In aquatic insects, emergence (ecdysis to the adult or subimaginal stage) varies widely in temporal pattern. The comparative study of this pattern is feasible and informative in orders such as Plecoptera, Ephemeroptera, Odonata, Diptera and Trichoptera in which all members of a population pass through the water-surface when emerging. Methods by which emergence rate can be measured are discussed. Four basic temporal patterns of emergence exist. Emergence may be (1) continuous with irregular fluctuations in rate; (2) rhythmic, with a lunar period; (3) sporadic, occurring at irregular intervals of a few days; or (4) seasonal. Examples of each of these patterns are given, and reference is made to the proximate and ultimate environmental factors which may be maintaining the patterns observed. Diurnal rhythms of emergence are excluded from consideration. When emergence is restricted seasonally in temperate latitudes, the degree of its synchronization within the emergence period varies widely but is usually constant and typical for a given species. This has provided the basis for an ecological classification of British Odonata, the validity of which is examined in the light of recent research.

The first part of this paper discusses the characteristics of the arctic environment as it influences insect life. The most important features are the low temperature, the low annual heat budget, the severe and often variable weather and the continuous daylight of the growing season. A certain uniformity in aspect of the tundra environment is mentioned. Some microclimatic and regional differences are considered and a definition of the low arctic life zone is proposed.

The make-up of the arctic fauna is considered in the second section. In the low arctic there may be up to 5% of the number of species found in a comparable temperate area, and in the Queen Elizabeth Islands about 1%; but the representation of the various groups of insects in the arctic varies greatly, the Diptera, and especially the Chironomidae, being by far the most numerous. The fauna is derived from that of the north temperate zones, by extensive reduction and by a limited development of endemic forms up to, but rarely beyond, the specific level. Many of the endemic forms seem to be adaptations to the special features of the environment, especially the low temperature and short season; while others appear to show a loss of specialized features, e.g. of colour or pattern, in relation to the small number of species in their environment and the consequent lower level of biotic interactions. The arctic Lepidoptera are frequently cited in these sections and a number of species are figured.

The paper concludes with a discussion of the general reasons why so few species are found in the arctic. The arctic environment is not inherently simple and does not forbid a greater diversity, yet in fact very few species occur. It seems to lie beyond the range of physiological tolerance of all but a very few of the forms of temperate origin. But, given time, there is no apparent reason why a greater diversity should not develop, although the evolutionary process is probably slower in the arctic and is always liable to be set back by climatic disasters, from the last of which the fauna has only just begun to recover. It is suggested however that the arctic will always tend to lie towards the limit of the physiologically possible range because in fact the greatest continuity of evolutionary history has been in the tropics and most forms of life have thus been shaped in response to tropical conditions.

The underlying causes of grasshopper outbreaks in Western Canada are of interest both as an academic problem in insect population dynamics and as a practical problem in modern agriculture. Grasshopper populations reach their annual low point in early July when they are in the adult stage and increase sharply in the fall at the time of oviposition. Any factor that influences the grasshoppers during this period may have a marked effect on the outbreak of the following year. The relationship between grasshopper population numbers and proceeding fall temperatures may either control grasshopper populations directly by affecting their physiology or indirectly by affecting their parasites, predators, and diseases. Studies at the University of Saskatchewan aimed at solving this problem include investigation of the microclimate to which grasshoppers are subjected under field conditions, a survey of their feeding habits as shown by crop analyses, and a study of their oviposition behaviour. On the basis of available information an attempt is being made to predict future outbreaks at least 6 months in advance.

The functional morphology of the insect compound eye is reviewed with special reference to its surface and volume relationships with the rest of the head and its evolutionary development. Measurements of the more important parameters of the eyes of 28 species representing 14 major orders are given and interpreted in relation to this review. Recent histological and biophysical work on insect vision is also reviewed and some conclusions, especially those concerning the limit of sensitivity in the ultra-violet, are shown to be consistent with current theories of the early history of the oceans, the atmosphere, and of life.

