Culturing the Curly winged Fly Musca domestica
The Curly winged fly (plate 1) seems to be relatively new to the world of livefoods. It only came to my attention a couple of years ago. It is sold by a few live food suppliers and is an excellent livefood for a variety of small reptiles and amphibians. It typically averages about 7mm in length. As such it is considerably larger than the large fruitfly (D. hydei) and can be used as a larger alternative to that animal. Unfortunately, it does not seem to be widely kept at present and I have seen no instructions as to its culture. Dealers either don’t know how it is cultured or aren’t telling. This reduces its usefulness considerably, as it is prohibitively expensive when purchased from commercial suppliers. As a result I started experimenting with its culture about a year ago and have now developed an effective system after considerable trial and error.
Plate 1 – An adult fly – note the curled ends to the wings and sponge-like mouthparts
The Curly winged Fly is a variety of the common house fly (Musca domestica). The wild type of this animal is well known as a common pest that occurs throughout the world. The full life history of M. domestica is available at http://creatures.if.as.uyl.edu/urban/flies/house_fly.htm. Its larvae develop on carrion and manure and are well known as one variety of the angler’s maggot (the squat). The Curly winged Fly differs from the wild type in the fact that it is flightless. In fact some individuals can fly small distances but their flying ability is severely limited by the curled wings which give the animal its name. There are many similar species of flies ( like Bluebottles and Greenbottles) which are easily cultured, but they are winged and, as such, difficult to handle. Being flightless this fly is much more convenient as a livefood. I have no information on the origin of the variety and would welcome information from anyone who has such information. Such a versatile and ubiquitous animal as the house fly might be expected to be flexible in its requirements and be easy to culture. Furthermore as it is cultured in large numbers for bait its culture should be well developed and documented. Unfortunately it is not quite as simple as this.
Culturing of the flies is simple enough if they are raised on carrion such as chicken, cat food or any form of meat. However, such cultures smell appallingly. Furthermore, such cultures are ideal breeding grounds for the bacterium Clostridium botulinum, which is the cause of botulism. This is one of the most lethal of the food poisoning bacteria. Botulinum toxin is one of the most potent toxins known to man and even small quantities are lethal to both people and other animals. The true risk of botulism infecting a culture is difficult to determine accurately. It may be more theoretical than actual. Certainly most anglers (including myself) regularly warm up the larvae in their mouths and I’ve never heard of any poisoning from this source. Furthermore, botulism will not generate spontaneously, but has to be introduced from an existing source. Nonetheless, the potential danger of this organism behoves us to take every precaution and rotting flesh is a prime source for botulism.
Thus it is not ideal to culture the flies on carrion even though it is their natural food. It is then quite problematical to find a medium which will provide the nutritional requirements of carrion yet be of vegetable origin. I have experimented extensively with numerous alternatives and have eventually identified a mixture which meets the flies requirements.
There are also other difficulties in the culture of this organism. Firstly, it does not survive well in the kind of small containers used for the culture of Drosophila. These are too moist for the adult flies, which rapidly die. The pupae similarly prefer relatively dry conditions in which to complete their development. However the eggs and young larvae need moist conditions to prosper. Thus, whereas an animal such as Drosophila can thrive in one environment for all of its lifecycle the Curly winged Fly needs different conditions at different stages of its life. Any culture method must take account of these characteristics. Experience has also shown that the characters of the media make it very susceptible to attack by mites. The culture method must also take this into account.
The following method has been developed after much trial and error and has proven to be effective. However, the method is still in its early stages of development and I am continually refining it myself. It is likely that others will develop their own refinement, but I would advise anyone beginning culture of this animal to start with this method as I can assure them that it does work.
