The present status of our knowledge of the sub-species of anopheles maculipennis

Abstract The six varieties of A. maculipennis, as identified by their eggs, are each to be found in a number of different areas which are widely separated geographically. Within its own climatic range, the presence of a given sub-species is determined by the orohydrographical conditions of the area, or more particularly, the character of the surface water which the insect requires for oviposition. No observations as yet suggest that an environment or micro-environment suited to the adult of either sex can determine the presence or prevalence of any subspecies. It has not been shown that individuals laying the same kind of egg vary significantly in physiology or behaviour, even if they come from different regions. Our surveys of new zones proceed, therefore, on the assumption that the egg type will serve to identify the sub-species, which is broadly homogeneous throughout its range, in spite of adaptations to local conditions. Such modifications are apt to be quantitative (as in duration of sexual repose in winter, or range of micro-climatic tolerance) rather than qualitative and genotypic. The biological differences between sub-species are such as apparently to preclude hybridization in nature and while cross-mating has occurred in the laboratory, successive hybrid generations have never been obtained. The sub-species differ also in the frequency and regularity with which they bite man under the present conditions of rural life, and since this determines the amount of malaria transmission, some races are more consistent vectors than others. Thus, maculzpennis (typicus) and melanoon have little contact with man, labranchiae and elutus are continually penetrating into houses in relatively large numbers wherever these races occur, while messeae and atroparvus are easily deviated by domestic animals but overflow into human habitations in search of food under various circumstances, among which are a disproportionate density of anophelines, a scarcity of animals, or the stimulus of certain physical conditions, such as temperature, humidity, etc. Under identical conditions the various sub-species behave quite differently, being drawn by diverse instincts into different environments and to different hosts. This in general explains their separate roles in the spread of malaria, but whether a given sub-species becomes a vector or not in a given locality depends also on a complex of local conditions, which may in one place lead it into continual contact with man, or in another restrict it to animals to such a degree as to render malaria transmission highly improbable. The range of adaptation of labranchiae and elutus is so wide, however, that there is no region in which either is known to be effectively deviated by domestic animals.