Summer saw the garden thronged with marmalade hoverflies (Episyrphus balteatus), our commonest species and one which most people will recognise on sight. They are the small flies, with thin, broad bodies like a cuttlebone or a knapped flint. Their thorax is marked with yellow and black – typical warning colour mimicry but the marmalade hoverfly is entirely inocuous, unable to sting, bite or otherwise inflict the damage that its stripes would suggest. The white etchings between the warning bands give them an edge of elegance, and perhaps give them away as the imitators they are.
The garden is quieter now, but November sun still brings pollinators to visit the few remaining flowers. I first thought that the hoverfly in its midnight blue insignia was a new species, but it turned out on closer inspection to be the cool incarnation of the familiar marmalade, now a darker shade of its summer self.
The amount of yellow on their bodies waxes and wanes as the year progresses, starting low in March and rising to a peak in midsummer, declining again into the autumn and early winter. The arc it describes almost tracks daylength and this is not far from the mark; the darker pigments act like a black shirt on a hot day and help the hoverfly to absorb as much heat as possible when warmth is scarce. The temperature at which the larvae develop is key to the colour of the adult and this sleek specimen must have hatched into this early November chill.
The Batesian bands of black and yellow are lost – staying warm must be more important to the hoverflies than warding predators away once the summer abundance is passed.