Weekly observations on two populations of Culicoides impunctatus Goet. were made in 1948 at Bannachra and Wester Bannachra, near Loch Lomond, throughout the whole of the C. impunctatus season by means of “ sticky ” traps. Each site had an area of 5 acres of which half was composed of woodland and half of moorland. Twenty-two catching stations, each consisting of three traps placed 2 ft., 6 ft., and 10 ft. above the ground were erected on each site, equal numbers being placed in the woodland and moorland. There were therefore 132 traps to be changed weekly and a total of 67,033 Culicoides were trapped of which 65,576 (97·9 per cent.) were C. impunctatus. In 1949 observations were repeated only in Bannachra woodland.

The results have been analysed in the light of two alternatives (1) that C. impunctatus is a single biological entity and (2) that C. impunctatus is composed of two biological races.

When the seasonal effect was eliminated the stations showed very real differences between their weekly catches. Some stations recorded a large catch every week whilst others always recorded low catches. The total seasonal catch at each station was plotted on plans of the two sites and it then became apparent that there were two centres of high C. impunctatus density at Bannachra, one in the woodland and the other on the moorland, whilst at Wester Bannachra there were two foci in the woodland and a very diffuse zone of high midge density on the moorland. It has been shown that the centre of high midge density in Bannachra woodland was associated with a concentrated breeding place which supplied all the adults for the woodland (2½ acres) and moreover this centre remained unaltered in 1949. The importance of these observations on the control of C. impunctatus is discussed.

After the breeding place in Bannachra woods was discovered it became clear that the majority of adult C. impunctatus moved in an eastward direction from the breeding place and very few went westwards. This was attributed to the influence of the prevailing SW. wind. Knowing the centre of breeding in Bannachra woods it was possible to calculate the regression of the midge density on the distance from the breeding site. It was found that there was a linear relationship between the logarithm of the catch and the distance from the breeding site. A useful statistic is the reciprocal of the regression which may be designated as the density coefficient. It is the distance at which the density has decreased to l/10th of its initial value. The density coefficients for male and female C. impunctatus are 62·5 yards and 64·9 yards respectively. An attempt has been made to separate the effects of “ dilution ” and “ mortality ” on the density of C. impunctatus.

The mortality of male and female C. impunctatus is shown to be normally distributed with respect to the logarithm of the distance flown. These distributions are male l·867±0·197 (=73·6 yds. ; 46·8–115·9 yds.) and female 1 ·885±0·197 (=76·7 yds. ; 48·8–120·8 yds.).

From the density/distance formula the average distances flown by male and female C. impunctatus have been calculated. They are 79·0 yards (males) and 81·4 yards (females).

The analysis of the vertical distribution of C, impunctatus revealed that each station had its own definite pattern. The stations sited amidst the birch saplings showed very real differences between themselves which were not related to the ground flora. It is suggested that these are related to the height of the leaf canopy. Evidence was also produced which pointed to the fact that in the oak wood where the base of the leaf canopy was about 20 ft. from the ground, C. impunctatus had a self-imposed “ ceiling ” of little more than 10 ft.

The ground flora was found to have an appreciable effect upon the vertical distribution of C. impunctatus on the moorland. Thus where the ground flora provided only very shallow shelter, i.e., about 6 ins. in depth, equal numbers of adults were captured at each height, but when the stations were sited amongst Juncus sp. or in small sheltered clearings in the herb layer then more adults were caught on the 2-ft. trap. However, the shelter provided by bracken Pteridium aquilinum appeared to be less attractive and there were fewer adults caught on the 2-ft. traps at stations sited in dense bracken than at other moorland stations.

In the woodland the females were slightly more abundant on the 2-ft. trap and the males more numerous on the 10-ft. trap but the differences in distribution, although significant, were small. On the moorland the males were more abundant at 2 ft. and the females at 6 ft.

Although the general pattern was very similar there were highly significant differences between the horizontal and vertical distributions of the two races of C. impunctatus.