NO. I BLACK FLIES OF GUATEMALA — DALMAT 365 



SPECIES ASSOCIATIONS 



When collecting larvae and pupae of a particular species, it is often 

 interesting to know what other species might be expected to be found 

 in the same stream. Knowing the type of stream (width, depth, sub- 

 stratum, temperature, current speed, and volume flow) and the alti- 

 tude of the region, a study of tables 23 through 30 should enable the 

 collector to make a provisional list of such species. The probabilities 

 could then be checked by referring to table 33 in Appendix III, and 

 a more accurate prediction made. Table 33 was prepared from the 

 findings of the author after making approximately 4,000 collections 

 of larvae and pupae in over 500 streams throughout Guatemala. Each 

 species has been listed along the vertical and horizontal margins of 

 the table. To the right of each species listed along the vertical mar- 

 gin, there has been given the number of times it was collected with 

 species listed along the horizontal margin. Likewise, the number 

 given below each species listed along the horizontal margin will tell 

 how many times that species was collected with the species listed 

 along the vertical margin. Thus, by mere observation the relative 

 probability of finding a particular species in the same stream with all 

 other species can be determined. For example, along with S. (D.) 

 acatenangoensis, S. aiireum was found on 6 occasions, C. aguirrei on 

 3, S. callidum on 30, capricornis on 23, carolinae on 4, downsi on 18, 

 ethelae on 3, rubicundulum on 32, etc. Therefore, it can probably be 

 said that 5'. acatenangoensis will be more likely to be found associated 

 with callidum, capricornis, and rubicundulum than with C. aguirrei, 

 S. carolinae, or ethelae. 



It will be seen in table 33 that along with 5. ochraceum, in its rather 

 specialized habitat, the other species most commonly found are metalli- 

 cum and callidum. However, although metallicum and callidum do 

 breed along with ochraceum, they are much more likely to be found 

 in larger young to adolescent streams in which there is prolific breed- 

 ing of downsi, rubicundulum, and mexicanum. 



RELATION OF OXYGEN CONTENT OF STREAMS TO SPECIES BREEDING 



The dissolved oxygen content of 62 streams in the Municipality of 

 San Pedro Yepocapa was determined by the Winkler method (Amer. 

 Publ. Health Assoc, 1936) five times during the period of a year. It 

 was found that all streams in this region were very high in oxygen, 

 especially at the end of the rainy season. The lowest oxygen content 

 was recorded during the month of April, at the end of the dry season. 

 Since the seasonal variation in dissolved-oxygen content was found to 



