Drosophila development: A prepattern for sensory organs

The bristles and wing veins of Drosophila are ideal for studying the generation of spatial patterns. Sensory bristles occupy stereotyped positions of importance to the animal, as the neuronal specificity of bristles depends upon the site in the epithelium at which they are born. The thoracic bristle pattern has been conserved throughout the 2000 or more species of this genus and is well over 50 million years old; there has thus been a long period for stabilization by natural selection. Primitive insects were covered with uniformly spaced bristles. The Dipteran thorax is characterized by rows of spaced, large bristles, known as macrochaetes (Fig. 1a; the rows of small bristles, or microchaetes, are not shown). During the evolution of Diptera, however, many of the large bristles have been lost from the pattern, so that in Drosophila some rows have been lost entirely and only vestiges of most of the other rows remain (Fig. 1b,c).