Nick Baker’s Hid­den Bri­tain

BBC Wildlife Magazine - - Contents - NICK BAKER Re­veals a fas­ci­nat­ing world of wildlife that we of­ten over­look. NICK BAKER is a nat­u­ral­ist, au­thor and TV pre­sen­ter.

Get­ting cosy with clus­ter flies

S ynan­thropic species fas­ci­nate me. In a world where nat­u­ral habi­tats are in dis­ar­ray and de­cline, they buck the trend. If I want some kind of pos­i­tive af­fir­ma­tion that all is not bad in the world, then I look to­wards the an­i­mals and plants that thrive in man-made spa­ces.

One such species that gives me this kind of re­as­sur­ance is found in my at­tic. Here, sep­a­rated from the ev­ery­day warmth and do­mes­tic­ity chez Baker, is an even stranger world – in­hab­ited by a crea­ture whose en­emy is the de­hy­drat­ing ef­fects and fluc­tu­a­tions of cen­tral heat­ing, and which finds sanc­tu­ary in this cool but frost-free space ly­ing above us.

Creak­ing up the loft lad­der, I en­ter the dark. For the most part, this species is widely scat­tered, so barely reg­is­ters, but right now I just need to look at the side of the chim­ney breast. As I shine my torch, I re­veal bricks car­peted by a twitch­ing veil of in­sect life. Hun­dreds of flies sit silently wait­ing for spring. The golden fuzz of their col­lec­tive tho­raxes tells me that they are com­mon clus­ter flies, Pol­lenia rudis.

Dur­ing the rest of the year, I hardly see any, apart from on one warm au­tumn day when they con­gre­gate on the sunny side of my chim­ney stack. Are they queu­ing to get in­side? Some­where there must be a chink, a crack in the mor­tar, or a gap in the barge­boards, that acts as a por­tal to winter safety, be­cause over the next weeks these flies come in­side. In­sects are sen­si­tive to in­finites­i­mally small changes in mi­cro­cli­mate, and clus­ter flies are no ex­cep­tion. Look closely at the ‘face’ of one of your dozy, do­mes­tic denizens and you’ll see two very fine pro­jec­tiles be­tween its eyes. These are the ‘arista’ – feath­ery parts of their an­ten­nae that are highly sen­si­tive to en­vi­ron­men­tal cues. By us­ing these, clus­ter flies are able to pick out the per­fect spot for their winter so­journ, choos­ing the same spot ev­ery year.

The clus­ter­ing ef­fect is pos­si­bly am­pli­fied by the pro­duc­tion of ag­gre­ga­tion pheromones, which pull in­di­vid­u­als into these utopian win­ter­ing spots. Here they re­main un­til spring, when the whole mass of them re­verse the ef­fect and sev­eral hun­dred of these golden-napped flies let them­selves out again.

What they do then is dis­perse into the garden and sur­rounds, where they take part in a num­ber of ac­tiv­i­ties. Their sci­en­tific name, Pol­lenia, in­di­cates that, like many other blowflies, they visit flow­ers and so are im­por­tant pol­li­na­tors. But spring is when the re­ally in­ter­est­ing part of their life­cy­cle hap­pens. After mat­ing, the flies lay their white lozenge eggs on the sur­face of soil and, when these hatch, the mag­gots slide down into the soil with the sole aim of find­ing the mu­cus trails and bur­rows of their prey.

On the hunt

They may not look like it, but these lit­tle, pal­lid mag­gots are preda­tors of earth­worms. Us­ing a sharp set of jaws, they bur­row in be­tween the soft seg­ments of their host. In some in­stances, they lit­er­ally climb into their din­ner, with noth­ing but the tip of their ab­domen stick­ing out.

Over the sum­mer months, these pro­lific flies can breed sev­eral gen­er­a­tions, and only when the day length draws in and tem­per­a­tures drop will they re­turn to our homes.

Clus­ter flies can be seen returning to the same spots each year, in search of refuge from the harsh­ness of winter.

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