Odonata generally in that part of Essex. Interestingly, in the rest of East Anglia, to the
north of Essex, the distribution of E. cyathigerum is very patchy. It is at its most abundant
at recently established sites, such as farm reservoirs, flooded pits, and dredged drainage
dykes, but it holds its own as these sites become more richly vegetated and support other
species. It is one of the few species which tolerates the brackish conditions often
encountered in the coastal and estuarine borrow dykes and also inhabits moving-water
sites, such as the Roman River, the River Lee and the River Blackwater.
Erythromma najas (Hansemann, 1823) Red-eyed Damselfly
This is another of our predominantly blue-and-black species. In both sexes the
colouration is mainly black, though the hindmost segments of the male abdomen are
bright blue. This latter characteristic could lead to some confusion with our commonest
damselfly, Ischnura elegans, but in the latter only segment 8 is coloured blue dorsally,
whilst segments 8 and 9 are so coloured in E. najas. In addition, the red eyes of mature
specimens are very noticeable, even at a distance, and E. najas is a larger, more robust-
looking species.
Erythromma najas is said to favour ponds or lakes with floating leaved water-plants
such as Water-lilies or Broad-leaved Pondweed. This is generally true in Essex,
although they are sometimes found on flooded sand and gravel pits without such
vegetation. Here. I have observed them to use floating patches of algae, and debris, such
as detached Reed-mace stems and leaves. The males, especially, use these as 'basking'
sites and they may have territorial and mate-location functions.
As early as 1897. F. A. Walker noticed what he interpreted as the pugnacity of this
species, in chasing off members of other species which settled on the same lily-leaf. My
own observations suggest that males of E. najas are subject to frequent 'buzz-bombing'
attacks from males of Enallagma cyathigerum when both species are present at a site.
Often more than one individual of E. cyathigerum is involved, and the attack usually
results in a retreat by E. najas (which may or may not return a little later). Winsland
(1983) has also observed this inter-specific competition between E. najas and E.
cyathigerum. He speculates that E. najas may be unable to withstand the fierce
competition at water margins, and that this may explain their tendency to use the more
inaccessible weedbeds (or. presumably, lily-patches) on areas of open water, away from
the pond margin. It also seems possible to me that inter-specific competition of this
kind, combined with the physical structure and pattern of vegetation of sites may well
explain why E. najas is so very localised in our county, when apparently suitable sites
remain uncolonised.
Another interesting observation relating to this species is that several adults of E.
najas were found dead, or hopelessly entangled in webs of the spider Tetragnatha extensa
along the Cornmill stream during a visit to this site in June 1983. No other species of
damselfly was observed to have been caught in this way, though several other species
were present. A. E. Le Gros (1954), in an interesting note on predation by the spider
Araneus cornatus on various Odonata species mentioned T. extensa as a possible
predator of damselflies, though he had not himself observed it. The recorded flight-
period of E. najas in Essex between 1980 and 1985 is from the beginning of June until the
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