Lasius flavus

Identification
Clear yellow to brownish yellow. Body hairs on dorsum of gaster and alitrunk long; appendages and body covered with more or less thick adpressed pubescence, more dilute on head. No erect hairs on tibiae, scapes or genae. Scale thin in side view, low and broad in front view with dorsal margin mildly convex straight or in larger specimens occasionally emarginate. Size very variable in North European populations. Length: 2.2-4.8 mm (Collingwood 1979).

Distribution
United States to Japan; North Africa to Arctic (Collingwood 1979).

Distribution based on Regional Taxon Lists
Nearctic Region: Alberta, Canada, Canada, United States. Palaearctic Region: Albania, Andorra, Armenia, Belgium, Bulgaria, China, Croatia, Czech Republic, Democratic People's Republic of Korea, Denmark, France, Georgia, Germany, Greece, Iberian Peninsula, Iran, Italy, Japan, Kyrgyzstan, Mongolia, Montenegro, Poland, Portugal, Republic of Korea, Romania, Russian Federation, Slovenia, Spain, The former Yugoslav Republic of Macedonia, Turkmenistan, United Kingdom of Great Britain and Northern Ireland.

Regional Notes
Wilson (1955) - The nesting habits and habitat preferences of flavus are subject to marked geographic variation. In Germany, Gosswald (1932) found the species to be highly adaptable, occupying moist forest floors, forest borders, hedgerows, grassy paths, cultivated areas but does not nest in gardens. In a random field sample, Gosswald recorded 835 colonies under stones, usually in dry situations, 300 in mounds, mostly in meadows, and 30 in dead tree trunks in woodland. The mounds reach their largest size in swampy areas, and may exceed 60 cm. in height. Gosswald judged this species to be more adaptable, although not more abundant, than Lasius niger. He encountered 6 colonies that he determined as "myops", all under rocks in open, dry ground. It sounds likely that these were depauperate colonies living in a habitat affording only marginal existence.

Many other authors have made similar observations concerning the diverse nesting habits of flavus in northern and central Europe. O'Rourke (1950) found it in Ireland mostly in dry, sunny situations with fine soil, but never encountered it in marshes or in rotting wood in forests. Skwarra (1929) found it to be a very successful ant in the Zehlau Moor of East Prussia, exceeded in abundance there only by Lasius niger, she notes the general preferences of this species for open, moist, grassy land, in fields, marshes, along the shores of inland lakes and ponds, and on riverbanks.

The mounds which the European flavus builds have been described in the literature many times. In Switzerland they occur mostly on eastern and southern mountain slopes, tending to increase in height and size with elevation (Wheeler, Forel, et al.). They are typically elongate in shape under these conditions, with the long axis east-west and the east face precipitous. According to Linder (1908) this peculiar shape is caused by the ants inhabiting and building only in the east end of the mound.

In southern Europe, in the lowlands at least, the mound-building habit is lost, and the species nests almost exclusively under stones. Zimmermann (1934), for instance, found it limited to this latter nesting site in the islands around the Quarnerolo. At Miao T'ai Tze, Shensi, China, W. L. Brown (pers. commun.) found flavus nesting under stones. This is the only type of nesting site I encountered in several dense populations in the Sierra Nevada of California, and is by far the predominant type through the eastern U.S. I do not know of any cases in North America of flavus constructing mounds in open soil.

European observers are in agreement that flavus is completely subterranean. Its mounds ordinarily lack external openings and workers are rarely seen above the ground. In Ontario and California I watched for signs of activity around flavus nests at night, but was never rewarded with the sight of a foraging worker. It has been generally assumed that the main food source of this species consists of the secretions of Homoptera maintained in the nests (cf. Eidmann, 1926), but food habits have never been well investigated. Indeed, I have only occasionally found evidence of any food source, including Homoptera, in a number of nests I have excavated, although workers and brood were turned up in abundance. The utilization of some amount of insect food seems likely. Donisthorpe (1927, p. 258) mentions the presence of insect remains in flavus galleries under stones, and Richards (1953, p. 128) has observed flavus workers dismembering a caterpillar on top of a mound.

