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Experiments, in which alate aphids were marked immediately after eclosion from the last nymphal instar and observed until they took flight, gave information which, together with the measured rate of moulting and a continuous record of temperature, has been used to construct theoretical daily curves for numbers flying from a bean crop. These curves have been matched against the observed curves for the same days.The differences between curves so constructed for a period of 17 days and the observed curves for the same period are of the same order and kind as the differences between two measured curves in different sites at the same time.The different types of daily flight curve can thus be accounted for in terms of the rate of production of alatae by moulting, the effect of temperature on the teneral ('drying out') period and the effect of low light intensity and temperature on takeoff behaviour.

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In a natural population of alienicolae of Aphis fabae Scop. on field beans in July 1952, the frequency of moulting of the last larval instar into the alate form was usually relatively high between about 04.00 and 08.00 hr. G.M.T. This high rate was followed by a decline to a lower fluctuating rate for the rest of the day. T h e moulting rate at night was usually very low.The high rate of moulting in the morning, just as the temperature is rising, may be due to acceleration of nymphal development which leads to a synchronization of moulting, much as teneral development of alatae, if accelerated, producesflight peaks during the day (Johnson, Taylor & Haine 1957): data on the relations of moulting and nymphal development to temperature are, however, insufficient for making a complete analysis along these lines. Changes in moulting rate during the rest of the day are correlated with temperature and with time, both independently. A rise of I " C. ambient temperature is associated vith an I I .6 0" rise in moulting rate : a fall of I ' C. with a drop of 10.5 $& and a lapse of I hr. in time by a 5.4 "/o drop in the rate. Sunshine and humidity show no simple correlation with moulting rate.The migratory flight of alienicolae of Aphis fabae away from the beans on which they have bred shows a diurnal periodicity which is often bimodal. The alatae in this flight are produced some hours before by the moulting of the last larval instar and this moulting rate is also periodic (Johnson, 1954). An analysis of the association between moulting and flight was therefore undertaken and data on flight and on moulting rates during the day were collected. In dealing with the whole problem it is more convenient and less confusing if the data on moulting, and the methods used to obtain them, are treated separately. This has been done in the present paper; the relationship between the moulting and flight rhythms is the subject of a companion paper (Johnson, Taylor & Haine, '957). METHODS Counting the numbers moultin#Observations were limited to the moulting of the last instar of A. fabae into the alate form in a natural infestation of alienicolae on field beans in July 1952. Fortyeight leaves were taken, more or less at random, along the south and west sides of a small plot of beans (40 x30 ft.) grown in a sheltered garden at Rothamsted Lodge.Each leaf was tagged and numbered. As the nymphs on the leaves moulted and