Arising out of a routine mosquito survey, carried out in a coastal district of the Gambia in 1946, it was discovered that specimens of typical A. gambiae showed a variation in maxlllary indices, suggesting the possible differentiation of the species into two races distinct from the known variety “ melas ”.

Material from catching stations, considered beyond flight range from possible breeding places of “ melas ”, was subjected to statistical analysis as were also collections of adults emerging from different categories of fresh-water breeding places.

The analysis showed that the maxillary indices of the samples obtained from control house catches were distributed in an abnormal manner and were therefore unlikely to have been drawn from the same population. At the same time, the samples collected by window-traps and spray-catches were found to differ significantly in their means, and this supported the view that a mixed population was being sampled. The samples obtained by trapping emerging adults from different categories of breeding places, where any adulteration by “ melas “ was a physical impossibility, showed significantly different mean maxillary indices. It was therefore very unlikely that they could have been drawn, by random sampling, from a pure population. When the samples, obtained by trapping over-breeding places, were divided according to their mean indices, it was found that those falling into the group with a mean index of 13·107 were taken from collections of water which could be classified as casual and those falling into the group with a mean index of 15·33 were collected from permanent pools.

Preliminary research on the micro-flora of fresh-water pools has shown that certain distinct phases, marked by dominant plants, can be distinguished. The distinction between casual water and permanent water is capable of more accurate definition by reference to the dominant species. It has been observed that, within the limits of statistical differentiation, the race having the larger mean maxillary index is never found in water where photosynthesis, and hence chlorophyll, is absent whereas the race with the smaller mean maxillary index is never found in water where photosynthesis has been established amongst the micro-flora.

The apparent difference in behaviour between the two races, as shown by the predominance of the 13·107 index group in the window-traps, and the predominance of the 15·33 index group in the spray-catches, cannot it is thought, be accepted as evidence unless the existence of a fatigue factor, accounting for house-resting habits in members of the latter group which have travelled far and exhausted their energy reserves, can be excluded.

It is considered that the results of the work described in this paper may be of value in permitting control house catches to be analysed. From such analysis it should be possible to assess the proportion of adults which had originated in casual water and the proportion coming from more permanent breeding places. If, of course, it should be proved that the apparent differences in adult behaviour are, in reality, linked with differences in mean maxillary indices, it will be necessary to take this into consideration in assessing the value of residual insecticides and mass spraying. It is further argued that, even if the differences in behaviour can be explained on the fatigue factor hypothesis, it will still be necessary to take them into account in attempting to assess the value of mass spraying technique.

It is considered that the method of statistical analysis could be more widely utilised in exploring many problems arising in the field, and that other variations, apart from those of maxillary dentition, might prove even more fruitful as a basis for analysis, although they would call for more time and labour than can usually be spared.