The paper gives a detailed account of the structure and function of the mouth parts of Phryganea striata L., followed by a comparative study of these structures throughout the order Trichoptera. Observations on the feeding of caddis flies are reviewed. Consideration is given to homologies and phylogeny. In Phryganea the head is produced ventrally into a proboscis to which all parts of the mouth complex contribute. A detailed account is given of external and internal structure, musculature, and nervous system of the head and mouth parts. The central area of the anterior surface of the head capsule is interpreted as a frontoclypeus because of the origin of muscles to the foregut. The elongate labrum covers a sclerotized groove or sitophore. Mandibles are reduced to small lobes. The cardines and stipites of the maxillae contribute to the base of the proboscis. The single maxillary lobe is interpreted as a lacinia on grounds of musculature. The distinctive protrusible haustellum is regarded as derived from the hypopharynx. It is traversed by a common salivary duct, provided with a muscular valve. The anterior surface of the haustellum is covered with a system of channels which converge to the sitophore. These channels are formed by cuticular outgrowths arranged in lines and drawn out into filaments which roof the channels thus formed. These outgrowths, which are named pectinate hairs, differ in form according to their position on the haustellum. The labium forms part of the base of the proboscis. There is no ligula. Extension of the proboscis is brought about both by muscle action on sclerites and increased blood pressure affecting the flexible areas of cuticle. Relaxation results from reduction in blood pressure, and contraction of retractor muscles. The haustellum functions as an organ for taking up liquids. A direct drinking and a lapping attitude are described. The comparative study includes observations on fifty-three species, which are representative of each of the thirteen families found in Britain. All species examined have a protrusible haustellum, and are capable of drinking. The most highly developed condition is seen in the Phryganeidae and Limnephilidae. A channelled haustellum is also found in the Sericostomatidae, Beraeidae, Molannidae, Odontoceridae, Leptoceridae and Polycentropidae. A simple granulose haustellar surface, devoid of channels, is present in the Hydropsychidae, Psychomyidae, Philopotamidae, Rhyacophilidae and Hydroptilidae. The mandibles are of doubtful function. They are largest in the Hydropsychidae and Rhyacophilidae, and most reduced in Limnephilidae. Small lobes, which are thought to represent the ligula of the labium, are seen in the Philopotamidae, Hydropsychidae, Psychomyidae and Polycentropidae. These differing conditions of the mouth parts are shown to accord with views on the phylogeny of the Trichoptera, which are derived from other data. An account is given of published descriptions of modified mouth parts in some exotic species. The nature of these modifications is discussed. Published observations on the feeding of caddis flies are reviewed. It is concluded that using the haustellum to drink nectar and water is a normal activity of caddis flies.