Changes in attachment sites of a commensal chironomid,Nanocladius (Plecopteracoluthus) sp. (Diptera: Chironomidae), on the body of its host,Nigronia serricornis(Megaloptera: Corydalidae), were observed in laboratory streams over a 13-week winter period and a 6-week spring period to determine seasonal movement patterns and emergence success. All chironomid commensals were lost from their hosts by the end of the winter study and no emergence was observed. During winter, chironomids resided in the same tube location on hosts for approximately three weeks before changing positions or being lost from hosts. Vacated tubes remained attached to hosts for an average of 3.6 weeks. Eight re-position movements were documented on host bodies and six midges colonized new hosts in the stream. New colonizations occurred most frequently on previously colonized hosts. During spring observations, commensal midges migrated from the abdomen and ventral thorax to the lateral thorax prior to pupation. Emergence occurred between 6 June and 2 July in the laboratory streams. Emergence success was strongly influenced by commensal attachment site when more than one commensal resided on a host, but not when midges occurred singly. Overall, midges exhibited about 41% emergence success. Unsuccessful emergence was attributed to re-attachment failure following a host molt, host death, or unknown causes. These data suggest that commensal midges have high rates of movement after attaching to a host and attachment sites may determine, in part, emergence success.