The adult flies are placed in a small glass aquarium (plate 2) about 8" x 8" x12". The top of this is covered by fine mesh. I use ladies tights with one leg knotted off and the other cut to about 8" length. This acts as a means of access to the tank without removing the top or giving the inmates opportunity to escape. A perspex top is placed over this. By sliding it partly to one side the humidity in the tank can be controlled quite precisely with a little practise. The flies themselves can survive quite happily with minimal humidity once they have plentiful water; in fact they prefer this. However, a reasonable humidity needs to be maintained for the egg-laying site which needs to kept quite moist, both to encourage oviposition and to prevent the eggs drying. The tank contains a small dish (I use the top of a small jam jar) containing a small quantity of ordinary granulated sugar. This provides food for the adult flies. The tank also contains a water fountain of the type used for small birds such as canaries. The trough at the base is filled with a small wad of tissue paper. In this way the fountain will provide water to the flies without drowning them. Be aware they will drink quite a lot of water and if they don’t have both water and food they will die with extreme rapidity. In any case the lifespan of the adults is typically less than 2 weeks. They should begin to lay eggs after about 5 days and even if they survive beyond 10 days will probably not be producing many eggs any longer. As with all insects the exact duration of their life history is extremely dependant on the temperature of their environment. The live cycle can be completed in anything between 8 and 20 days , depending on the temprature. The housefly is quite adaptable as to its requirements; typical room temperatures are probably a good average for them. I keep mine on top of a vivarium where they get quite warm in the day (from the lights) but are cooler in the nights. As a result I can’t easily give an average temperature for this environment, but it does not seem to be critical.
The flies will produce copious droppings in the form of fly spots on the glass. There will also be rapid mortality in even the most ideal environment because of their short lifespan. As a result the debris (mostly dead flies) should be ‘shovelled’ up every couple of days. The tank should be emptied, cleaned and disinfected every couple of weeks depending on the number of flies being kept. This is quite a tricky operation as the flies are quick and also have a tremendous grip on the smooth surface of the glass. This makes them difficult to manipulate, but with care and effort the flies can be shaken into a large container such as a bucket and thus transferred to another tank. However, be prepared for a few escapes.
Plate 2 – Tank for adults – note damp tissue in the foreground for oviposition; tray for sugar and water fountain are in the background. Many of the flies will be found on the underneath of the mesh covering the top.
When the flies have been in the tank a few days and are ready to oviposit I add a piece of tissue (plate 3) which is used as an oviposition site. This is two pieces of quilted toilet paper or a sheet of lightweight kitchen towel. This is thoroughly soaked in a mixture of the medium made up to a soup- like consistency with milk. The milk is needed as the flies are attracted to oviposit on decaying material. Milk decays very quickly and generates the odours needed for this. Milk that has been out of the fridge for several hours is even better than fresh milk for this purpose. However, be aware that even this small amount of animal protein increases the risk of the culture smelling or becoming host to botulism. If you are concerned about this you can mix the medium with water, but rates of oviposition will be much lower. This can be improved if the tissue soaked in medium is left to ‘mature’ for a day before use. The tissue is left in the tank for about twelve hours. The longer it is left the more attractive it is to the flies, but it will dry out after 12 hours in even ideal conditions and need water adding.
After 12 hours in the tank there should be many eggs around the tissue. If there are not then the adults are probably either too young or too old. Alternatively the tissue may have become too dry or the medium is not mature (smelly!) enough. It is necessary to keep the tissue at least slightly moist. At the same time the tank must not become too moist or the adults will suffer. There must be no sign of condensation on the inside of the tank. The correct level of humidity can be maintained by adjusting the Perspex sheet covering the tank. The exact position can only be determined by trial and error, as it will depend on the temperature and the number of flies. 200 flies (about the maximum population of the tank) generate considerable humidity and will need the top about 25% open. However, if there are only a handful of flies in the tank (as in a starter culture) then the tissue will rapidly dry out if the tank is not completely covered. Typically the moist tissue should remain adequately moist for about 12 hours and I usually change them morning and evening. There is no real problem in keeping the tank drier, but the tissues will then have to be moistened with water more frequently and this is time consuming and inconvenient.
Plate 3 – Tissue moistened with dilute medium is used as an oviposition site and is covered in adult flies.
The eggs are hard to see on white tissue, as they are also white. However, usually most eggs are not laid on the tissue but under it or next to it, attached to the glass. For the next stage of the culture the eggs must now be removed to a separate container. This not only breaks the cycle of any pests such as mites, but also allows us to provide the two different types of environment needed by adult and larvae.
The containers used for the next stage of the culture are something like a 700ml jar (plate 4). The lid of the jar needs a hole cut about 3 cm in diameter, which is sealed with fine mesh. The mesh needs to be both fine and stiff (stainless steel mesh about 60 threads per inch is ideal) as the larvae are masters of escape. They have no real skeleton and can force their bodies through incredibly tiny apertures. The moist tissue from the tank is transferred to this container. If the tissue is nearly dry then moisten it, but don’t soak it. The remaining eggs adhering to the glass base of the tank can be collected easily using a tissue moistened with water and added to the same container. They are quite tough and seem to survive even quite robust handling.
