Jessica Davidson, of Balclutha Primary School, asks :-

Are there any blind insects?

David Warton, a zoologist at the University of Otago, responded.

Most insects have eyes, which are of two types (simple and compound) but are completely different to the eyes of humans.

The compound eyes consist of a number of units, each with its own lens and light sensitive cells, that form overlapping images. Compound eyes are good at detecting movement but not so good at forming an image. In fact their ability to form an image is so poor that if the insects that possessed them were humans they would be considered legally blind.

There are, however, some insects that are completely blind; the best known being those found in caves. True cave dwellers (called troglobionts) live permanently in caves and cannot survive outside them. Insect troglobionts include beetles, crickets, cockroaches and flies. Being constantly in the dark has caused their eyes to become small or to be lost altogether. A springtail (a wingless insect) called Plutomurus ortobalaganensis was discovered by the Ibero-Russian CaveX team in 2010 living in the world’s deepest cave in a remote area of the Western Caucusus Mountains near the Black Sea. It has no eyes at all and so is totally blind.

However, not all cave-dwelling insects are blind. The New Zealand glow-worm is the larva of a fly and lives in a variety of damp places such as the moist banks of streams, as well as in caves and old mine shafts. It has two small eyes on each side of its head that may be used to distinguish between light and dark.