Population ecology requires realistic and precise analyses of whole systems, or processes, and not just fragments of them. This poses some difficult problems because of the distinctive complexity of these processes. Recent studies of predation have shown, however, that it is possible to achieve great analytical depth and to simulate whole systems in the form of realistic and precise mathematical models. This is accomplished by ignoring the degree of simplicity traditionally required of population models and by emphasizing the need for reality. Extensive experimentation is required to suggest and test possible explanations for the action of each component of the process so that the explanation evolves in gradual steps to include one component after another. The form of the explanation and the resulting equations is hence dictated by the process itself and not by the need for mathematical neatness. The considerable complexity of the predation model arose from features common to many biological processes i.e. the prevalence of limits and thresholds, the presence of important discontinuities and the historical character of biological events. These features can be analyzed effectively only by establishing an intimate feed-back between experiment and theory. Mathematical models incorporating these features are admirably solved using digital computers. Computers, and the languages used to program them, seem to be ideally suited to handle the distinctive type of complexity shown by population processes.

A knowledge of ecological principles is essential to the successful development of future research on pest control problems in Canadian orchard entomology. Such knowledge should be especially helpful in spotlighting omissions in past studies as well as indicating new areas of study in the future. In particular, the ecosystem concept should serve not only as a basis for evaluating and bringing together existing data but also as a guide in the collection, integration and interpretation of new data.

Our approach should be to study the population dynamics of major arthropod pests in Canadian apple orchards to obtain data of the fundamental kind on natural populations of these species. Since this approach is dependent on precise measurement of the population and its mortality factors, as well as on the mathematical modelling of the data obtained, unambiguous deduction and greater understanding and utilization of the results would then be possible.

Examples from a recent eight-year study of the population dynamics of two pest species in Canadian apple orchards reveal that the fundamental approach to pest problems in Canada is both feasible and practical, and that the results can yield considerable insight into the role of mortality factors in population regulation as well as in control of pest stages of economic importance.

Three different types of historical population data on the fall webworm, Hyphantria cunea Drury, are examined graphically. Oscillations in population density have peaks at intervals varying from 8 to 16 years, and are generally synchronous over very large areas in eastern and central Canada. Temperature in late summer is a key factor affecting the rate of change in density, particularly during the development of gradations, and this explains the synchronism of the oscillations. Climate and vegetation appear to be the main factors determining differences in mean density from one area to another. However, oscillations about the mean density have discrete limits, which are usually below the limits imposed by the food supply even where climate is generally favorable, and population decline often occurs despite favorable weather. These limits are imposed by other factors, such as the parasite Campoplex validus Cress. which has a delayed response to webworm density in eastern Canada.

It is concluded that similar analyses for other insects, perhaps based on the sort of data being accumulated each year by the Forest Insect Survey, would contribute to the development of population principles and would suggest many useful short-cuts when more intensive work is undertaken on additional species.

Among the purposes of classification are association and relocation of data, expression of degrees of similarity, prediction of unobserved data, and expression of functional relationships. Classification must conform to the characteristics of the organism classified and also to the thought processes of the classifier. Analysis reveals hidden patterns and acts as a check on intuition, which though valuable is of uneven reliability.

The Linnaean system is well adapted to modern needs, though we have had to modify its theoretical basis. Modern information may force us to a fundamentally different system at levels at and below that of the Linnaean species.The data of conventional taxonomy are voluminous, but the obvious need for using more characters greatly increases the problem of recording and processing information. All characters, morphological or otherwise, of a species have potential taxonomic value. The latter are often less convenient but may yield special information.

Functional relationships must be distinguished from non-morphological characters. There is a problem of harmonizing classifications based on functional relationships with those based on similarity and difference of attributes. This problem takes different forms at the species level and at the level of higher categories.

