ANATOMY AND PHYSIOLOGY 79 



Under nervous stimulus a muscle shortens and thickens because its 

 component fibers do, and this in turn is attributed to the shortening and 

 thickening of the longitudinal fibrillae. When the stimulus ceases, the 

 radial fibrillae, by their elasticity, possibly pull the longitudinal ones 

 back into place. The last word has not been said, however, upon this 

 perplexing subject. 



Muscular Power. The muscular exploits of insects appear to be 

 marvellous beside those of larger animals, though they are often exag- 

 gerated in popular writings. The weakest insects, according to Plateau, 

 can pull five times their own weight and the average insect, over twenty 

 times its weight, while Donacia (Chrysomelidae) can pull 42.7 times its 

 weight. As contrasted with these feats, a man can pull in the same 

 fashion but 0.86 of his weight and a horse from 0.5 to 0.83. How are 

 these differences explained? 



It is incorrect to say that the muscles of insects are stronger than 

 those of vertebrates, for, as a matter of fact, the contractile force of a 

 vertebrate muscle is greater than that of an insect muscle, other things 

 being equal. The apparently greater strength of an insect in propor- 

 tion to its weight is accounted for in several ways. The specific gravity 

 of chitin is less than that of bone, though it varies greatly in both sub- 

 stances. Furthermore, the external skeleton permits muscular attach- 

 ments of the most advantageous kind as compared with the internal 

 skeleton, so that the muscles of insects surpass those of vertebrates as 

 regards leverage. These reasons are only of minor importance, how- 

 ever. Small animals in general appear to be stronger than larger 

 animals (allowing for the differences in weight) for the same reason that 

 a smaller insect has more conspicuous strength than a larger one, when 

 the two are similar in everything except weight. For example: where 

 a bumblebee can pull 16.1 times its own weight, a honey bee can pull 

 20.2; and where the same bumblebee can carry while flying a load 

 0.63 of its own weight, the honey bee can carry 0.78. Always, as 

 Plateau has shown, the lighter of two insects is the stronger in respect 

 to external manifestations of muscular force in the ratio of this muscu- 

 lar strength to its own weight. 



To understand this, let us assume that a beetle continues to grow (as 

 never happens, of course). As its weight is increasing so is its strength 

 but not in the same proportion. For while the weight say that of a 

 muscle increases as the cube of a single dimension, the strength of the 

 muscle (depending solely upon the area of its cross-section) is increasing 

 only as the square of one dimension its diameter. Therefore the 



