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The aim of this work was to study the distribution of the Simuliidae of Scotland, and to investigate the ecological characters of Scottish species more extensively than had been attempted by former workers. Credit for having been the first to look into the problem of the distribution and ecology of black-flies in Britain is assigned to Edwards (1915, 1920), and Smart (1936) in a paper on the Simuliidae of Fortingal, Perthshire, furnished a foundation for the distribution of the species in that locality. It was realised, however, that as certain important details were omitted in those papers, further study was desirable. More recently in England the important question of the effect of the chemical composition of stream water on the immature stages of Simulium was considered by Pentelow (1935, 1937) in a survey of the non-tidal reaches of the river Tees.

The morphology of the rays of the primary fans, the main food collecting organ, of 25 species of black-fly larvae (Diptera: Slmuliidae) were studied using the scanning electron microscope. The form, arrangement, spacing and dimensions of the microtrichia on the rays of these species was recorded. The flicking action of the primary fan was found to be frequent and irregular. The actions of the primary fan and mandible were also studied and their mode of action outlined. Experiments on black-fly larvae of four species (Simulium omatim Mg., S. variegatum Mg., S. monticola Fried, and S. reptans (L. ) ) were conducted in an artificial environment in which current velocity, food concentration and light level could be controlled. Each species was found to differ from the others in its rate of food intake over a range of current velocities. Species with similar patterns of miorotrichia on their head fans had comparable rates of food Intake at the same current velocities. Experiments were done, using a dye-tracer technique, on S. omatum, S. variegatum, S. reptans, S. venustum and S. pictipes- longistylatim in the natural stream habitat of these larvae to determine the rate of intake of natural food. Intake rates varied widely from species to species and within a species depending on current velocity and probably the amount of suspended matter in the stream Larvae of P. ferruglneum (Wahlb.) from Eastern Norway were found to predate on other aquatic arthropods including other species of black-fly larvae. Experiments were conducted on the space requirements of S. omatum larvae in an artificial environment. Densities of up to l4l larvae per sq. cm. were obtained. These densities were much higher than those found in nature. Densities in nature varied widely. Analysis was made of the movements of larvae on an attachment site of limited area.

An investigation has been made of the role of the host plant in reproductive behaviour of the adult female Frit fly Oscinella frit L. Individual variations in oviposition of females kept in the presence of the oat plant are described and classified. Ovary maturation in virgin females in the presence and absence of the host plant is also described and related to normal reproduction. Daily changes in weight from emergence are interpreted in terms of food ingested, ovary development, and ovary development, and oviposition. The effect of the host plant are interpreted in terms of the current theories of humora control of reproduction.

The thesis makes a distinct contribution to knowledge of both the systematics and the ecology of the species of psychodidae associated with aquatic habitats. The systematic work (Section I) was carried out as a necessary precursor to ecological investigation, aiming at the precise identification of every individual encountered. In this section there are descriptions of new species, and of previously unknown larvae and pupae; revised descriptions of other species, and some information on geographical variation. The problem or species pairs in the Family psychodidae is discussed at length. It is shown that Pericomu trivialize and P.nubila are true species, and that there are no grounds for considering these and the members of other species pairs as cases of male dimorphism, or any other than long established species. In section II an account is given of the ecology of the larvae and adults of species described in the first half of the thesis, considered from a number of new aspects. Several congeneric species normally occur together in the same breeding area, and although there is often a degree of spatial isolation between the larvae of these species, there are examples in which the larvae appear to live together in exactly the same microhabitat. Species of Pericoma and Telmatoscopus are all either univoltine, bivoltine or trivoltine, and there is a well defined seasonal succession of species. Larval adaptation is considered, with particular emphasis on the structure of the mouthparts. It is shown that the same adaptive features often arise independently in a number of separate evolutionary lines. Information is given on the adaptive significance of many structural characteristics, including reduction of setation, and the form of the mandible, labrum, hypostome and other mouthparts.

The investigations described in the first part of this thesis were carried on the biology of blood-sucking midges (Culicoides Latreille - Diptera: CERATOPOGONIDAE = HELEIDAE) in Scotland. Midges, at times a source of annoyance in almost all parts of Britain, are, as is well known, particularly troublesome in Scotland, especially in the Highlands. Here, during the summer months, they are a severe source of irritation to both local inhabitants and tourists. Domestic animals are also attacked and may be adversely affected; cattle subjected to heavy attack, for example, are said to suffer a reduced milk yield. In consequence of its great importance, two research assistants were, in 1946, appointed in the University of Glasgow to investigate the biology of Culicoides in Scotland. The author held one of these appointments until 1948, when he left this field of investigation. New facts have since come to light, and such of these as are relevant will be commented on in the discussions following the sections in which the author's own results are presented. In Part IA are presented the results of an investigation into the habits of adult Culicoides in the field, the investigation being carried out mainly at the University of Glasgow Insect Field Station, situated at Rossdhu, Loch Lomond, The main object was to study variations in incidence occurring at different times of day and year. Subsidiary investigations were the effect of weather conditions on adult activity, and the relative readiness of different species to attack man. Part IB is a description of laboratory investigations primarily aimed at obtaining the eggs of as many species of Culicoides as possible, observing them under normal conditions, and, assessing their resistance to adverse conditions of drought and heat. Data on the maintenance of adults in the laboratory for egg-laying purposes are also presented. Part II is an account of an investigation undertaken while the author was Research Entomologist at the West African Institute for Trypanosomiasis Research, a post which he assumed in 1950. The principal vectors of trypanosomiasis in Africa are, of course, tsetse flies (Glossina Wiedemann - Diptera: MUSCIDAE ). Although numerous field observations on the breeding sites of these insects had been recorded, practically nothing was known of the stimuli to which the pregnant female responded in choosing these sites. The investigation described was principally an attempt to determine the nature of these stimuli by laboratory experiments, supplemented, where necessary, by field observations. The species investigated was G. palpalis Robineau-Desvoidy, an important carrier of human sleeping sickness; this species was the one most readily available in the neighbourhood of Kaduna, Northern Nigeria.

The literature relevant to mosquito genetics is first reviewed. In the present work, 51 different strains of Aedes aegypti have been examined for colour variation in particular. Methods of rearing, handling and routine are briefly described. Colour of the dorsal abdomen was so variable, that a scheme was devised to classify it, according to paleness, in 37 grades and sub- rades. These could be assiened to 15 numerical colour values to enable the paleness of a population to be quantitatively defined. Photography of variants was adopted as a routine. In Linkage Group I, two partially sex-linked factors controlling eye colour and one controlling abdominal colour were isolated, and 4 linkage distanoes determined. This is the first example of partial sex-linkage in A. aegypti and the first three-point, linkage estimation in any mosquito. In Linkage Group II one new mutant control ing thoracic colour was isolated and its recombination with the previously described s locus measured. At least 4 mutant alleles are shown to occur at the s locus sld one is identical with a gene previously ascribed to another locus. Two new mutants were isolated in Lirucage Group III and the crossover distance of one measured from the previously described blt locus at which a second mutant allele was isolated. A further three potentially useful mutants were obtained and other variation mentioned. The variation in abdominal colour in 39 strains is described and discussed. The frequency of genotypes in populations polyworphic for an s allele suggests,though not significantly, some degree of heterosis. Hybrids were successfully obtained between three pairs of Stego yia species. The relationship between A. aegypti and A. mascarensis, one of the two crosses giving fertile hybrids, is discussed more fully. A number of gynandromorphs and intersexes is also recorded.