Scaling of sense organs that control flight: Size and sensory cell number of dipteran fly halteres

An animal with a large body can have larger sense organs than a smaller one, collecting more sensory information. However, many structures scale with body size to be relatively smaller in larger animals. This occurs with eyes, however the situation for mechanosensory organs is less clear. I investigated scaling of particular mechanosensory organs unique to dipteran flies, halteres, which replace hind wings and are important for the aerobatic ability underlying the success and diversity of dipterans. Halteres are shaped like drumsticks and beat up and down in time with the wings, acting as gyroscopes because yawing, pitching or rolling generate twisting forces in them. These forces are detected by campaniform sensilla (CS), dome-shaped sensory cells in the cuticle that are mainly clustered into three fields. Unusually for a sense organ, individual CS are visible on the cuticle surface. I compared how haltere size, shape and CS number scale with body size in four clades representing different branches of dipteran phylogeny: tipulids (craneflies); tabanids (horseflies); syrphids (hoverflies) and calyptrates (e.g. houseflies, blowflies). In all clades, haltere length scaled with body mass raised to the power 0.23, similar to that for eyes of insects and other animals. It was directly proportional to wing length, probably enabling halteres to beat in time with wings. In relation to body size or wing length, tipulids had longer halteres than other clades. In contrast to haltere length, the number of CS was not related to body size within each clade but did vary between clades, suggesting a basic pattern for each clade was laid down early in its evolution. Tipulids had the smallest number, associated with relatively ponderous flight. Tabanids had more CS than calyptrates or syrphids, reasons for which require further investigation.