1. The respiratory horn of Psychoda severini Tonn. subsp. parthenogenetica Tonn. is described. Within the tubular horn lies the felt chamber, which meets the horn wall at a number of thin circular regions, the pits, and communicates, through the annulated base of the horn, with the tracheal extension. This has a characteristic retiform thickening and is continuous, at the lower end, with the spiracular chamber, from which a trachea runs to the prothoracic spiracle of the imago.
2. At the upper end of the tracheal extension lies the closing mechanism consisting of a lever, and a muscle surrounded by a cuticular sheath. It is suggested that the mechanism functions by compressing the tracheal extension, and that the sheath serves to protect the muscle from the action of the moulting fluid.
3. The respiratory horns of fourteen of the British species of Psychoda are described, and it is shown that Ps. spreta Tonn., Ps. albipennis Zett. and Ps. trinodulosa Tonn. are like Ps. severini in possessing long lateral tubes in at least the basal half of the horn, whereas Ps. alternata Say, Ps. surcoufi Tonn., Ps. cinerea Banks, Ps. gemina Eaton, Ps. lobata Tonn., Ps. grisescens Tonn. and Ps. setigera Tonn. have only short lateral tubes. The two species Ps. phalaenoides L. subsp. elongata Tonn. and Ps. crassipenis Tonn. are alike in having a two-branched felt chamber, and Ps. brevicornis Tonn. has a flat triangular horn with a three-branched felt chamber and a cluster of air-containing tubes at each corner.
4. An account is given of the development of the respiratory horn of Ps. alternata Say. The horn arises as an outgrowth from the larval prothoracic imaginal disk, and the felt chamber forms by an invagination of its posterior wall. The tracheal extension arises as an invagination of the disk and subsequently acquires a connexion to the felt chamber.
5. The development is compared with that of Chaoborus plumicornis, and it is concluded that Weismann's theory of the origin of the horn from two distinct cell layers is not applicable to Psychoda.
6. The various terms that have been applied to the respiratory horn are discussed, and the conclusion is reached that though none of them is satisfactory, it is premature to devise a new scheme of nomenclature.
Grateful acknowledgements are due to Dr Ll. Lloyd for his help and advice in the course of this work. Some of the material was collected whilst the writer was in receipt of a grant from the Department of Scientific and Industrial Research, to whom acknowledgements are also due.
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