ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 387 



it rises to the surface; note the water repellent surface; 

 compare with the surface of the nymph. Compare the two as 

 to what happens when they stop swimming; which one 

 rises to the surface like a cork? Compare with one of the 

 free-swimming forms in respect to this. Then examine the 

 whirligig more carefully i) as to the differentiation of its 

 feet; 2) the extreme specialization of its hind feet; 3) the 

 form of its body and 4) the differentiation of its divided 

 eyes, into an upper eye to look into the air, and a lower one 

 to look down into the water ; all are express adaptations for 

 living on the surface. Then examine the other, as to its 



FIG. 225. The nymph of a damselfly (Ischnura verticalis). 



climbing feet, the gills upon its abdomen and its protective 

 coloring. 



Then compare the representatives, of the groups i to 4 as 

 to i) position in the water, and movements; 2) mode, if they 

 have any of carrying air; 3) air repellence of the body sur- 

 face, and 4) weight. Air carried externally can be recog- 

 nized by its shine. Push a skater or a water-spider (or even 

 a housefly) under water and see the layer of air enveloping 

 its whole body. 



