Ectosymbiosis between Nanocladius downesi (Diptera: Chironomidae) and Acroneuria abnormis (Plecoptera: Perlidae) in a Michigan Stream, USA
Bottorff, Richard Lee; Knight, Allen Warner
All larval instars and pupae of the chironomid Nanocladius downesi (Steffan 1965) are found living ectosymbiotically on the stonefly Acroneuria abnormis (Newman 1838) in a Michigan stream, USA. Midge larvae occur year-round on the stonefly; pupae occur from April-September. Infestations are highest in autumn-winter and lowest in summer. Overwintering midge larvae synchronize their development to emerge prior to the May-July stonefly emergence. Midge density and instar influence attachments sites on the hosts; both larvae and pupae are most abundant on the mesothoracic wing sheaths. The spatial dispersion of midge larvae on the host stonefly population alternate between random (May-Aug) and regular (Sept-Apr). Territoriality between midge larvae is indicated by (1) changes in attachment site with midge density and instar, and (2) the spatial dispersion of midges on the host stonefly population. Possible benefits and costs of this ectosymbiosis are discussed.