The strategy of systematic investigation is influenced by the purpose. For identification a simple array of characters, in a dichotomous table, may be adequate. Such tables are well adapted to digital-computer operations. Where large arrays of characters are to be studied, as where functional relationships, variation or capabilities of organisms are important, time becomes critical, and priority should be given to characters that can be observed quickly or that are known to be specially informative. Methods of multivariate analysis are being applied increasingly in systematics. Methods using unweighted arrays of homologous characters are likely to be supplanted by those that permit controlled weighting, use of nonhomologous characters, and incorporation of sequential and functional relationships. Electronic computers will permit study of problems that would formerly have been too large or complicated. Trained observers will have to provide data, but special types of information, such as chromatograms, electrophoretic and serological observations, and biogeographic data, might be read automatically into a computer. With increased complexity and automation of computer operation, sound initial planning and checking will become more important. The whole subject of systematist-computer relationships needs experience, careful thought, and perhaps a reorientation of attitudes.

Systematics is an approach to biology rather than a department of it; it is intimately related to evolutionary genetic studies, and is in this generation uniting with them in a general science of population biology. Perhaps in the next generation population biology and individual biology will be united in an integrated body of biological science.

After defining the entomological use of the term mycetome a short general account is given of the various supposedly beneficial relationships between insects and internally harboured micro-organisms and the metabolic balance needed. Not all beneficial micro-organisms are harboured in mycetomes or even in mycetocytes, but the present paper is concerned only with insects having mycetomes. It is shown that there is good evidence for accepting the premise that true micro-organisms are found in insect mycetomes and that the micro-organisms supply the insect host with some useful metabolite (s), Special consideration is given to the genus Sitophilus, Here the mycetomal micro-organisms probably supply a useful nutrient that is "essential" only when the diet available to the weevils is inadequate. Whole wheat grain appears to be an adequate diet. Some strains of weevils apparently free of mycetomal micro-organisms are pale in colour, light in weight and particularly susceptible to dietary changes. Weevils reared at temperatures too much above the optimum lose their mycetomal micro-organisms. Although the organisms are pleomorphic there is evidence that their morphology in any one strain of weevil, reared under standard conditions, is constant: and thus it seems the micro-organisms may have taxonomic value in entomology. The possible origin of mycetomes and of the mutualistic associations are discussed and suggestions made about future research.

In the embryo, as in the postembryonic insect, morphological development is accompanied by an increase or change in enzyme activity. The appearance of certain enzymatically active proteins in a particular tissue or organ may indicate when that tissue or organ has become functional. The time and place of appearance of several types of esterase has been determined in the tissues and organs of the developing embryo of the large milkweed bug, Oncopeltus fasciatus (Dall.). The appearance of acetylcholinesterase in the neuropile of the nervous system reflects the morphological development of this system; it may also determine the onset of functional activity. Similarly, the appearance of non-specific esterase in many tissues and organs at a definite time during their morphological development suggests a relationship between their functional differentiation and the appearance of the enzyme. Evidence for such a relationship will remain circumstantial until the physiological significance of these esterases is understood.

A brief account of the history of insect cold-hardiness studies shows that interest has been sparse and sporadic, and advancement contributed by a very small number of people. Three distinct aspects of cold-hardiness research are used as examples to show their basic dependence on physico-chemical principles and consequently the preferred direction for future research.

An attempt is made to re-evaluate the data on the origin of the ovipositor in insects and to explain its mode of development in living forms. Comparative developmental data from other groups of animals is cited to substantiate the claim that part of the insect ectodermal genitalia is appendicular rather than sternal in origin. It is suggested that the primary abdominal segmental appendages have provided a source of competent tissue which through subtle changes in selection, has evolved along many pathways, to form the gonocoxae, the pleuropodia, the pseudoplacenta and perhaps the prolegs in many different taxa.

It is shown, by aid of sections through the insect embryo and larval stages, that the primary embryonic segmental appendages on the abdomen, do not differentiate; there is no loss of tissue and it cannot be proven that such appendages have been lost in insect phylogeny. The fact that they are represented still in the modern embryo, indicates that they have been retained. To explain the observable developmental details, it is suggested that abdominal limb histogenesis is arrested or suppressed in normal development, but this limb tissue retains its competence to differentiate. Thus development may be initiated again at a later time in postembryonic life. In this manner, the original limb tissue is available for organ formation in the maturing insect.

