56 M. F. Plateau on the Position of 



following differences in tlie relative positions of the centre of 

 gravity : — 



Agrion puella ? . First third of the third abdominal seg- 

 ment. 



Agrion sanguinea. Posterior margin of the second abdo- 

 minal segment. 



Lihellvla conspurcata $ . Posterior margin of the meta- 

 thorax. 



Libellida vulgata $ . Furrow between thorax and abdomen. 



Cordulia metalUca $ . Posterior margin of the metathorax. 



jEschna grandis $ . Middle of the second abdominal seg- 

 ment. 



4. The centre of gravity does not occupy the same position 

 in the two sexes of the same species ; it is sometimes more 

 and sometimes less backward in the females than in the males^ 

 and its situation depends upon the relations existing between 

 the various dimensions of the individuals. 



It might have been supposed that the centre of gravity was 

 always situated further back in the females, the abdomen of 

 which is generally more voluminous than that of the males* 

 I have observed the opposite condition in the females of 

 Oryctes nasicornis^ Ltbellula vvlgata, and Agrion puella. 



5. During the metamorphosis of the larva into the perfect 

 insect, the relative centre of gravity approaches the head ; the 

 absolute centre of gravity, on the contrary, departs from it*. 



This apparent contradiction is easily explained. The tho- 

 rax of larvaj is generally very much reduced and the segments 

 of the abdomen numerous. The centre of gravity therefore 

 falls inevitably in an abdominal seg^ment. In the perfect 

 insect the thorax has acquired considerable dimensions, and 

 the number of abdominal segments has diminished. The 

 thorax, thus being more prolonged posteriorly, has advanced, 

 in a manner, to meet the centre of gravity, which remains 

 plainly in the median region of the body ; and the abdomen 



* In my memoir I have given the name of the relative positioti of the 

 centre of (/ravity to its position with relation to some one of the parts of 

 the body (segment, coxa, &c.), and that of the absolute jjosition of the 

 centre of (/ravity to the number which is obtained by calculating the 

 relation between the distance from the centre of gravitv to the posterior 

 extremity of the body and the total length of the animal. The quotients 

 0-50, 0-67, for example, obtained in this manner, signify that the distance- 

 from the centre of gravity to the posterior extremity \&Jive tenths or sivty- 

 seven hundredths of the length of the body. They show at once, and 

 independently of the form and extent of the segments, whether the centre 

 of gravity is at the middle of the insect, mare approximated tb the head, 

 or nearer to the anal orifice. 



