Common Blue Damselfly

Enallagma cyathigerum

In Gateshead are neither as widespread or numerous as their only confusion species, the similar Azure damselfly. They inhabit most of the same sites but tend to be more numerous on the larger, more open water-bodies better than the Azure, being more powerful flyers they are often seen far out from the shoreline whereas the Azures tend to hug the margins.
Both have the similar flight periods but locally the azures tend to appear first. The ponds at Stargate and Kibblesworth hold good numbers by early June, and at Gibside they tend to be found at the larger Octagon Pond rather than the Lily Pond which is home to the azures. Clockburn Lake outlet stream and Far Pasture hold far fewer, and I've yet to record one at the smaller Thornley Woods Pond which too hosts a good population of azures.

Common Blue damselfly (male) from side and above
Plenty of subtle differences with Azure, note the tail end appears
brighter blue as segment 9 does not have any black on it,
the thorax side lacks the black semi-stripe of the azure, and the
shape on segment 2 is like a mushroom rather than the U-shape
of the Azure. 
Look out for very pale males, as in cool conditions they turn grey, but immatures are also very pale.
Close-up of the 'mushroom' shape on segment 2 of the abdomen
Common Blue damselfly (immature female)
Note the black markings along the back of the abdomen are
distinctly torpedo or bomb shaped, and the black on segment 8
is diagnostically triangular.

When the female matures (both sexes take 10 - 14 days) it takes
one of three colour forms. Thy are typically green, but above is
the brownish yellow form, and there is also a blue form.   

Tandems are formed on sunny mornings, usually well away from water.
Copulation takes 20- 30 minutes, either well away from
or close to the breeding site.
Note this female is the typical green form. 

Oviposition can take place close to shore or far out, usually in tandembut
the female may climb down plant stems until totally submerged, when
the male will separate.
Females returning to the surface often struggle to escape the water and
are vulnerable to predators.

  Larvae live among submerged water plants, not venturing out as they tend to share waterbodies populated by fish, so they feed by ambushing passing prey and can take 2-4 years to develop.

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