Fruit flies are an excellent food for many surface feeding killifish, closely resembling their natural diets in the wild. They are easy to culture, quickly, cheaply and in large numbers. Fruit flys are rarely available in pet shops, but are widely available online, and very much worth trying, even if you don't bother culturing them on an ongoing basis.
Fruit flies are small flies that live on and around decaying fruit and vegetation. They occur worldwide, except the very coldest regions. The species most familiar to scientists, aquarists and herpetologists are in the genus Drosophila which literally means 'dew-lover' in Greek. Also known as 'bar fly' or 'vinegar fly', these little flies are commonly found around waste bins, bars, orchards etc, and they are widely used in laboratories as research animals. The species we feed to our fish however, have been bred to have shorter 'vestigial' wings, and as such can not fly. These are typically from two species Drosophila melanogaster and Drosophila Hydei.
Although dealing with highly mobile flies can be trickier than with worms, there are some great reasons to consider culturing them.
Adult flies lay a lot of eggs. A single female can lay over 20 eggs in a day, and can store sperm within her body for up to two weeks, to lay fertilized eggs at will.
Eggs take 23 hours to hatch at 25 deg C. Once they do hatch, the larvae bury into the media and grow. They will stay as larvae for five days, in which time they undergo three moults.
Eventually they will start to climb the sides of the vial, and when fully grown will settle near the top, and form a chrysalis. The change from maggot to fly takes around three days, but again, is temperature dependent.
The image to the right shows a newly formed chrysalis with the lava's segments still visible (1), an older chrysalis with the fly clearly visible. The red dots are its eyes (2) and an empty chrysalis from which a fly will have hatched (3).
Once the flies have emerged they must be moved to new vials, as there is now no food suitable for them in the old vials.