The Fruit Fly

John Skillcorn
Costa Rica
Last update: 21 September 2002

The fruit flies, Drosophila, have long been studied in Biology, and in particular with regard to the study of genetics. One species, D. melanogaster, is by far the most extensively studied organism of all in this respect, and almost all of its four pairs of chromosomes have been mapped. As a result, we know exactly where on the chromosomes each gene occurs.

In order to map the chromosomes, it has been necessary to cause mutations or 'freaks' to occur in a much higher frequency than is normal in nature. This is brought about by exposing the parent flies to various chemicals and gases, or by giving them very high doses of radiation, such as X-rays, g-rays or simply ultraviolet light. By breeding from these parent flies, their offspring produce very high frequencies of mutants. Their wing type affects at least two of these mutants. One is wingless and the other shows the characteristic vestigial wing. It is the latter that we are particularly interested in here.

As can be seen from the photograph left, vestigial wing flies do have wings, but after hatching from the pupa (or chrysalis) the wings fail to open and expand. The flies are, therefore, flightless, and for this reason are much easier to culture than the normal, wild type. They are identical to wild type in all other respects, however. They are only a few millimetres long, and have a brown-yellow body. The females are slightly larger than the males, but the shape and colour of the abdomen easily identify the sexes.  The photograph left shows a male.  Fruit flies are very easy to culture, but do take up a little time to cultivate continuously.  

The normal culture vessel is the small half-pint milk bottle (still delivered to schools, but steal your own!) although small coffee jars can be used.  Milk bottles can be plugged with cotton wool, but for coffee jars you will have to use kitchen towel and elastic bends.  All glassware should be washed in hot water, thoroughly rinsed free of all detergent and then dried.

The medium can be made from a variety of foods.  In schools we use a ready-mix flake which, when mixed with water forms a paste in the bottom of the bottle.  A very good substitute is a 50/50 mix of ripe banana and dry Ready-Brek, with liberal helpings of Vionate, SA-37 and Stress thrown in for good measure. This should be mashed into a paste and placed carefully into the culture vessels, avoiding messing up the sides. The paste should be thick and not sloppy – using more Ready-Brek can control this. In fact, I would tend to make it a bit on the dry side at first, as you will find that once the maggots begin to 'work' the medium, it will become wetter due to the moisture which they exude.  Whatever food you use, though, an essential item is a sprinkling of yeast granules on the surface of the medium.  Without this, the larvae will not flourish.  For other media, including instructions for fungus feeders, click here.

Some people place paper towelling into the medium in order to absorb excess moisture and/or to form pupation sites, but I find that after a little practice this is not generally necessary. Indeed, the paper can be the source of mould being introduced into the cultures. Also, for this reason, some recipes demand that everything be sterilised in a pressure cooker. However, I have found that if you are careful in your preparation, and you introduce healthy flies from a healthy culture, mould will not be a problem, at least not until the culture is past its best anyway and will then need to be thrown away.

By experience, I have found the ideal incubation temperature to be 24°C. Temperatures should not exceed 26°C, although lower temperatures are permissible, within reason. The life cycle, however, takes longer to complete.  At 24°C, it is almost exactly 10 days.  Several cultures should be set up at the same time, and will produce a profusion of flies.  The first few dozens to emerge, however, should be used to start new cultures.  Each culture will produce two or three generations, but you will notice that the flies produced from each successive generation will get smaller and smaller as the culture medium becomes exhausted.  In spite of this, these tiny flies can be used to start new cultures, and the next generation will be normal size.

Although easy to maintain, there are three main hazards you will need to be aware of:

Contamination by mites
If you keep crickets, keep your fruit flies well away from them, as Histiostoma mites will most likely then contaminate the flies. Cultures so affected must be destroyed, after saving only a very few flies to start new cultures, making sure as far as is possible that the flies chosen are free of mites. Bottles and jars may be stood in water as a precaution, but if mites are anywhere nearby they will eventually find their way into the cultures.  If you re-use bottles/jars, make sure they are washed very carefully between uses.

Culture bottles and population cages can be kept free from mites by spraying them with a 2% solution of benzyl-benzoate in alcohol.  This chemical does not harm Drosophila, but should be kept clear of Dendrobatid frogs.

Mould
This has been mentioned before, but should not be a great problem. There are various mould inhibitors available, Nipagin being the best known, although it is difficult to obtain in small quantities. You will most likely find that the constant churning or 'working' of the medium by the maggots will be sufficient to keep the medium clear of mould.

Contamination by wild flies
The characteristic vestigial wing is a recessive mutation. Put simply, this means that if wild male flies get into your cultures and mate with the female residents, the result will be cultures yielding large percentages of normal-looking, winged flies.  Similarly, if wild females get into your cultures, the end result could be worse.  Obviously, if this happens you are defeating the object of culturing pure vestigial wing flies. Not only that, but your fruit bowl may never be clear of flies again - and you will forever be picking flies out of your wineglass! 

Although D. melanogaster is the usual species of fly cultured, I have used other species from time to time:

D. subobscura, D. mojavensis - these look similar to D. melanogaster but are larger.

D. virilis (complex) - a very good, large, black species of Drosophila, much larger than D. melanogaster and almost half the size of the common Housefly.