By R. C. RAINEY Anti-Locust Research Centre, London
Desert locusts, like many birds, commonly make flights over distances of thousands of kilometres; and their flight-behaviour presents a number of major problems both of pure and applied biology. When observations are made on birds or insects in flight, or when their movements and changing distribution are mapped, these data represent the combined effects of what the air is doing to the animals, as well as of what they are themselves doing; and one of the most familiar types of problem in such work is the assessment of the relative contributions of animal behaviour and of air movements to the phenomenon which is actually observed, usually in circumstances in which neither behaviour nor environmental conditions can be recorded in sufficient detail to exclude ambiguity of interpretation. In problems of this general type the consideration of simple theoretical models has often been found to provide a useful approach.
An introductory example is provided by Plate 1, which shows European storks included by chance in part of one of a series of vertical photographs taken during the passage of a locust swarm which the birds were accompanying. Using a technique due to Sayer ( 1956), two successive exposures of sec. were taken at an interval of sec., giving two images of each bird. Measurement of these separate images, and of the distance between them, gives the speed of flight, relative to the ground (ground-speed), in terms of the linear dimensions of the bird; from data on the latter, the actual ground-speed of each bird can be estimated, together with its height, from the size of the image and the focal-length of the lens used (14 cm. on this occasion).
For this purpose, the overall wing-span is the obvious measurement of choice, while ornithologically the best-documented measurement is the wing- length, from the tip to the carpal joint, a point which cannot be satisfactorily distinguished in such a photograph of the fully-extended wings. Wing-length, so defined, is recorded as ranging from 53 to 63 cm. in this species (Witherby et al. 1939), and, in two living specimens measured at the London Zoo,