NATURE 



297 



THURSDAY, FEBRUARY 15, 1S72 



THE POSITION OF THE CENTRE OF 

 GRAVITY IN INSECTS 



A /TY researches on the conditions of equilibrium in 

 -'•'•'- living beings, have led me to the conclusion that 

 a complete knowledge of them is only possible when the 

 position of the centre of gravity in each is known. 



At present the knowledge of the mechanism of the Ar- 

 ticulata has made considerable progress, thanks to the 

 use of processes of investigation borrowed from Physics ; 

 and it appeared to me, that there would be real utility in 

 the description of an easy method for the discovery of the 

 centre of gravity in the Articulata, and the results which its 

 application to insects has allowed me to obtain. I am, 

 unfortunately, unable in a simple resume to give a descrip- 

 tion of the instrument which I have employed. A very 

 short description without an engraving is necessarily 

 obscure, and loses all utihty. I shall simply say that the 

 instrument in question is a reproduction, on a small scale 

 and with some improvements, of that which Barelli has 

 invented for the determination of the centre of gravity in 

 man. And with regard to the results of my experiments, 

 I must also renounce the idea of giving them in the form 

 they assumed in my work , that is, without the considerable 

 number of figures combined in tables. I shall confine 

 myself to the enunciation of the general conclusions I 

 have been able to deduce, and to supporting them, as re- 

 quired, by several examples. 



(i.) The centre of gravity in an insect is situated in 

 the vertical and medial plane which passes along the 

 longitudinal axis of the body. 



(2.) It occupies a position almost identical in insects of 

 the same species, the same sex, and in the same attitude. 

 (3.) The exterior form of the body rarely permits the 

 determination of the exact position of the centre of gravity 

 ■withoict experiment. I shall cite the results with which 

 the family of Odonates have furnished me as examples. 

 All its representatives have nearly the same exterior 

 aspect ; and yet, notwithstanding this quasi identity of 

 structure, I have found in the relative position of the 

 centre of gravity the following differences : — 



Agrion puella, female I st third of the 3rd abdominal ring. 



„ sanguinea ,, Posterior border of the 2nd abdo- 



minal ring. 

 Libellula conspurcata ,, ,, ,, of melathorax. 



Libellula viilgata ,, Groove between thorax and ab- 



domen, 

 .(^ischna grandis ,, Middle of 2nd abdominal ring. 



(4.} The centre of gravity does not occupy the same 

 position in the two sexes of one species. It is sometimes 

 less and sometimes more to the rear in the females than 

 in the males ; and its situation depends on the relations e.x- 

 isting between the different dimensions of the individuals. 

 One would suppose that th; centre of gravity would 

 always be situated further back in females than in males, 

 as the abdomen of the former is in general more bulky 

 than that of the males. During the metamorphosis from 

 larva to perfect insect, the relative centre of gravity ap- 

 ^•0L. v. 



proaches the head ; the absolute centre, on the contrary, 

 recedes from it.* This apparent contradiction is very easily 

 explained ; the thorax of the larva is generally much re- 

 duced, and the abdominal rings numerous. In the perfect 

 insect the thorax has acquired considerable dimensions, 

 and the number of abdominal rings has diminished. The 

 thorax, prolonging itself more to the rear, has approached, 

 so to speak, the centre of gravity, which also remains in 

 the medial region of the body ; and the abdomen shorten- 

 ing itself, the distance of its extremity from the point in 

 question diminishes. 



(5.) While standing, the centre of gravity is placed at 

 the base of the abdomen, or in the posterior portion of 

 the thorax, and usually in the centre of the length of the 

 body. 



(6) When an insect is walking, its centre of gravity under- 

 goes constant displacement about a mean point, but the dis- 

 tances of displacement are too small to be measured. In 

 fact, if experiments are made with leaping Orthoptera, 

 grass-hoppers, or Acridians, it is ascertained that the dis- 

 placement of their enormous posterior members leads to 

 changes in the situation of the centre of gravity, but 

 these changes are so small that one arrives at the con- 

 clusion that it is impossible to measure them in ordinary 

 insects. 



(7.) The displacement of the centre of gravity, when 

 the insect passes from the state of repose to that of flight, 

 cannot be ascertained except with those species where the 

 wings lie folded on the back when in a state of repose. 

 The displacement is horizontal and from back to front. 



For example, in the following species the displacement 

 is : — 



Dytiscus dimidiatus 

 Hydropliilus piceus 

 Melolontha vulgaris 

 Notonecta glauca 

 Locusta viridissima 

 Vespa vulgaris 

 Plusia gamma 

 Eristalis tenax 



0-045 of the total length of the body. 



0'02S ,, ,, 



°°53 



°'°3- )> ), 



0-054 ,, ,, 



0-023 ,1 ,, 



0-025 ,, ,j 



o -03 7 



(S.) During active flight the centre of gravity oscillates 

 continually about a mean position, which corresponds 

 with the instants when the extremities of the wings pass 

 the point of crossing of the 8-shaped curve which they 

 describe in the air. 



(9.) In aquatic insects the centre of gravity is nearer 

 to the lower than to the upper surface of the body. 



(10.) During swimming, the movements of the posterior 

 feet, acting like oars, determine the oscillation of the 

 centre of gravity around a mean position, which answers to 

 the position of the swimming feet placed at the middle of 

 their course. These oscillations of the centre of gravity 

 lead to a continual swaying of the body about a transverse 

 axis passing through the mean centre of gravity, and it 

 ought, consequently, to follow a gently undulating course. 

 Felix Plateau 



* In niy work I have called the rel jtive position of the centre of gravity, 

 its position as regards any portion of the body, as rings, hip (hatichcj, &c. : 

 and I have named the absolute position of the centre of gravity tlie number 

 which is obtained by calculating the relation between the distance of the 

 centre of gravilj^ from 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, mean that the distance of the centre of gravity from the posterior 

 extremity is i^^ or iV^ of the total length of the body. They show imme- 

 diately, and independently of the form and thinness of the rings, whether the 

 centre of gravity is in the centre of the insect, nearer to the head, or nearer 

 to the anal orilice. 



