The epidermis of a D. melanogaster adult has on the order of 500,000 cells, ∼5,000 of which (∼1%) make bristles. A priori, it would seem reasonable to expect bristles to sprout as randomly as the hairs on a human arm. However, even the most scattered bristles – the tergite microchaetes (mCs) – have fairly uniform spacing. At the other extreme of precision are the 40 macrochaetes (MCs) on the head and thorax, whose basic layout has been conserved for 50 million years (Fig. 3.1).

Except for the MCs, the bristles of each body region tend to vary in number and position from one fly to the next. Interestingly, most bristles are organized in rows that run parallel or perpendicular to axes of the body or limbs. Within such rows, the bristles are aligned more or less accurately and are spaced more or less evenly. Different rules govern different patterns. Thus, notal mCs form jagged rows along the anteriorposterior axis, while wing bristles form straight rows along the margin, eye bristles arise at alternating vertices of each ommatidium, and belly (sternital) bristles are spaced at intervals proportional to their shaft lengths.

Why do such patterns exist? Surely, some are adaptive. For example, flies use “brushes” (parallel transverse rows) on the legs to wipe dust from their eyes, and other patterns appear to map air currents, prevent wetting, or act as shock absorbers. However, many may simply be accidents of evolution.