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.