ARTHROPODS AND MOLLUSCS i6i 



which swim with back uppermost, and are greenish gray, 

 with fine black mottling. Both. kinds come to the surface 

 for air, and carry a supply of it down with them. Observe 

 this, and note the difference in' the disposi|;ion of the air 

 (revealed by its silvery appearance) in the two kinds. What 

 is the favorite resting position of each? Which pair of 

 legs do the back-swimmers use for oars? Which pair do 

 the water-boatmen use? Water-bugs are predaceous, suck- 

 ing the blood of captured insects by means of a piercing beak. 

 On the under side of stones, in brook "riffles," and in 

 pools and watering-troughs 

 not too frequently used are 

 to be found commonly the 

 nymphs, i. e., young (fig. 

 71), of Mayflies, recognizable 

 by the rapidly vibrating 

 flap-like tracheal gills along 

 each side of the flattened 

 dehcate body, three pairs of 



legs, and two or three long, Fig. 70. Water-boatman, Corisa 

 ,1 j-1 . • .• sp. (Twice natural size; after 



slender filaments projectmg /enkins and Kellogg.) 



from the tip of the abdomen. 



Those found in ponds or other quiet water can easily 

 be kept alive in the school aquarium (see Appendix II). 

 Examine a live specimen in water in a watch-glass 

 with a magnifier. The body-wall is so transparent that 

 many of the internal organs can be seen. Note especially 

 the beating of the heart, a slender tube running along 

 the middle of the back. See the dark air-tubes (tracheae) 

 running out into the thin gills, and note the rapid vibration 

 of these gills to keep in contact with fresh water. The 

 young Mayflies feed on minute organisms such as dia- 

 toms and other algae. They live as nymphs for a year, 

 or even two or three years in some species, and then crawl 

 out "of the water on a stone or plant-stem, or come 