The mass of published data on nuptial flights by this species in Europe has been well summarized by Donisthorpe (1927). The flights occur in the late afternoon from July to September and predominantly in August. They are often concurrent with flights of niger. Winged forms are found in the nests from June to October. I have seen in nido North American collections of winged forms ranging from July 21 (Penobsquis, New Brunswick) to August 30 (Rochester, New Hampshire).

Europe
Collingwood (1979) - This species is very widely distributed and one of the most abundant in North Europe where it is a characteristic earth mound builder in pastures and along the periphery of woodlands but also nesting under stones in rocky areas. Colonies are started by one or more queens with primary pleometrose quite frequent. In North Europe nests in exposed places and in northern extremity of its distribution, L.flavus exhibits a wide range of worker size. On warm sites in southern areas usually in sandy lowland heath, worker size is small and much less variable. Eye ommatidium number is correlated with size and series of small workers with eyes with low ommatidium number are sometimes referred to Lasius myops Forel. However, queen size is constant regardless of worker size. L. myops is therefore regarded as a synonym of L.flavus. Individual nests may contain several thousand individuals and favourable nest sites, e. g. pasture sloping with a southern aspect, may be crowded with mound nests. This species, as with Lasius niger, tends to swarm on the same day in any one area and in years of abundant production of sexuals huge mating swarms may occur during late July or August. This species is hypogoeic, seldom occurring above ground, feeding on small insects and the exudate of subterranean root feeding aphids.

Nomenclature

 *  flavus. Formica flava Fabricius, 1782: 491 (w.) EUROPE. Latreille, 1798: 42 (q.m.); Wheeler, G.C. & Wheeler, J. 1953c: 152 (l.); Hauschteck, 1962: 219 (k.); Imai, 1966: 120 (k.). Combination in Lasius: Mayr, 1861: 50; Emery, 1925b: 231; Kuznetsov-Ugamsky, 1929b: 36; in Donisthorpea: Donisthorpe, 1915d: 216; in Formicina: Emery, 1916b: 241; in Acanthomyops: Forel, 1916: 460; Kuznetsov-Ugamsky, 1927e: 187; in Lasius (Chthonolasius): Ruzsky, 1914a: 59; in Chthonolasius: Ruzsky, 1925a: 288; Ruzsky, 1936: 90; in Lasius (Cautolasius): Wilson, 1955a: 112. Senior synonym of ruficornis: Roger, 1862c: 285; of ibericus and material of the unavailable name sancho referred here: Wilson, 1955a: 112; Seifert, 1990: 12; of apennina, brevicornis, fuscoides, helvus, microps (and its junior synonym claripennis), morbosa, odoratus, olivacea: Wilson, 1955a: 112. See also: Bernard, 1967: 359; Kutter, 1977c: 229; Collingwood, 1979: 96; Yamauchi, 1979: 160; Kupyanskaya, 1990: 222; Atanassov & Dlussky, 1992: 241.
 * brevicornis. Lasius brevicornis Emery, 1893i: 639, pl. 22, fig. 22 (w.q.m.) U.S.A. Junior synonym of flavus: Wilson, 1955a: 112.
 * fuscoides. Lasius flavus var. fuscoides Ruzsky, 1902e: 16 (w.) RUSSIA. Combination in L. (Chthonolasius): Ruzsky, 1914a: 61; in Chthonolasius: Ruzsky, 1925a: 288; in Acanthomyops: Kuznetsov-Ugamsky, 1927e: 187. Junior synonym of flavus: Wilson, 1955a: 112.
 * odoratus. Lasius flavus var. odoratus Ruzsky, 1905b: 282 (w.) RUSSIA. Junior synonym of flavus: Wilson, 1955a: 112.
 * claripennis. Lasius (Formicina) flavus subsp. claripennis Wheeler, W.M. 1917a: 527 (w.q.m.) CANADA. Junior synonym of microps: Creighton, 1950a: 422.
 * microps. Lasius (Formicina) brevicornis var. microps Wheeler, W.M. 1917a: 526 (w.) U.S.A. Wheeler, G.C. & Wheeler, J. 1953c: 152 (l.). Combination in L. (Chthonolasius): Creighton, 1950a: 422. Subspecies of brevicornis: Wheeler, G.C. & Wheeler, E.W. 1944: 253; of flavus: Creighton, 1950a: 422. Senior synonym of claripennis: Creighton, 1950a: 422. Junior synonym of flavus: Wilson, 1955a: 112.
 * morbosa. Formicina flava var. morbosa Bondroit, 1918: 28 (w.q.) FRANCE. Junior synonym of flavoides: Emery, 1925b: 231; of flavus: Wilson, 1955a: 112.
 * apennina. Lasius (Chthonolasius) umbratus var. apennina Menozzi, 1925d: 34 (w.) ITALY. Menozzi, 1932a: 8 (q.m.). Subspecies of flavus: Menozzi, 1932a: 8. Junior synonym of flavus: Wilson, 1955a: 112.
 * ibericus. Lasius (Chthonolasius) umbratus st. ibericus Santschi, 1925g: 349, fig. 2 (w.) SPAIN. Junior synonym of flavus: Wilson, 1955a: 112.
 * olivacea. Lasius (Lasius) flavus var. olivacea Karavaiev, 1926e: 194 (w.) CAUCASUS. Junior synonym of flavus: Wilson, 1955a: 113.
 * helvus. Lasius helvus Cook, 1953: 326, figs. (w.) U.S.A. [Also spelled helveolus, on p. 327.] Junior synonym of flavus: Wilson, 1955a: 113.