Plate 4 – Jars used for culturing larvae
The jars with the egg-covered tissues are then left in the same environment as the tank. Within about 24 hours the eggs should have hatched and tiny larvae are visible roaming around the jar. At this stage there is still enough nutrient on the tissue to sustain them for their first few hours. If medium is added earlier it is not consumed and grows mould. However after 24 hours it is about time to add the medium on which the larvae will develop.
The medium I use is a vegetable-based recipe, which I have developed through extensive trial and error. I sell it through my website at www.jangala.co.uk; the price is less than half the cost of commercial Drosophila medium. If you don’t want to use this medium then it is quite possible to use moist cat food in its place and use a tissue moistened with milk for oviposition.
The medium is mixed with water to a consistency of stiff dough; that is it should not be too moist. The amount needed will depend on the amount of larvae present. Usually I use a heaped dessertspoonful of medium (this is for a thriving culture; less would be needed for a starter culture). When I add the medium I add it on top of a dry paper towel, which prevents the medium becoming too wet. This is one of the major pitfalls in culture failure. The medium needs to be slightly moist, but not wet. If the medium looks too wet or is solidifying into a solid lump (a result of excess moisture) add dry bran or tissue to absorb the excess moisture and loosen the medium.
If the larvae start swarming up the sides of the container in large numbers (a few are normal) whilst still small this means they are probably searching for food and need more medium. More can be added as needed during the larval development. It is generally safer to have too little medium and add more as needed than to have too much and for it to go ‘sour’. The precise amount needed will be learned by experience. The jars can comfortably hold about 50 larvae or even 100 with care (plates 5 & 6). Generally the larvae do well if they are quite crowded, but one can have too much of a good thing. 50 larvae per container is probably ideal until one is experienced with the culture method. If the culture seems overcrowded simply split it into several jars. The larvae will pass through three instars and then start to pupate about 7 days after oviposition .
Plates 5 & 6 – Fly larvae (second instar). If they are this crowded then you’ve probably got too many in the container unless you’re an expert with them.
Prior to pupation the larvae will swarm up the sides of the container, as they naturally disperse prior to pupation. The larvae will pupate on the medium (plate 7) if it is sufficiently dry, but it is safer to arrange a drier site for pupation. To this end add a few (~6) centimetres of dry bran or a couple of kitchen towels (or both). I prefer the tissue as this allows the adults to be shaken out of the jar whilst the tissue holds other debris in the jar.
Plate 7 – Fly pupae on the surface of the medium. These pupae are really too wet, but you can get away with this if you’re lucky.
The larvae will take about 5 more days before they emerge as adult flies. If tissue is used for pupation the adults will gather on the top of this and can easily be shaken into another container for addition to the tank used for oviposition. Once a culture is established a tank can be maintained with about 100 to 200 adults. These will produce in the region of 200+ eggs per day. The tank can be topped up with about 75 flies twice a week. The other flies produced can be used to feed to your animals. They can be gut loaded and dusted with vitamins and fed in a similar way to Drosophila. Being considerably larger than Drosophila they are suitable for larger animals for which Drosophila would be too small. As such they add flexibility to the armoury of live foods. They have the advantage that they are quick to breed and prolific once their culture has been mastered.
Be aware that the culture medium can also attract other species of fly or wild type house flies. These can oviposit on the medium in your culture (They can sit on the vent at the top and drop eggs onto the medium like bombs!) leading to you culturing a batch of these flies instead of (or as well as) Curly wings. If you don’t notice this you can end up with a room full of bluebottles when you open up the jar! Worse yet you may cross your flies with the wild type and have your culture revert to fully winged flies. There are also occasionally some Curly winged flies which develop an ability to fly. I let these escape when they are being emptied out of the jars as in Drosophila cultures such adults can take over the culture is not weeded out and it is likely that the same is true of Curly winged Flies.
In ending it should be noted that the Curly winged Fly is not the easiest animal to culture because of the several stages needed in the culture method and the need to maintain humidity within fairly narrow limits during parts of their development. They are particularly difficult to get started. With a starter culture the flies all tend to oviposit at the same time so that one ends up with a culture operating in cycles. The larvae also do better when they are present in large numbers. When there are only a few larvae the amount of medium and the humidity are difficult to control and the medium can go ‘sour'. However, perseverance usually pays off and anyone familiar with the culture of Drosophila should be able to master the method without too much difficulty. If anyone does have trouble with the method I would be happy to advise them. I can be contacted through my web site at www.Jangala.co.uk.
Dr Robert Brown