POND LIFE. 



97 



ing prey. Make a drawing of one of these legs and 

 label the following parts: coxa, trochanter, femur, 

 tibia, tarsus, and claws. 



11. Study the wings. These wings are of espe- 

 cial interest, for, judging by the powers of flight of 

 these insects, they must be extremely well fitted for 

 their use. Although large they are very light ; yet 

 notwithstanding their lightness they are strong, be- 

 ing strengthened by a large number of veins and 

 veinlets, and the margin of the wing is strengthened 

 throughout by an ambient vein. Near the middle of 

 the length of the frontal (costal) margin of each 

 wing there is a jointlike structure : this is called the 

 nodus ; this structure is characteristic of the wings of 

 dragon-flies and damsel-flies, but its use is not known. 

 The front part of each wing, which is the part where 

 the greatest strength is required, is folded in plaits 

 somewhat like a fan. This adds greatly to the 

 strength of the wing. Take a flat piece of writing 

 paper and observe how easily it can be bent. Now, 

 fold this piece of paper like a fan and see how much 

 stiffer it is. Study the cross-veins extending back 

 from vein I (i. e., the front margin of the wing) and 

 note that some of them extend straight across the 



O 



furrow to vein I II (the second vein from the margin), 

 and are in the form of triangular braces that tend to 



o 



preserve the form of the furrow. 



12. The abdomen is long, and usually quite slen- 

 der. The necessity for the great length of the abdo- 

 men is not quite clear; it may be that the abdomen 

 serves to steady the flight like the shaft of an arrow ; 

 and, too, there may exist a relation between the form 

 of the wings and that of the abdomen. With most 