The study has suggested that the appendages on the eighth and ninth segments of the abdomen initiate but do not complete their development in the polypod embryo. Possibly the potential limb tissue is arrested in development because it has not undergone some vital change as regards its capacity to respond (competence) to an inductor, perhaps the inductor is not available or perhaps it is not available in the correct form.

There is evidence to suggest that the developmental capacity of the limb anlagen are reduced with time, so that full limb formation is not possible in postembryonic life: this can explain the development of abdominal coxae in the Thysanura and hence gonocoxae in higher insects. It is noted that should Gustafson's suggestion that the eversible sacs and gonapophyses are homologous with primary segmental genitalic ampullae prove acceptable, then the female ectodermal genitalia in insects would appear to have a dual origin.

It is emphasized that the speculation expressed are being subjected to experimental study in an attempt to verify the suggested ontogeny and phylogeny.

Environmental instabilities may be grouped into three broad categories each with different conseqences for the organism which has to survive the stress of these instabilities. On one side are irreversible changes of the environment which will lead to complete adaptive compliance or conformity of the population, brought about by natural selection. On the other side are short-term recurrent instabilities, fluctuations, or oscillations; if their cycle is short enough so that all phases are experienced by all individuals of each generation, natural selection will promote the ability of each individual to withstand the whole range of environmental recurrent fluctuations. Between these extremes are recurrent instabilities that are not experienced by all individuals, or by each generation; here natural selection will evolve mechanisms that prevent the population from conforming with any temporary selection pressure. Polymorphism is such a mechanism, specifically polymorphism based on a gene potency balance system modelled on the same principles as the system that determines sex, the most common example of polymorphism. Instances of such polymorphism in a genus of tortricid moths include haemolymph pigment, adult wing colour, and rate of larval development. The latter exemplifies polymorphism of a quantitative character.

Apart from some heretical remarks about the modern scientific attitude, several areas of common interest to the entomologist and geneticist are discussed. They include immunological, electronmicroscopical, cytogenetical and behavioural studies.

Inactive moths of Malacosoma pluviale (Dyar) oviposit near their birthplaces, and most of their offspring also are inactive. More active moths can travel farther before they oviposit, and always have a higher proportion of vigorous individuals among their progeny.

Such polymorphism allows the insect to cope with environmental diversity; e.g., inactive residents exploit favourable habitats, and active migrants colonize more severe habitats, or replenish the vigour of other populations.

Because the most active moths usually export the most vigorous progeny, the population left behind becomes less vigorous during successive generations. This steadily decreasing vitality eliminates local populations that are not replenished by vigorous immigrants.

Qualitative changes in Malacosoma populations follow this basic pattern, but the rate of deterioration is affected by the habitat. Departing migrants fly too high to be stopped by small trees in farmland, bur many are stopped near their source by tall trees in forests. Deterioration therefore is slower in forests. Forests also delay return migration to nearby farmlands, and thus allow some farmland populations to deteriorate unchecked.

In a fluctuating climate, the size of the area tolerable for the species varies annually. When it begins to expand, the vigorous progeny of active moths can take immediate advantage of slight local improvements. Consequently, they are the first to exploit each marginal habitat that becomes tolerable. But while better climate persists, some less active descendants of these pioneers appear in all occupied habitats.

When the regional climate deteriorates, the tolerable area contracts, and most marginal populations are totally destroyed. Moreover, even within the contracted tolerable area, the harsher climate becomes intolerable for any deteriorated stock. In the next generation, therefore, the only regional survivors are vigorous colonies deposited in the tolerable area by some of the few migrants that escaped the widespread destruction of the preceding generation in the margins. Their descendants recolonize depopulated sections of the refuge, and so preserve the species in the region while the climate remains severe.