (Hymenoptera: Formicidae) y actualización del listado para Catalunya. Boletín de la Sociedad Entomológica Aragonesa. 42:189-192.