Observations over a period of two years in eight comparable villages, four each in malarious Pattukkottai taluk and in the contiguous non-malarious Tanjore delta, in both of which the malaria vector A. culicifacies is found, indicated that the chief factor explaining the phenomenon of anophelism without malaria in the delta was that of density of vector. No morphological, ecological, or behavioristic differences could be found between the A. culicifacies of the two areas. But standardized per man hour collections in human and animal dwellings, and the yield in uniform traps, indicated that the density of A. culicifacies was three to four times greater in the taluk than in the delta, owing to the much greater prevalence and extent of suitable breeding places for this species in the taluk. Intensive agricultural practice seemed to explain the lessened extent of such breeding places in the delta.
These observations bear out previous assumptions that there must be a critical density of a malaria vector species of anopheline, below which no transmission of the disease will occur.