Harebell carpenter bees (Chelostoma campanularum) are tiny solitary bees which are mostly found in or near bell flowers, especially harebells (Campanula rotundifolia), nettle-leaved and clustered bell flower (Campanula trachelium and C. glomerata). They are quite common in gardens, parks and other urban habitats in the South of England but often overlooked and can easily be mistaken for flies.
The bees are very small (6-7 mm) and black in colour. The males have a two-pronged peg at the last abdominal segment whereas the females have pale pollen-collecting hairs underneath the abdomen. They fly from mid-June to August and you will often see them in groups of many individuals swarming around bell flowers on warm and sunny days. Males often stay in one flower for several days in bad weather and even sleep in the flowers over-night.
Photo taken last summer.