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The nymphs that are here described belong to those species the biology of which has been discussed in a previous paper (Lal, 1934 b). The plan followed was the one originally adopted by Ferris (1923), in which descriptions and illustrations of all morphological features important in the identification of the species of the nymph, apart from the adult, were prepared from careful examination of specimens mounted for microscopic observation. The importance of such a study has been increasingly appreciated in the last decade, both from the economic and the systematic points of view. Harmful species of Psyllidæ are mainly injurious because of the feeding activities of their immature stages, and there are but two species known, Psyllia pyricola Först. and Paratrioza cockerelli Sulc., in which the adults have also been found to be capable of causing injury. On the other hand, the specific determination of immature stages has been rendered difficult owing to the fact (first) that the published descriptions of individual species of nymphs are often found to be applicable to species other than those to which the particular description refers; and (second) that sometimes the nymphs of two closely allied species are distinguished by such marked differences as to warrant the adults being assigned to different genera and even subfamilies. Such cases have been mentioned by Crawford (1919), Ferris (1928 a), Husain and Nath (1927), and have been summarised by Rahman (1932) in the introductory part of his paper. The present descriptions have, therefore, been prepared with the avowed objects of facilitating the determination of individual species, where comparison with allied species is impossible owing to lack of available material, and also of assessing the value of the various nymphal characters with a view to establishing a classification based on these as contrasted with one based upon the characters of the adults.

The insect parasites bred out of Psyllid hosts, in the course of studying their biology in Scotland, belong to four families of Hymenoptera and one of Diptera, the total number of species encountered being seven. Of these one and possibly two more of the hymenopterous species are hyperparasites. The hosts involved include four species, of which only one, Psyllia mali race peregrina, may be said to have been fairly heavily parasitised. As observed by Ferrière (1926), species of Psyllidae, in spite of their nymphs leading a comparatively sedentary life, are less susceptible to attack by parasites than are the other Homoptera. As a rule the hymenopterous parasites attack them in the nymphal stage, while the dipterous species parasites the adults as well.