One of the secrets of Drosophila's success as
a laboratory organism is its enormous developmental speed. Within only
10 days, it develops into an adult organism (at 25°C).
embryo already hatches after 24 hours. Its development is optimized to
meet this temporal requirement. The first 13 divisions occur in a sysncytium,
only the nuclei are dividing in a single cell. The omission of cytokinesis
allows these divisions to be extraordinarily fast, with a cycle time of
less than 10 minutes. At blastoderm stage, when cellularization takes
place, the embryo is genetically subdivided into 14 segments, all arising
at the same time. Not only in mammals, but also in most other insects,
these segments arise sequentially.
At the end of embryogenesis, the larva hatches. Drosophila goes
through three larval stages that are specialized for eating and growth.
Larvae increase their mass about 200 fold during the three days which
they spend feeding prior to pupariation. This increase in size is not
achieved by cell divisions but by a tremendous growth of larval cells
which is accompanied by an endoreplication of nuclear DNA.
The purpose of larval growth is to prepare for the pupal stage in which
metamorphosis takes place and a completely new organism emerges. Adult
wings and other appendages develop inside the growing larva. The imaginal
cells are the only cells that divide during larval life. At metamorphosis,
the body is restructured completely. Adult structures are everted (turned
outwards) and substitute the larval tissue, which slowly disintegrates.