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.

In the course of a search for criteria which would assist in deciding whether a particular series of numbers was randomly arranged, it was found necessary to investigate some of the more obvious characters of a random series of numbers. If the series is an infinite one and all the numbers are unequal (the series of decimals between o and 1 selected in some random fashion may be considered) certain of the numbers will be maximal, that is greater than their immediate neighbours, whilst others will be minimal, that is less than their immediate neighbours. Every maximal number is succeeded by a run down, that is by a series of descending numbers ending in the succeeding minimal number. The length of a run down is the number of numbers it contains, including the maximal number which begins it and the minimal number which ends it. The length of a run up is similarly defined. Clearly the minimum length of a run is 2.

I have read the observations of Dr R. A. Houstoun at the top of p. 164 of the current volume of these Proceedings, and I gather the impression that he has been dissuaded by general beliefs of what cannot be done from using the simplest of all possible methods. Forty-five years ago when I required a very high speed of rotation I completely ignored or defied these general beliefs with the greatest success, and it seems to me to be possible that a few words on my practice might be useful. If this is the case I should feel that as a newly appointed Hon. Fellow of the Royal Society of Edinburgh I had at least done what I could in recognition of that honour.

The land nemerteans of the world comprise only twelve species and at present all are referred to the genus Geonemertes Semper. Most of the species occur in the Australia, New Zealand, and Malaysian regions, with a single species in Bermuda. European occurrences are rare and apparently due to introductions by man. G. chalicophora Graff, for example, is unknown outside glass-houses in Germany, Austria, Czechoslovakia (Stammer, 1934), and the Botanic Gardens, Dublin, Ireland (Southern, 1911). One other species, G. dendyi Dakin, which was originally described from a single female discovered in “a valley in the Darling Range not far from Perth,” West Australia (Dakin, 1915), was later found in glass-houses in the Botanic Gardens at Breslau, Germany (Stammer, 1934). More recently this worm has been found by us to be breeding in the open in wild situations around Swansea, Wales. We believe this occurrence to be the first record of a land nemertean established in the temperate climate of Europe.

It is generally accepted that the function of the mutant genes which affect eye colour in Drosophila is to produce either quantitative or qualitative changes in the normal development of eye pigment. Wright (1932) believes that these genes act by interfering with some part of a chain of reactions which give rise to eye-colour characteristic of the wild type. Mainx (1935) suggested that recessive eye-colour genes (with the exception of sepia) reduce the total amount of pigment, and that the degree of reduction is characteristic for each gene. Transplantation experiments of Beadle and Ephrussi (1935, 1936, 1937 a and b) have led them to the view that certain eye-colour mutants lack specific substances which are necessary for the development of additional pigment present in wild type eye colour. Johannson (1924) and Schultz (1935) studied the histology of the adult eye of D. melanogaster and classified the mutant eye colours according to the distribution of granules in their pigment cells. Schultz (1932) had previously noted the time of pigment deposition and subsequent changes in wild type and some mutants of D. melanogaster. In an earlier paper the author (1936) described in detail similar colour changes in the eyes of developing pupæ of D. pseudo-obscura. Previous work in this laboratory (1932, 1934, 1935) by Crew and Lamy make it appear that each mutant eye colour is the expression of a gene which acts to suppress normal development during a certain period of time, allowing the rest of the development to proceed as in wild type.