I’ve moved just a few kilometres from the River Liza to the location for this post: Croasdale Beck, a stream which joins the River Ehen at Ennerdale Bridge. Croasdale Beck has featured in a number of posts in the past (see, for example, “That’s funny …” and “Croasdale Beck in February”) partly because it continues to surprise me. Maybe that reflects a level of complacency on my part: regular visits mean that I know what to expect which, in turn, means that I am alert to things that I do not expect. Seeing something new in a stream I have never previously visited is evidence of life’s rich pattern; noticing something that was probably there on previous occasions but which I overlooked is a more profound and, somehow, humbling experience. This post is about one of each of these sensations.
There are, for example, a number of turquoise-coloured boulders in the beck that were certainly not there when I first started visiting in about 2015. Most of the stones in the beck are cobbles rather than boulders, so these stand out both for their size and colour. The colour is, if you look closely, due to a thin surface film – a Cyanobacteria which I will call Lyngbya vandenberghenii although, because it is difficult to scrape off (the filaments live in amongst the rock crystals), and lacks any really eye-catching features, it is hard to be totally certain about this. Presumably it likes the stability that boulders confer in this very flashy little stream. I also see it in the River Ehen nearby but there its presence is easier to explain as it is confined to chunks of limestone washed in from the foundations of a section of the Coast-to-Coast walk.
Today, however, I’m interested in what is growing on top of the Lyngbya rather than in the Lyngbya itself: dense patches of what looks, with the naked eye, like small tan-coloured seeds. These are the tiny larvae of Simulidae, whose adult phases are the annoying blackflies that swarm around streams on summer evenings. They spin a web of silk on the substrate to which they anchor themselves using a ring of hooks at their posterior. Their mouthparts include a pair of fans (one of which can be seen in the image below) and, by extending themselves above the stone, they can trap tiny particles (including algae) drifting in the current. They produce a secretion which makes the fans sticky and also have mandibles adapted to brush the trapped particles from the fans into their mouths. Most descriptions of the Simulidae refer to this filter-feeding life-style but I’ve also seen them bent double so that their fans can brush up the algae which grow on the stone surfaces.
At some point, the larvae cease feeding and spin slipper-shaped cocoons with the closed end facing upstream and the open end downstream. Six white ribbon-like gills protrude from the open end, ensuring a ready supply of oxygen to the pupa inside. The adult develops inside this cocoon, eventually emerging with a duel raison d’être of having sex and irritating humans. “Adult” hardly seems like the appropriate word: “perpetual teenager” seems more apt.
Whilst the adult males feed on nectar, the females need a blood meal before mating, adding a dark Gothic twist to their natural history. This difference arises from the roles each plays in reproduction: the male only needs the spurt of energy that the sugary nectar confers whilst the female needs the proteins and minerals from the blood in order to nourish the eggs. In the south of England, bites from the Blandford Fly, a relative of the Simulium I watched in Croasdale Beck, can cause nasty rashes whilst in large parts of Africa the bites from other species of Simulium can inject the parasite responsible for Onchocerciasis, or river blindness. This was a common disease in the region of Nigeria where we lived in the early 1990s so I’ve seen the damage that these flies can cause. Much as we find black flies and midges to be a nuisance in this country, at least they are not vectors for potentially deadly diseases.
At a deeper level, knowing about the life cycle of Simulium reminds us that we are not just observers of aquatic ecosystems, we are, indirectly, part of these ecosystems too. We may like to think of ourselves as the ultimate predator (remembering that this power brings with it great responsibility) but sometimes, as here, we can be the prey too.
And thanks to Richard Chadd for identifying the Simulium from my photographs.
Some other highlights from this week:
Wrote this whilst listening to: The late great Toots Hibbert, remembering, in particular, Toots and the Maytals’ set on the West Holt Stage on a glorious summer evening at Glastonbury 2010
Cultural highlights: We’re in the Lake District this week and, having recently watched part of Simon Scharma’s BBC series on the Romantic Movement, I’m reflecting on the role that the landscapes around me played in catalysing the work of Wordsworth, Turner and others.
Currently reading: English Pastoral by James Rebanks, a thoughtful analysis of the state of British agriculture that does not shy away from criticism either of farmers or naïve ecologists.
Culinary highlight: James Rebank’s thesis hangs on the necessity of animal husbandry to maintain healthy soils. With that in mind, I ate a Lakeland lamb steak at the Shepherd’s Arms hotel in Ennerdale Bridge with a clear conscience.