84 NATURE STUDY. 



ids until every bit of available surface is covered by them, 

 spread their little "fans," and begin to take in food and to 

 grow. 



The black-fly, or Simulium. larva, which will be scarce- 

 ly more than a quarter of an inch long when grown, is al- 

 most microscopic in the beginning, but it is fully equipped 

 at the outset for catching and eating, and the particles of 

 food which it catches are ever so much smaller than itself. 

 For it feeds upon diatoms, which can only be seen with 

 the help of a powerful microscope, and which are brought 

 along in the swiftly running water from the more quiet 

 places in the brook above. 



The larva has a row of tiny hooks, arranged in the form 

 of a ring, at the lower end of its body, with which it can 

 hold on to a weed-covered or slimy stone, even when the 

 water is running very fast. When its hooks are fairly set, 

 it rises upright, looking like a tiny mealbag with the up- 

 per end a little the smaller, and spreads out its fans, one on 

 either side of its head, to catch the diatoms that it feeds 

 upon. I have found sixteen species, or kinds, of diatoms 

 in a single stomach, while the total number of individuals 

 was so great that it was a hopeless task to try to count 

 them. The cut, showing a fan, enlarged many hundred 

 times, gives a better idea of this wonderful little strainer 

 than any description could do. 



Of course only a small part of the great number of these 

 larvae live to grow up and become black-flies. Some lose 

 their hold, many are crowded off, some are swept away by 

 the water, and in the end most of them have been carried 

 into the nets of the caddis-flies, or have fallen into the jaws 

 of the many hungry things that are waiting for them. 



