Murk-dwelling bottom-breathers

I've posted about our garden pond before, but may not have mentioned that off to one side of it is an old stone fountain-top which has been plugged to provent leaking. Having no outflow unless there is heavy rain and it spills over, this means it becomes stagnant with leaves and other organic matter accumulating. The temptation might be to clean this out and add clean water, but no, it is there for a reason - habitat for larvae that are adapted for such conditions. An example of this is the rat-tailed maggot, the larva of hoverflies in the genus Eristalis, in particular E. tenax, and others such as the sun-flies Helophilus sp., plus Sericomyia, Mallota, Anasimyia and Myathropa. As adults, Eristalis are are excellent honey-bee mimics (hence the common name of 'drone-flies'), but their larvae, like those of the other genera listed, are very different. The most obvious feature is the long, telescopic posterior breathing tube - essential when living in stagnant, low-oxygen water among decaying vegetation.

Rat-tailed maggot, the larva of Eristalis sp. (probably E. tenax), approx. 10mm long excluding 'tail'.

This is a young larva - my identification is a little tentative as I can't see the key features yet, but the timing is right as E. tenax were frequenting the water long enough ago for eggs to now have hatched, whereas although there are Helophilus pendulus as well, they have only just started breeding activity as far as I can tell; I will be able to confirm their identity when they are more fully developed. They are filter-feeders and some of the gut contents are visible here, as are the well-developed prolegs.

Rat-tailed maggots showing how their breathing tubes are used.

The white section of the tube forms a sheath made from one extended segment and contains complex musculature used to extend and retract the breathing tube, as well as protecting it (the tip of the tube can be seen as a short white-tipped dark segment). The larvae can also swim slowly by undulating the body and tube. They may not be the most attractive creatures, but I do think they are interesting, and they are of evolutionary interest as these larval forms appeared quite recently in hoverfly evolutionary history (Rotheray, 1993), not to mention their role in consuming waste organic material. They do of course also develop into adults - the dronefly mentioned above, which has a role in pollination, so a valuable species too.

Eristalis adult on Buddleia davidii

Reference

Rotheray, G.E. (1993). Colour guide to hoverfly larvae (Diptera, Syrphidae). Dipterists Digest 9: 1-156.