It has been shown by various workers in recent years that the explanation of the phenomenon of anophelism without malaria in Europe depends largely upon the behavior, and subsequently upon the presence or absence, of particular varieties of Anopheles maculipennis, of which five have been recognized.
A further explanation has been offered by Martini and Teubner (1933). They claim that each type of Anopheles may have a preferred type of indoor climate (microclimate) for seeking food and shelter and that a general change in microclimates, resulting from a general change in type of housing for man and domestic stock may, therefore, prove an important factor in freeing a region of malaria.
The writer conducted various experiments in the Struma Valley, Greece, during 1933 to 1934 to test the microclimate preferences of mosquitoes.
By means of two tunnels, one with the entrance situated inside a stable, the other excavated in a clay bank facing due east, it was possible to create various types of microclimate stations. Temperatures and humidities in the tunnels were regulated simply by reducing light intensities by means of partial partitions of wood. The microclimate conditions could thereby be made to range from too unfavorable to optimal and then to conditions too dark and possibly too cool and humid for the mosquitoes to enter.
All of the stations were easily accessible to anophelines living in a state of nature, and therefore the type of microclimate they chose was in accord with their needs and desires. (The investigations reported upon deal only with “day anophelism,” or the daytime resting places of anophelines.) Three species An. sacharovi (elutus), An. maculipennis (varieties messeae and typicus), and An. superpictus are well represented in the records.
The results to date show that:
1.Light should be regarded as a definite microclimate factor, as in nature temperature depends largely upon the intensity and duration of light, and humidity in turn depends largely upon temperature. (Dark situations normally indicate decreased temperatures and increased humidities.)
2.The intensity of light at dawn serves as a more direct and safer guide than either temperature or humidity, as at this time, and until about one-half hour after sunrise, temperatures and humidities within all types of structures approximate those occurring outdoors.
3.A light intensity of 1 to 5 candle-foot power is suitable for anophelines under summer conditions; greater or less than this is avoided. (The intensities of light given are approximate, specific light intensities not being available.)
4.The influence of temperature, although not as direct as that of light, has a measurable effect upon anopheline behavior, as is revealed in the following abridged table which is based on a total of 9169 specimens:
PER CENT OF ANOPHELINES IN MAIN ROOM OF STABLE
PER CENT OF ANOPHELINES WITHIN ENTRANCE TO TUNNEL OPENING INTO THE SAME STABLE
4a.The combined percentages of a, b, and c show that 38 per cent of the anophelines were collected in the main room as compared to 62 per cent in the tunnel entrance. Those for A, B, and C (comparable temperatures) give 72 and 28 per cent respectively. Therefore over twice as many anophelines remained in the main room when the windows were closed as when they were open.
4b.A direct comparison of the percentages of the items a, b, and c with A, B, C, and D shows that a is equivalent to B, b to C, c to D. This indicates that at temperatures under 23°C. anophelines tolerate a stronger light intensity than at higher temperature; that under darkened conditions an approximate increase of 5°C. produces the same effect as occurs under an increase of 5 or more candle-foot power, e.g., an approximate 10 candle-foot power at 27°C. equals an approximate 3 candle-foot power at 31°C.
4c.Excessive temperatures tend to cause all anophelines to seek even darker and consequently cooler quarters. This is absolute at 40°C.
5.The distribution of the species as shown in the table indicates that superpictus is somewhat more tolerant of light and higher temperatures than the others while maculipennis is least tolerant.
Another type of experiment made several times shows more precisely the respective influences of light and temperatures. An earthenware jar containing both sacharovi and maculipennis was exposed to the direct rays of the midday sun. The anophelines remained in the jar until the interior had been heated by the sun to 36°, after which they were forced to brave the sunlight. This indicates that the respective forces of sunlight and temperature become equalized at 36°C. A few individuals have been observed to remain in the jar in spite of the excessive temperature, but none have been observed to survive 41°C.
The results of these investigations show that anophelines choose their daytime resting places in accordance with microclimate conditions; further, that each species here considered differs to some extent in its microclimate preferences. (Culex pipiens shows a much greater preference for darker and cooler microclimates. During the summer of 1933, 154 specimens of this species were found within the tunnel entrance but none were seen in the main room of the stable.)