I was up early enjoying the sun at breakfast, but someone was up before me with the same idea. For a couple of days I had been trying to get a picture of his sort or swipe at one of them with my net, but they were too fast for me, but now the tables were turned and I could see who he was without bothering him. Or rather, her – the eyes of the males meet above the antennae, whereas her eyes were separated by a shiny forehead. This is one of those hover flies that can be identified from by sight or from a photograph and is Eristalis pertinax, one of the first hoverflies to appear in the spring. Though I have never seen one, the larvae of flies in this genus (known as rat-tailed maggots) have a distinctive long breathing tube that allows them to live safely underwater without having to come up for air.
Which brings me to my greatest (and only) success as an investigative entomologist. As the Secretary of my local beekeepers’ association, I would often be called up about bees nesting under the garden shed (usually bumble bees) or the attic (usually wasps). After a few pertinent questions about hairiness, size and colouring I could work out which group was involved and dispense advice, usually along the lines of “If you can, leave them be and they will be gone in a few months”. However, one caller was insistent that it was honeybees that were getting into her bedroom, and could I come and have a look. There often isn’t much a beekeeper can do once bees have set up home in your house – it’s a case of put with them or kill them. However, I took pity on her and set off on my bike to investigate.
The scene of the crime turned out to be a top floor flat on the main street in town, and the bedroom had a low sloping ceiling that, from about waist height, sloped to follow the roof line. The honeybees, as you may have guessed, turned out to be Eristalis hoverflies, and it greatly reassured the damsel who had summoned me when I told her there was no risk of being stung. She tried to distract me with the offer of a cup of tea, but like Sherlock himself, I followed the clues wherever they might lead. The flies were only appearing in the bedroom of the flat, and I was assured that the flies would appear whether or not the window had been left open. Hence, they must be coming through the walls or floor, and there was no good reason for them doing this as adults whose only interest is in finding flowers and mates. But what if they had got into the walls or roof space as larvae and had then pupated there? When the adults emerged, some of them might take a wrong turning and up inside the house. That would mean that there would have to be a permanent pool of water nearby that was well supplied with rotting sediment. I opened the window and saw, directly underneath, a clogged gutter, brimming with water and blackened leaves and moss. Q.E.D.