This insect is shield shaped and is found mainly on the leaves and fruits of hawthorn so, not surprisingly, it has the common English name of ... hawthorn shield-bug (Acanthosoma haemorrhoidale). Although common on hawthorn and carrying that name it also frequents other hedgerow shrubs like dogwood, hazel, holly and even oak. They can be found in woodland settings, in hedgerows and are quite common in gardens.
The hawthorn shield-bug is mainly green but they can vary in colour quite considerably depending on age but the red triangle is usually visible. Take care though, some other shield-bug species also have a red/brown triangular shape on their backs, it is often more pronounced on this particular species. The green surface is also pitted and appears to have lots of black dots on it as a result.
Although at their peak in September they hibernate as adults and can be see as early as March if they awake in a mild spring and can be seen as late as November if conditions are such that they do not have to go into hibernation earlier. In their search for somewhere safe to spend the winter that can venture into houses but they do not pose a threat , they are quite harmless. One of the bigger shield-bugs, one of the most distinctive in appearance and and one of the more frequently seen, there is likely to be one near you soon!
Shield bugs are part of the order Hemiptera, sub-order Heteroptera; they are not flies or beetles, they are a separate taxonomical group.Acanthosoma haemorrhoidale: the hawthorn shieldbug