50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I25 



ceicm, the parasite apparently remains within the larva and pupa with- 

 out destroying either. 



In the highlands above 6,000 feet, large numbers of dead and dying 

 larvae and pupae have been encountered frequently on stones during 

 the months of January and February, when the water level of the 

 Streams is greatly reduced. The larvae all had a milky appearance 

 owing to great multiplication of a Protozoan parasite of the order 

 Microsporidia (probably Thelohania sp.). The dead larvae were soft 

 and in all stages of decomposition, while the live ones seemed ex- 

 tremely sluggish and with small swellings. The pupae were covered 

 with what seemed to be a network of fungal mycelia that also extended 

 over many of the stones. It may be that the fungus did not cause the 

 death of the simuliids, but took hold upon their dead bodies. In situa- 

 tions where the Microsporidia were widespread, the number of healthy 

 larvae and pupae seemed negligible and the simuliid population of the 

 entire stream seemed especially low. 



THE STREAMS 



In the foregoing sections, the following environmental factors and 

 their possible influence on the presence of onchocerciasis and its insect 

 vectors have already been discussed : Geography, climate, the inhabit- 

 ants and their customs, plant associations, and animal associations 

 (reservoirs and vectors, predators and parasites of the Simuliidae). 

 Considering the members of the family Simuliidae as the vectors of 

 the disease, it naturally becomes important to learn more about the 

 streams in which they pass their immature stages and how these 

 streams may afifect the development and, therefore, the presence of 

 particular species. 



As stated in the subsection "Reservoirs and Vectors," approximately 

 4,000 collections of larvae and pupae of Simulium species were made 

 throughout Guatemala in over 500 streams during a 3-year period. 

 Streams in the Municipality of San Pedro Yepocapa, which support 

 a preponderance of anthropophilic species, were revisited regularly. 

 With each collection, ecological data (see section on ecology) were 

 taken. A well-established relationship was found between the type 

 of stream and the species of Simulium that will breed in it. The opti- 

 mum stream conditions for each species are sufficiently well marked, 

 so that by mere observation of a stream we can predict rather ac- 

 curately what species of Simulium will be found in it, in which section 

 of the stream, and upon what substratum. 



There follows a general discussion of streams as a breeding place of 

 Simulium, the formation of the streams in Guatemala, their morpho- 



