This will be the medium for the flies to lay their eggs in, and will provide nutrition for the growing larvae. There may also be a wad of cotton wool, or strips of a wood like substance, curled up. This will provide a surface for the larvae to pupate on, and hatch out. The larvae will also swarm up the sides of the container, and on the lid.
When pupated, they need no further attention. If left in a warm place, they will hatch out after about 6 days or so. The warmer the culture is kept, the faster the pupae will hatch.
When the flies emerge, you can offer them to what you are feeding straight away. If you wish to keep a culture going, then you should keep the culture tub in the container where the flies are being fed. After a few days, when matured, the flies will then start laying in the original culture tub. If a lot of flies are offered, then it is likely that the flies will breed, before they get eaten. When all the flies have been eaten, remove the culture tub, and store it in a warm place, for the eggs to develop, and pupate.
Thus, the fly cycle goes on.

The fruit fly culture cycle

The best temperature to keep the flies at is around 20-25c. In these temperatures, the cycle will be at its fastest. To slow the cycle down, simply cool the culture. To prevent any mould, or fungal infections occurring, make sure the culture is very well ventilated. Piercing small holes in the side of the culture tub can do this. However, these holes need to be small enough, so the larvae cannot squeeze through.
A method that has proven successful is to use clear, plastic tubs or cups, with no lid. As a substitute for a lid, a piece of thin cotton like material can be fixed to the top, using an elastic band, or something of the sort. This way, the larvae cannot escape though holes in the side of the tub, or through holes in what could be mesh, used as a lid.
Humidity does not need to be high when breeding Drosophila. Depending on the mixture used, it is important that it doesn’t dry out. If the mixture does look dry, then simply spray some water in through the top, which will absorb down into the mixture. Do not spray too much water in, as this can drown the flies, and the pupae.

Creating the medium

There are various different culture mixtures that be easily made at home.
There is one very easy option, which is usually used to feed flies on mass. Get a large bucket, and put a few handfuls of rotting fruit in it. Keep this in the corner of your garden, or somewhere similar, and this will attract swarms of drosophila. The fruit has to be rotting, as they will not reproduce on fresh produce. The bucket should be covered with a fair fine mesh, large enough to allow the flies to enter, but to keep out any other pests, such as birds and other fruit eaters.

Another, smaller scale recipe is one that can be made easily indoors. Mix a table spoon of sugar, with a cup of instant or mashed potato, which is readily available from most supermarkets. This can be put in any size cup, jar or tub. When added, pour water in, that is half as deep the mixture that was put in. Mix this, until you get a soggy medium. Then sprinkle some dried yeast on the surface of the medium.

Just cups, jars or tubs of rotting fruit can also be used as a medium. Ripe bananas can also be used on their own. These will not dry out, because as they rot, they release liquid. As this is the case, it might be necessary to put in some cotton wool, which will absorb the liquid.

In all cultures, be sure that the cup, jar or tub you are using is sterile. This way, you will not encourage moulding, and other infections.