326 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



callidum. Under the ranges of relative humidity, the median point, 

 or reading above and below which 50 percent of the flies fall, is indi- 

 cated for each species. The following observations appear significant : 

 The medians of the three species are at different levels of temperature 

 in almost every range of relative humidity, especially in the high ones ; 

 S. callidum definitely bites at a significantly lower temperature than 

 either metallicum or ochraceum; at relative humidities between 81 and 

 100, ochraceum seems to bite at considerably higher temperatures than 

 metallicum or callidum; callidum has a threshold relative humidity 

 between 51 and 60, below which the species does not bite (also borne 

 out in table 18) ; while the medians for biting incidence of metallicum 

 and ochraceum vary considerably between each range of relative hu- 

 midity, the median of callidum remains fairly constant (except between 

 31 and 40, for which range the number of observations was too small 

 to be significant). (See also Appendix III, text figs. 16-21.) 



Relation of light intensity to biting activity. — As stated in the intro- 

 ductory paragraphs on the effect of environment on feeding habits, 

 readings of light reflected by the human subjects were taken every 10 

 minutes during the experiments designed to determine the effect of 

 various environmental factors on the feeding activity of the flies. 

 It was found that the three principal anthropophilic species would bite 

 when the reflected light readings (taken with a General Electric ex- 

 posure meter) ranged from 0-20 footcandles, the optimum being be- 

 tween 4 and 10. The intensity of reflected light, as would be expected, 

 is related to the time of day, the clarity of the sky, quantity of vegeta- 

 tion giving shade, and the color of the subjects' skin. Light-skinned 

 subjects reflected much more light, at times three or four times more, 

 than did dark-skinned subjects. In the early morning and in the eve- 

 ning, when the light readings were zero, few if any flies would bite the 

 dark individuals, while they would readily bite the fairer-skinned ones. 

 However, during the middle of the day, when the light intensity in- 

 creased, there was little difference between the number of bites received 

 by the light- and dark-skinned subjects. In open areas with clear 

 bright sun, the light intensity is often such that only dark-skinned 

 individuals are bitten. 



It has often been said that Simulium species will not bite indoors, 

 supposedly owing to inadequate light. However, Simulium ochraceum 

 frequently, and v9. metallicum on occasion, have been found biting 

 inside of our laboratory during the night at light intensities that were 

 greatly reduced although apparently sufficient for fly activity. The 

 lighting in the rooms where fly biting occurred consisted of two 20- 

 watt fluorescent bulbs which were ample to give a reading of light, 

 reflected from a fair-skinned subject, of 2 footcandles. 



