66 



NATURE STUDY 



water, so that oxygen may be brought in and the exhaled car- 

 bonic acid gas carried away. 



OTHER BROOK INSECTS. The young stone-flies, May- flies 

 and caddice-flies are the insects of the brook most certainly and 

 easily found. On the surface of quiet pools may be seen water- 

 striders and whirligig beetles, but these may more certainly be 

 found on ponds, and are described among the " pond insects." 



You may find, perhaps, clinging to the rock-bed of the 

 stream where the water is shallow but flowing swiftly, many small 

 black worm like animals (fig. 39, a) holding firmly to the rock 



by one end while the rest 

 of the body stands nearly 

 upright in the water wav- 

 ing about as the swiftly 

 running water strikes it. 

 These are the young or lar- 

 vae, of the black fly, a small 



J et black hump-backed, 



two- winged fly (fig. 40). 

 The black-fly is a biting fly of more vicious disposition and 

 more effective biting ability than the mosquito. Take some of 

 'these squirming black larvae to the school-room and examine 

 'them in a watch-glass of water with a magnifier. (They cannot be 

 ikept alive long in quiet water). Note the odd shape of the 

 body. Note the sucker at the posterior end of the body, by 

 which the larvae holds fast to the rock despite the swift current, 

 Note the two fan-shaped organs attached to the head, which arc 

 composed each of fifty hairs rising from a short, stout process; 

 these organs are waved about in the water, sweeping microscopic 

 water organisms into the mouth. You may find among these 

 black-fly larvae a number of odd little cornucopias (fig. 39, 5), 

 fastened to the rock by their lower end. From the upper end of 

 each cornucopia a pair of tiny tufts of short filaments (tracheal 



and pup* (*) of the 



