Enallagma cyathigerum (common blue damselfly or common bluet) is a species found mainly between latitudes 40°N and 72°N; It is widely distributed in the Palearctic, common in all European countries (including Portugal, Spain, France, Great Britain, Ireland, Germany, Sweden, Norway, Finland, Poland, etc.) and in Asia in Turkey, Iran, Russia, and South Korea. Damselflies are an important link between the health of the aquatic ecosystem and its response to climate change. The closely related Nearctic species Enallagma annexum (northern bluet) was at one time considered to be synonymous with it.
Habitat
These damselflies inhabit freshwater bodies whose conditions range, they have been seen in acidic fens as well as eutrophic ponds. They are important within the trophic levels as they are an intermediate predator. They consume smaller larvae and they are preyed on by fish and larvae bigger than them. They are efficient in both complexities equally, but the complex vegetation also serves as protection from fish. The damselfly larvae require a plant structure that can withstand the backward movement that occurs when the labium protracts to catch food.
The common blue damselfly can be easily mistaken for the azure damselfly (Coenagrion puella), but on the back and the thorax, the common blue damselfly has more blue than black; for the azure damselfly it is the other way around. The second segment of the thorax has a distinctive spot with a line below connecting to the third segment.
Another difference can be observed when inspecting the side of the thorax. The common blue damselfly has only one small black stripe there, while all other blue damselflies have two.
Behaviour
All Coenagrionid larvae have a broad range in diet, so they can live in many different habitats. The larvae are able to eat relatively large prey items, but the prey are typically less mobile and therefore easier to catch.
Human impacts
Pesticides have a large impact on this species due to its sensitivity. These pesticides are introduced to the water by runoff and by being added directly. Being exposed to pesticides made these organisms less likely to be successful in metamorphosis with the effects being worse the longer the organism is exposed.