Lasius umbratus

Lasius umbratus workers are yellowish and rarely seen due to their underground habits. Unlike the common yellow meadow ant Lasius flavus, L. umbratus workers have numerous erect hairs on their scapes and tibia. Queens are reddish brown and have heads that are broader than the maximum width of the alitrunk. Males are brownish black and have denticulate mandibles. Adult workers are typically 3.8 to 5.5 mm in length.

Lasius flavus and Lasius mixtus.

The length of the longest hairs on the posterior half of the first gastral tergum (excluding those along the posterior strip) are less than ½ the maximum width of the posterior tibia at mid-length. / The hairs on the first gastral tergum are spaced far enough apart that the distances between the tips of most or all hairs are greater than their lengths. 

Unless identified by a recognised expert, a photo is required and the specimen should be examined with a microscope. In the comments box, state the key or ID method used and describe the size and identifying characters.

Lasius umbratus can be found in most areas where their host nests exist including open woodland and urban areas. They avoid very dry and very wet habitats. It nests under boulders, in tree stumps and at the base of old trees. 

Mating flights typically occur between in August and September. Adult workers can be seen from early spring through to autumn.

Workers are rarely seen above ground. They forage underground on small invertebrates and tend root-feeding aphids. Nests are often carton like structures of thin walled chambers made of fibres and minerals bonded together with secretions and fungal hyphae. New colonies are founded by single queens invading nests of Lasius niger, L. platythorax Seifert or L. brunneus. In late summer queens are sometimes seen wandering over the surface of a L. niger nest carrying a dead worker as a prelude to securing adoption.

Common - Throughout British Isles to central Scotland. This species is not regarded as scarce or threatened.

Currently uncommon in Leicestershire and Rutland though this is almost certainly due to lack of recording and difficulty in observation due to their subterranean nature. The presence of Jet Ant, Lasius fuliginosus, in VC55 confirms the presence of Lasius umbratus as its host species.

http://www.bwars.com/ant/formicidae/formicinae/lasius-umbratus