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B Barber |
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Science |
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Middle Years Science Program
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Drosophila St Joachim's (Sam Alcock) The vinegar fly, Drosophila melanagaster, is widely used in studies of inheritance. One reason is that it produces a large number of offspring in a short time (6-10 adults may produce 200-300 offspring within 8 days). Their offspring take about 12 days to develop from egg to adult in a reasonably warm room). The duration of each stage is affected by several environmental factors. Temperature is the most important: at 20 degrees celsius, the complete life cycle takes about 12 days; at 25 degrees celsius (the optimum temperature) it takes about 10 days. Drosophila come with 2 types of wings: wild type (fully developed and workable wings) and vestigial (small squashed up useless wings). Life Cycle: The eggs are small, elliptical structures (like footballs) about 0.5mm long, each bearing 2 filaments one one end; with practice you can see eggs with the naked eye. Usually they are laid on the surface of the food or agar within the tube. The larvae hatch from eggs in about 24 hours and, as they increase in length, moult twice. They eat almost continuously; in culture their black mouth parts can easily be observed moving back and forth. The larvae tunnel through the medium as they eat; the presence of numerous channels in the medium shows that the food has been "worked", and is an indication of a successful culture. The larvae stage is followed by an inactive stage, the pupa. When ready to pupate, the mature larvae usually climb up the side of the bottle containing the culture. Unlike the larvae of moths and butterflies (which spin a cocoon), the larvae of flies pupate within the larval covering, which then becomes harder and darker. The change to an adult fly can be observed through the pupal case: the eyes, wings and legs are particularly easy to see. When the adult fly finally emerge, they are fragile, light in colour, do not have fully expanded wings, and have a rather elongated, pale abdomen. In a few hours the body colour darkens and the wings expand. Adults may live a month or more. Females do not mate for a least six hours after emerging. After mating, sperm is stored within the body of the female; fertilization occurs later, at the time the eggs are laid. Distinguishing the sexes: Although.the male flies are usually smaller than the females, this is not a reliable guide for distinguishing the sexes; there are a number of more reliable characteristics. Females have a more pronounced abdomen compared with the remainder of their bodies. Females also have a longer mid section of their foreleg. |
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