NO. I BLACK FLIES OF GUATEMALA — ^DALMAT 323 



range and between 41 and 70 ; at less than 40, the biting activity is the 

 same in the sun and shade. 



Simulium metallicum seems to bite almost as actively in low relative 

 humidity as in high humidity, although its optimum also falls between 

 81 and 90 ; there is a slight reduction in activity at relative humidities 

 above 90, but not as marked as for ochraceum; this species generally 

 prefers to bite on shaded regions, except at relative humidities below 

 40, when the preference is definitely for areas exposed to the sun (no 

 biting at all in shaded areas), and within its optimum range of 81 to 

 90, when it slightly favors these sun-exposed areas. 



Table 18. — Relation of relative humidity to biting habits of adult Simiiliidae 



Expressed as number of flies biting per observation period at different ranges of 



relative humidity 



(Flies collected while feeding on human subjects who were exposed to bites on 

 69 days from 6 :3G a.m. until 5 :30 p.m. Relative humidity was recorded every 30 

 minutes. Sun or shade readings indicate that the flies were biting on a part of 

 the body that was either shaded or exposed to the sun.) 



Number of Number of flies biting per observation period 



observation , '^ ^ 



Relative periods ochraceum metallicum callidum 



humidity , ^- s , -^^ ^ , ^ ^ , '^ , 



(percent) Sun Shade Sun Shade Sun Shade Sun Shade 



91-100 254 600 9.9 9.2 3.2 3.5 0.2 0.6 



81-90 74 221 41.3 23.4 8.8 7.9 0.2 0.6 



71-80 114 214 13.7 19.3 3.2 6.0 o.i 0.8 



61-70 81 139 26.9 34.8 6.9 8.2 o.i 0.9 



51-60 54 58 26.0 25.6 5.0 5.7 0.9 0.6 



41-50 II II 22.0 29.4 7.8 8.3 0.0 0.4 



31-40 2 2 3.5 3.5 7.5 0.0 0.0 0.0 



Simulium callidum differs from ochraceum, and metallicum in that 

 its preference for biting on shaded regions over regions exposed to the 

 sun ranges from 3-9: i, while in the latter two species it never quite 

 reaches 2 : i (table 18) ; this preference for the shade is demonstrated 

 in all ranges of relative humidity except between 51 and 60, where 

 there is a favoring of regions exposed to the sun ; this latter range is 

 also its optimum for biting; callidum does not bite below 31-40, but 

 its activity between 91 and 100 is almost equal to that shown at any 

 other range of relative humidity, except from 51 to 60; the negative 

 data on feeding below 40, as seen in table 18, may be due to the small 

 number of observations made in this range of relative humidity, as 

 well as to the relatively low population of callidum as compared with 

 ochraceum or metallicum. 



Table 19 presents the relationship of both air temperature and rela- 

 tive humidity to the biting activities of S. ochraceum, metallicum, and 



