426 MORPHOLOGY OF THE ABDOMEN IN THE ODONATA 



remarkable swelling or constriction of certain parts in many 

 groups, and the great reduction of the muscles, especially the 

 sternal muscles. Some of these problems have been discussed in 

 the introductory pages of this paper. It seems probable that 

 most of these variations are correlated with the elongation of the 

 abdomen. 



The writer thinks that we cannot explain the nature of these 

 modifications, or even decide whether they are adaptive or not, 

 without much more knowledge than that set forth in the section 

 on comparative anatomy. A great deal of field study of the 

 exact mode of functioning of the parts above described is nec- 

 essary before valid conclusions can be drawn. And in order to 

 bring out clearly the nature of the problems, a preliminary view 

 of the main adaptive possibilities is added below. 



1. Flight. The dragonflies (Anisoptera) are probably the most 

 expert of fliers: powerful, quick and exact of manipulation, and 

 tireless. The damselflies (Zygoptera) are much weaker. Sev- 

 eral distinct modes of flight are seen in the difl"erent groups; 

 fluttering or sculling in the Zygoptera, darting in the Aeshninae, 

 and soaring or skimming in the Libellulinae. Thus far we do 

 not know the mechanical principles involved in each of these 

 modes, and but very little of the general principles having to do 

 with elongation of the abdomen and proportions of the thorax. 

 Such studies of flight as those of Amans (1883-84, 1885) and Need- 

 ham (1903) though carefully done include but little beyond the 

 structure of the wings and thorax. Hankin (1913, chapter XX) 

 has dealt with the purely observational side of the question. 



The elongation of the abdomen may be related to flight in 

 several ways: 



(1) In controlling the position of the center of gravity. In his 

 investigation of the center of gravit}^ in insects Plateau (1872) 

 long ago showed that this point is in the vertical median plane 

 through the long axis of the body, that it is the same for all mem- 

 bers of each species though different for the two sexes, that it is 

 not determined by the external form of the body, that at meta- 

 morphosis the relative center of gravity approaches the head, 

 that in standing the center of gravity is placed at the base of the 

 abdomen or in the metathorax, that in walking there is but slight 



