Fly haltere scaling abstract and data.rtf
An animal with a large body could have larger sense organs, and so collect more sensory information than a smaller animal. However, many structures scale with body size so they are relatively smaller in larger animals. This occurs in eyes, but the situation for mechanosensory organs is not so clear. I investigated scaling of particular mechanosensory organs unique to dipteran flies, halteres, which replace hind wings and are important for the aerobatic ability that underlies the success and diversity of these insects. 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 large fields. 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 (eg. houseflies, blowflies). In all clades, haltere length scaled with body mass raised to the power 0.23, similar to insect eyes, and was directly proportional to wing length. In relation to body size or wing length, tipulids had longer halteres than other clades. Body size was not related to the number of CS within each clade, which was consistent for different sizes of dipterans within the same clade. This is in contrast to ommatidium numbers in insect compound eyes, but similar to numbers of auditory receptors in some clades of bush cricket. Tabanids had the highest numbers of CS on their halteres, whilst tipulids had the smallest.