Summary.1. The economic importance of the Thysanoptera as an order of insects and the necessity for a revision in the classification of the order are pointed out.2. The past confusion between some common and destructive species of the genusThripsis unravelled and the reasons for this confusion are set out.3. The range of variation, within the species, of characters which have in the past been used for the separation of species in the genusThripsis illustrated, this relating especially to measurements of the antennal joints and of setae upon the body. New structural characters of true specific value, especially relating to the chaetotaxy of the abdomen, are illustrated by examples.4. Variation in colour and the shape of the head are referred to.5. Characters of a constant nature for the species and of value in determination are set out, together with some account of the abdominal structure.6. Quick and reliable methods for the mounting of specimens for microscopic examination are given.7. Specific characters, which enable them to be distinguished from one another, and from other species, are given for some common British species of the genusThrips.8. A general account of the biology and a more detailed account of that relating to some common species is given.9. Lines upon which a sounder classification of the Thysanoptera than at present exists are outlined.