ARTHUR DEWITT WHEDON 427 



displacement, that there is no displacement of the center when 

 the insect passes from repose to flight and that a very slight 

 oscillation occurs during flight. In aquatic insects the center of 

 gravity is nearer the lower than the upper surface of the body 

 and a slight oscillation occurs during natation. He also used 

 the Odonata to illustrate his discoveries: in Agrion the center of 

 gravity is in the first third of the third abdominal segment, in 

 Aeshna in the middle of the second segment, in Coi-dulia on the 

 posterior margin of the metathorax, and in Libelhda Ijetween the 

 metathorax and the first abdominal segment. It would seem 

 that we might safely infer that stability would be greater in 

 forms having the center of gravity a little back of the wing center; 

 this is true of dragonflies. 



(2) In the production of a rudder. Hankin has suggested that 

 the wings of dragonflies are used in steering the body to right 

 and left and in keeping the horizontal position. This seems the 

 probable mechanism as there is little freedom of movement to 

 the sides in the abdomen. Ascent or descent, however, must be 

 affected by the position of the abdomen. Again Hankin noticed 

 that the abdomen of Pantala was allowed to droop under con- 

 ditions of easy flight but was straightened out or even elevated 

 under more difficult air conditions. In this connection the 

 greatly flattened abdomen of the higher Libellulinae and the 

 expanded tip in the Gomphinae must be taken into account. 

 Either form should furnish an excellent rudder. 



(3) In Modifying the Contour: 



That excellence in flight is dependent upon the relation of 

 abdomen to thorax may be inferred from the table on page 387. 

 The thoracic equipment of wings and muscles is very necessary 

 l)ut the length and nature of the abdomen are just as important 

 though more passive factors in successful flight. The rapid 

 development and present importance of the monoplane for war 

 purposes should act as evidence in this direction. 



Some of the most marked improvements in the aerojilane 

 during the past three or four years (Lefranc, 1918) have to do 

 with that portion of the mechanism that corresponds to and 

 resembles- in appearance the abdomen of the dragonfly: the 

 enclosure of the body, the production of a very smooth surface, 



TRANS. AM. ENT. SOC, XLIV. 



