British distribution: Widespread, but predominantly in the north and west; scarce or absent in much of central and eastern England and only a vagrant in Ireland.
Cordulegaster boltonii is a true dragonfly (i.e. a member of the suborder Anisoptera, including the hawkers and darters, rather than the suborder Zygoptera, the damselflies), and is one of our largest species (wingspan 10 cm, overall length given as 84 mm in females, males slightly shorter). Like other members of the suborder, it is a strong, agile and rapid flyer and, within its size scale, a fearsome predator, taking sizeable insects on the wing, including beetles, wasps and bumblebees. Typically it patrols along streams and along the edges of other waterbodies, keeping low over the water.
There are several similar species of Cordulegaster in Europe but most are eastern and unlikely to reach Britain as vagrants. Macromia splendens is a rare and very local species of south-western Europe that also resembles C. boltonii. Distinguishing characters are given in Dijkstra & Lewington (2006).
Ecology and biodiversity assessment
Dragonflies have become a group regularly featured in biodiversity assessments. This is partly (probably substantially) because they are attractive, relatively easily identified and with not too large a number of species in Britain (42 breeding species of Odonata listed in Hammond (1985), including 3 thought to be extinct). However, they can also be considered a monitor of the overall 'health' of wetland habitats. They are sensitive to management practices, including drainage or manipulation of water levels and clearance of marginal vegetation. The nymphs require unpolluted water and agricultural runoff is a major threat. Beyond the immediate confines of waterbodies, intensive agriculture, including conversion of scrub and old meadows into 'improved' grasslands, substantially reduces the availability of insect prey. It follows that a rich dragonfly and damselfly fauna is a sign of a high grade site. It should be noted, however, that many species are southern in their British distribution, irrespective of availability of apparently suitable habitat, Cordulegaster boltonii being unusual in being more northern and to a substantial extent seeming directly to replace some of the Aeshna species.