Using the inverse square law, estimates can be made of the distances at which illumination from a light-trap is equal to that from background sources. From these distances an index of trap radius can be constructed which can be considered as a measure of trap potential. Between new moon and full moon trap radii vary, depending upon the times of the night at which the trap may be operating, in ratios from about 10:1 to 15:1. A comparable index of light-trap catches can be calculated which allows catches to be examined in relation to changing radius of the trap. Analysis of a series of catches in Uganda and Ghana shows that many species are more abundant than expected in periods of moonlight, particularly at and near full moon, the biggest difference between new moon and full moon being about 10:1 for Marasmia trapezalis (Gn.), whereas the Isoptera, Bostrychidae and Spodoptera triturata (Wlk.) are 3-4 times more frequent at full moon. The ratios between catch at new moon and catch at full moon suggest that the primary determinant of catch is the frequency with which insects cross the boundary of a region of influence whose size is determined by a radius of equal energy. Deductions about the pattern of insect activity through a lunation, and nightly, and the general agreement between curves describing the change in radius of the trap and those of trap catches suggest that changes in catch over a lunation can be explained by changes in the effectiveness of the trap. When corrections are made to allow for such changes, all taxa show some increase in numbers in moonlit periods and in many taxa this increase is substantial. Correction of catches should take account of flight periodicity and this periodicity should, if possible, be confirmed by methods independent of light-traps.

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Emergence and life-span of wheat bulb fly, Leptohylemyia coarctata (Fall.), have been studied by the use of a field-cage-marking technique. Emergence was investigated by observing the numbers of flies emerging daily from an area of infested wheat enclosed by a cage of fine mosquito-netting, and life-span by making a daily census of marked and individually recognisable flies which had been liberated in the cage. Flies were handled only when being marked and in the later part of the work all observations were made without touching either them or the wheat.Flies were chilled to render them comatose for marking and under certain circumstances this and the marking was-harmful. Attempts were made to reduce these harmful effects.Emergence dates varied from year to year depending on the temperatures of spring and early summer, and there were also considerable differences between the emergence dates of populations of adjacent fields in the same year. Consistently, males appeared before females.The ratio of the number of flies seen to the number known to be alive on each day varied according to weather, flies being more difficult to find on windy days than on calm ones and on bright days than on dull ones.The observed life-spans of both sexes varied greatly, up to a maximum of 75 days for females and 55 days for males. An exact statement of mean life-span was not possible because there was a tendency for flies emerging later in the season to be less long-lived than those emerging earlier. Most flies of both sexes lived for over 30 days.The observed life-spans fall short of the true life-spans by amounts that depend on the proportions of living flies seen each day. Two methods are shown by which the mean unrecorded life-span can be calculated.