NO. I BLACK FLIES OF GUATEMALA DALMAT 321 



from dawn to about 9 a.m. and again from 3 or 4 p.m. until twilight, 

 with very little activity during the middle of the day. (See also 

 Appendix III, text fig 15.) 



Relation of air temperature and relative humidity to biting habits. — 

 It can be seen in table 17 that S. ochraceum feeds in the sun at air 

 temperatures from 15° C. to >40° C, with its optimum between 

 34° C. and 35° C, above which there is a marked reduction in ac- 

 tivity. In the shade, this species will start feeding at slightly lower 

 temperatures than in the sun. S. metallicum starts actively feeding at 



Table 16. — Relation of time of day to biting habits of adult Siimdiidae 



Expressed as number of flies biting per observation period 



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

 95 days from 6 :30 a.m. until 5 -.30 p.m. 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 



observa- Total ochraceum metallicum catlidum 



tion number , ^ , , ^ v , '^ , 



Hour periods of flies Shade Sun Shade Sun Shade Sun 



6-7 74 983 4-7 2.7 2.5 0.5 2.4 0.3 



7-8 95 5,526 29.9 14.3 6.2 3.4 3.4 i.o 



8-9 95 8,605 44-1 24.1 12.2 7.9 1.9 0.3 



9-10 95 8,029 39.4 23.4 12.7 8.0 1.0 O.I 



lo-ii 95 6,272 31.2 17.8 11.2 5.4 0.4 0.1 



11-12 92 5,220 28.5 14.7 8.8 4.1 0.5 0.1 



i2~ 1 87 5,142 29.7 15.4 9.8 3.7 0.4 0.1 



1-2 83 4,727 27.7 14.6 10.3 3.4 0.6 0.2 



2-3 80 4,149 26.9 11.9 9.4 3.0 0.6 0.1 



3-4 69 3,479 25.1 lo.i 10.9 2.9 1.3 0.1 



4-5 65 2,290 16.8 5.2 9.0 2.8 1.2 0.3 



5-6 43 1,529 14.2 5.3 lo.o 2.5 2.8 0.8 



about 22° C. in the sun and at 17° C. in the shade, and then continues 

 to feed with almost equal avidity until above 40° C. There is a slight 

 rise in activity between 27° C. and 28° C. in the sun and between 

 25° C. and 27° C. in the shade. 6^, callidum starts feeding actively at 

 temperatures well below 13° C, especially when in the shade, and 

 continues almost evenly until 30° C. in the sun and 34° C. in the shade, 

 above which it does not feed at all. This would explain its lack of 

 activity during the great part of the day when the temperatures are 

 above its optimum. 



Considering only the relationship of relative humidity to the biting 

 activity of the flies, the following observations can be derived from 

 table 18: At high relative humidities, ochraceum is much more active 



