( Latin: Syrphus ribesii)
During the late summer of some years the house may be swarming with enormous numbers of rather attractive insects, resembling small wasps. These are hoverflies which on a sunny day can be seen hovering almost motionless in the air, and suddenly darting off so quickly that it is very difficult to follow them with the eye.
Like all the true flies they have only one pair of wings, whereas the wasps with which they may be confused have two pairs of wings.
Their resemblance to wasps provides a good example of the phenomenon known as mimicry, in which a peaceful; harmless species may closely resemble another species which is dangerous or aggressive. Wasps can be said to be dangerous, and of course not just to man, and they advertise the fact by having a striking pattern of yellow and black which serves as warning coloration.
Over a period of perhaps millions of years certain hoverflies have developed a pattern of warning colours similar to that of the wasps. This has not been a conscious effort on the part of the hoverflies but is the result of natural selection over a long period of time, based on the principle that those hoverflies which resemble the model best will be less likely to be eaten. These will be the hoverflies that survive to pass their mimicking coloration to their descendants.
Hoverflies are sometimes seen perched on flowers, particularly umbellifers, for they feed on the flower nectar and pollen, and play an important role in pollination. Their larvae are very effective consumers of aphids (greenfly), so in addition to being attractive hoverflies are also extremely beneficial, and with them in mind it may sometimes pay not to spray the garden with an insecticide.
When hoverflies come indoors it is simply because the house, with open doors and windows, acts as a large trap, from which they have difficulty in finding their way out.