Population Regulation in Quill Mites (Acarina: Syringophilidae)

Abstract

The wing quills of the House Sparrow (Passer domesticus L.) may host as many as three species of quill mites, suggesting ecological differences in the quill mite habitat. Detailed investigation of a single species of quill mite, Syringophiloidus minor (Berl.) reveals adaptations that maximize mite success within the confines of the feather calmus and suggests a mechanism that allows partitioning of the wing by two or more different—sized quill mite species. Quill mites live inside the feather calmus and feed on host tissue fluids by piercing the quill wall with long styletlike chelicerae. Mite reproduction and development take place within the calmus. Only adult female mites disperse. Females can move into newly developing quills during host moult in the fall, or into the developing feathers of nestlings in the spring. The quills in which the mites live represent a closed system, with calmus volume limiting the number of mites that may mature and calmus wall thickness limiting the ability of the mites to feed. Female mites cease egg production before the quill contains enough mites to fill it. As a result, the quill does not become filled with immature forms that would be incapable of mating and dispersing. Males of S. minor are smaller than the females and have a frequency of exactly 1:11. Thus over 95% of the limiting resource, calmus volume, is occupied by females, the potential dispersants. The size of a female is evidently limited by two opposing selective pressures: the smaller they are, the more can disperse from a given quill, but if they become too small the mouthparts will not be able to reach through the quill wall to feed. Thus each quill size has an optimum mite size, and the mite species occur in a corresponding diversity of sizes.