Fig. 2: Life cycle of Drosophila (from Carolina Biological Drosophila manual).
Fruit flies are holometabolous insects; that is, they undergo complete metamorphosis during their life cycle. The life cycle consists of four distinct stages: egg, larva, pupa, and adult. The rate of development is dependent on temperature, being more rapid at higher temperatures. For instance, at 20oC, the life cycle is completed in 14 or 15 days, but at 25oC, the cycle lasts about 10 days.
Mating and Eggs: Mated females store sperm to fertilize eggs that are subsequently laid. Therefore, to ensure that the desired cross is achieved, it is necessary to place females that are virgins with their intended male mates. Female flies are unable to mate for several hours after they have eclosed as adults from their pupal cases. Therefore, virgin females can be obtained by clearing all of the flies from a vial and collecting all newly-eclosed females several hours later. These virgin females can be kept separated from males for several days until needed for crosses.
Oviposition by the female starts as early as the second day after its emergence from its pupal case. It increases for about a week until a female adult may be laying 50-75 eggs per day for a total of approximately 400-500 eggs in 10 days. The egg is ovoid, covered outside with a thin but strong envelope (chorion) from which project anteriorly two thin stalks whose terminal portions are each flattened into a spoon-like float. The latter serve as "water-wings" to prevent the egg from sinking and drowning in a semiliquid medium. At the anterior end of the egg is a minute pore (micropile) through which the spermatozoa enter the egg as it passes down the oviduct into the uterus. Although many sperm may enter the egg as it passes down the oviduct, only one fertilizes the female pronucleus and the others are soon absorbed in the developing embryonic tissue.
Larva: The larva is a white, segmented, worm-shaped burrower with black mouth parts (jaw hooks) in the narrower head region. For tracheal breathing it has a pair of spiracles (air intakes) at both the anterior and posterior ends. Since insect skin will not stretch, the young small larvae must periodically shed their skins (cuticle) in order to reach adult size. There are two such molts in Drosophila larval development that are accompanied by shedding of the mouth parts as well as the skins. During each period between molts, the larva is called an instar, i.e. the first instar is between hatching and the first molt. Both the size of the larva and the number of teeth on the dark colored jaw hooks are an indication of which instar the larva has reached. After the second molt, the larva (now third instar) feed until ready to pupate. At this stage, the larva crawls out of the food medium onto a relatively dry place, ceases moving, and everts its anterior breathing spiracles.
Pupa: Soon after everting its anterior spiracles, the larval body shortens and the cuticle becomes hardened and pigmented. A headless and wingless prepupa forms. This stage is followed by the formation of the pupa with everted head, wing pads, and legs. The puparium (outer case of the pupa) thus utilizes the cuticle of the third larval instar. The adult structures that seem to appear first during the pupal period have actually been present as small areas of dormant tissues as far back as the embryonic stage. These localized preadult tissues are called anlagen (or imaginal discs) and because of the ease in which they can be isolated have often been used in studies of developmental genetics. The main function of the pupa is to permit development of the anlagen to adult proportions. The breakdown of larval tissues to furnish material and energy for this development is therefore a prime feature of pupal metabolism.
Adult: Adults exhibit a typical insect anatomy, including compound eyes, three-part bodies (head, thorax, and abdomen), wings, and six jointed legs. The various types of bristles and hairs found on the body are characters that we will use to identify different phenotypes of flies.
Using the naming conventions described previously, a fly displaying the three mutant traits of white eyes, miniature wings, and forked bristles is noted as: wmf, while a fly that has the wild-type phenotype at those three loci would be noted as: +++.