NO. I BLACK FLIES OF GUATEMALA — DALMAT 5 1 



logical age in relation to black-fly breeding, and the characteristics of 

 a stream that determine which species of black flies will be found 

 therein. In the section on ecology the actual relationship between 

 stream characteristics and the species groups will be discussed. 



The immature stages of the Simuliidae seem to be extremely adapt- 

 able to aquatic environmental conditions. Thus, as a family, they are 

 encountered in Alaska, Labrador, Newfoundland, in many areas of 

 Europe and Asia north of the Arctic Circle, to the Equator and then 

 south to the southern tips of South America, Africa, Australia, and 

 New Zealand. They occur from sea level up to the snow line in 

 mountainous areas. Perhaps the only type of area in which they are 

 lacking is the desert, and that because of the lack of aquatic breeding 

 haunts. It is usually stated in the literature that Simulium species re- 

 quire for their larval development rapidly flowing, shallow, clear, cool 

 water ; that the bodies of water must be of a permanent nature ; and 

 that there must always be emergent or floating vegetation, stones, or 

 debris on which the larvae can anchor themselves. These are good 

 generalizations, but they do not cover exclusively black-fly habitats. 

 In the course of the present work, black-fly larvae have been found in 

 extremely slow-flowing streams, in almost stagnant water, in pools 

 with only a small overflow, completely covered by mud with no at- 

 tachment to any fixed object, and attached to the cement walls of man- 

 made waterways and to discarded tin cans. They have also been col- 

 lected in streams immediately below a spot where clothes were being 

 washed with extremely alkaline soap. The pH of the water in which 

 they are found ranges from 5 to 8. This variability of habitat will 

 become more apparent in the section on ecology. 



FORMATION OF THE STREAMS 



Water that falls on the surface of the land as rain either (i) evapo- 

 rates, (2) becomes channeled immediately to form surface wash, flash 

 (ephemeral) streams, temporary (intermittent) streams, or to swell 

 permanent streams, or (3) percolates through the soil, collecting in 

 underground deposits which form somewhat defined channels, later 

 reappearing at other sites as permanent springs and rivers. The runoff 

 water that immediately becomes channeled reaches permanent streams 

 only by flowing over the surface of the ground until it washes into 

 already established tributaries. It can be classified as transient insofar 

 as its distinctive existence is concerned, while the water that is chan- 

 neled underground, feeding already formed streams, can be classified 

 as sustained, permanent, or perennial. 



The surface wash and flash streams (pi. 8, fig. 2) can be disregarded 



