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Worker. Bicoloured with dark markings on head and promesonotum varying in size and intensity - generally not as brightly coloured or as large as F. rufaHNS. In the typical form distinct outstanding hairs fringe the posterior border of the head but do not occur forward towards the eyes as in F. lugubrisHNS. In many samples from South Norway and South Finland these hairs may be hard to find or absent. Erect hairs on gula and dorsum of alitrunk variable, usually short and sparse. Eyes with distinct short hairs but much less prominent than in F. lugubrisHNS. Frons rather dull with close dense microsculpture, gaster very closely punctured. Suberect hairs on extensor surfaces of hind femora and tibiae always present but sometimes few. Antennal scapes bare. Head width of largest workers less than 2 mm. Length: 4.0-8.5 mm.
Queen. Bicoloured, scutellum rather dull but gaster always shining. In normal samples short hairs project from the posterior border of the head and on the basal face of the gaster but are absent from the upper surface of the scale and propodeum. Gaster always with fine close micropunctures. Size generally smaller than other species. Length: 8.0-10.0 mm.
Figs. 236-241. Formica polyctena FoersterHNS. - 236: worker in profile; 237: male in profile; 238: queen in profile; 239: head of queen in dorsal view; 240: head of male in dorsal view; 241: head of worker in dorsal view. Scale: 1 mm.
Figs. 242-247. Formica aquilonia YarrowHNS. - 242: worker in profile; 243: male in profile; 244: queen in profile; 245: head of queen in dorsal view; 246: head of male in dorsal view; 247: head of worker in dorsal view. Scale: 1 mm.
Male. Black, appendages paler. Genae with a few outstanding hairs below eyes. Eyes always with short hairs. Gaster tergites 2 to 4 generally without dorsolateral hairs. Fringe of short hairs always present on upper surface of hind femora. Erect hairs present on whole of head and alitrunk. Size generally smaller than other species. Length: 8.0-10.0 mm.
Distribution. Denmark: EJ, Dokkedal (Bisgaard leg.). - Sweden: Dlsl. and Vstm. northward. - Abundant throughout Norway and Finland. - Locally common in Scotland. - Range: Eastern Alps to Siberia, North Italy to North Norway.
Biology. This is undoubtedly the commonest wood ant in Fennoscandia. Large tracts of forest in the centre and north are dominated by this species which is usually found in large multicolonial groups with isolated nests being very rare. This is one of the least aggressive species of the F. rufaHNS group. Long compact trails radiate from each nest to other nests or to aphid bearing trees and antagonism between neighbouring nests has not been observed. In South Norway and South Finland comparative hairlessness in many populations makes for confusion with the rather similar F. polyctenaHNS. Usually, however, if enough workers are collected a majority of at least 60 % of individuals will be found to have some projecting hairs at the back of the head.
However there is a form of this species found locally in the western suburbs of Helsinki to the Sjuntio district of Ab, within an area of about 40 km by 10 km, which is almost completely hairless in all parts of the body given as species specific by Yarrow (1955) for F. aquiloniaHNS. All castes moreover tend to be somewhat larger and more brightly coloured. This could well be a subspecies or species in the making. Its foraging habits have been studied by Rosengren (1971, 1977a, 1977b) under the name of F. polyctenaHNS. This consistent degree of hairlessness has not been found elsewhere within the range of F. aquiloniaHNS, except perhaps in Esthonia, according to samples sent to H. Wuorenrinne by Professor V. Maavara.
The characteristics are as follows: only about 5 % or fewer workers in a series have an indication of short hairs projecting from the occipital corners of the head. Queens have no such hairs but occasional microscopic hairs have been detected on the basal face of the gaster in a very few specimens of about 50 examined. In the few males examined only 1 in 12 has projecting genal hairs. The reasons for retention as an infraspecific form of F. aquiloniaHNS include the close sculpturing of the worker frons, small eye hairs which are always present as in F. aquiloniaHNS while hairs on the extensor surface of the femora form a more or less close fringe as in F. aquiloniaHNS in a majority of the workers. The queen, which appears more brilliant than F. aquiloniaHNS, has extremely close micropunctures on the gaster as in that species (and in this character alone is quite unlike F. polyctenaHNS) while the male has fringing femoral hairs although specimens are also somewhat larger and more shining than F. aquiloniaHNS.
Found most commonly in these habitats: 4 times found in production forest.
Found most commonly in these microhabitats: 4 times nest, 1 times nest 8.
Collected most commonly using these methods: 1 times search.
Elevations: collected from 208 - 1100 meters, 622 meters average
Type specimens: Holotype of Formica aquilonia: casent0903277