The larva, the larval case and the silk ofRheotanytarsus musdcolaKieffer was examined with scanning electron microscopy. The structures of mouthparts and the larval case are described in relation to feeding habits and silk production. Three methods of silk formation seem to be used. One is used in the capture net, and is produced by the act of grooves on the anterior edge of the ventromental plates. The other, which has a finer texture and is used in case construction, seems to be produced by the action of the ventromental and maxillary lamellae. The third silk type is formed as the silk secretion is ejected through the mouth cavity, producing a thicker silk strand, which is used in the framework of the capture net.