Description
Wilson (1955) - Worker and queen. In the eastern United States, where Lasius flavus occurs sympatrically with Lasius nearcticus, it can be separated from this and other Cautolasius by a host of characters, but elsewhere these are subject to much geographic variation and tend to break down and lose their diagnostic value. Only one character has been found which will consistently separate all Nearctic and Palaearctic flavus populations from Lasius nearcticus (no. 1 below).

(1) Maxillary palp segment V as long as segment VI or longer.

(2) A much weaker character is found in the petiolar outline. In flavus the dorsal margin in frontal view is usually emarginated to flat, while in the majority of nearcticus it is convex.

(3) In addition, flavus can be separated from the related species Lasius fallax and Lasius talpa by the following character: scapes and outer tibial surfaces lacking standing hairs.

Worker
Wilson (1955) - Mandibular dentition follows certain recognizable trends specific at least for the subgenus. In large specimens from northern Europe there are commonly four basal teeth, with either the second or third from the base frequently reduced in size. As body size decreases the common basal tooth number becomes three and then two; in the latter case the median tooth is frequently reduced. Superimposed on this allometric variation is the frequent, non-allometric loss of the second intercalary tooth. Clypeus with a well defined median carina, which tends to become obsolescent in small workers. Anterior border of median clypeal lobe broadly and evenly rounded. Head tending to be more massive relative to body than in all other members of the genus with the exception of L. brunneus. Color highly variable, from straw yellow to dark yellowish brown. Minor workers are nearly aways clear yellow, medias show various degrees of light infuscation, and very large workers (found in northern Eurasia only) are often deeply infuscated.

Male
Wilson (1955) - Isolated individuals cannot be separated with certainty from other members of the subgenus.

(1) The subgenital plate tends to be subquadrate, with a protruding posteromedian setiferous area. This character will separate a majority of series from Lasius nearcticus.

(2) The outer femoral surfaces in Lasius flavus are ordinarily bare of standing hairs, separating this species from Lasius fallax and doubtfully from Lasius talpa.

Mandible form highly variable, ranging from the presumably primitive Lasius pallitarsis type to the Lasius niger type. The variation is partly allometric, i.e. the largest males usually have the pallitarsis type, while the smallest males always have the niger type or some degenerate modification of it.