It is an early experience of the Field Entomologist in the Tropics, whether his work lies in countries such as the East African Protectorates, which have well defined seasons, or where the seasons are less clearly marked, as in the Federated Malay States, that a seasonal prevalence of the commoner insects is quite as definite as it is in the Temperate regions. Outside the Tropics the main factor determining prevalence would seem to be suitability of temperature, but in the Tropics, where temperature variations may be slight, moisture is the controlling factor. Purely terrestrial insects which feed, whether as larvae or imagos wholly on vegetable material, breed and are most abundant at such times as the food-plants are at their best, that is when, as the result of favourable conditions of temperature and moisture, there is a full flow of tissue-building sap. The law of seasonal prevalence is valid also for many insects whose imagos derive their food directly from the larger animals, a source of supply available at all seasons. For these insects, too, the same conditions hold good, but affect chiefly the larvae.