the Centre of Gravity in Insects. 57 



becoming shortened, the distance from its extremity to the 

 point in question diminishes. 



6. In standing, the centre of gravity is placed at the base 

 of the abdomen, or in the posterior part of the thorax, and 

 usually at the middle of the length of the body. 



7. In walking, the centre of gravity of an insect is con- 

 stantly displaced around a mean position, but by too small an 

 amount to be capable of measurement. 



In fact, if we make experiments by means of Saltatorial 

 Orthoptera (locusts or grasshoppers), we find that the move- 

 ments of their enormous posterior limbs induce changes in the 

 situation of the centre of gravity ; but these changes being- 

 very slight, we an-ive at the conclusion that it would be im- 

 possible to measure them in ordinary insects. 



8. We do not detect any displacement of the centre of 

 gravity when an insect passes from the position of repose to 

 that of flight, except in those species in which the wings are 

 decumbent or crossed upon the back in a state of repose. The 

 displacement is horizontal and from behind forward. For 

 example, this displacement is as follows, in the following 

 species : — 



Dytiscus dimidiatus 0'045 of the total length of the body. 



Hydrophilus piceus 0'028 „ „ 



Melolontlia vidgaris ? 0*053 „ „ 



Notonecta glauca . . . 0'032 „ „ 



Locusta viridissima. . 0*054 „ „ 



Vespa vulgaris .... 0*023 „ ,, 



Plusia gamma .... 0*025 „ „ 



Eristalis tenax .... 0*037 „ „ 



9. During active flight the centre of gravity oscillates con- 

 tinually around a mean position which answers to the moments 

 when the extremities of the wings are at the point of crossing 

 of the figure-of-8 curve which they describe in the air. 



10. In aquatic insects the centre of gravity is nearer to the 

 lower than to the upper surface of the body. 



11. During natation the movements of the oar-like posterior 

 legs cause oscillations of the centre of gravity around a mean 

 position, which answers to the situation of the natatory feet 

 at the middle of their course. These oscillations of the centre 

 of gravity induce a continual balancing of the body upon a 

 transverse axis passing through the mean centre of gravity, 

 and cause it, consequently, to traverse a slightly undulated 

 path. 



