THE COMMON FLEA 267 



proportionately far greater than that of vertebrate 

 animals. Thus a hive-bee, weighing .09 gramme, was 

 found to be able to lift a weight equal to 23.5 times that 

 of its own body; while a large humble bee, weighing 

 more than four times as much, had a relative muscular 

 force only a little more than half as great, or, more 

 exactly, could lift only 14.9 times the weight of its body. 

 Similarly, a large cockchafer, weighing .94 gramme, 

 could not raise more than 14.3 times its own weight; 

 while a much smaller, but allied species, weighing only 

 .153 gramme, i.e., about one-sixth part as much, was 

 strong enough to raise 24.3 times its own weight, or 

 more than half as much again as its lumbering relative. 

 If now we compare these results with those derived 

 from the higher animals, we find that, while insects can 

 raise from about 14 to 23.5 times their own weight, the 

 muscular force of a man or a horse, when expressed in the 

 same way, are represented by the ridiculously small 

 numbers .86 and .53 respectively. This high relative 

 muscular force of insects, however, is not due to any 

 superiority in the quality of their muscles, but is simply 

 a direct consequence of their small size. For, with 

 muscles similarly constituted, the contractile force de- 

 pends, of course, on the number of muscular fibres, i.e., 

 it varies as the cross section of the muscles, and is there- 

 fore proportional to the squares of linear dimensions; 

 the weight, on the other hand, depends on the volume, 

 and is therefore proportional to the cubes of linear 

 dimensions. Hence the ratio of the contractile force to 

 the weight decreases rapidly as the animal increases in 

 size, or, in other words, the smaller the animal, the 

 stronger, relatively, it must become, by virtue of that 

 very decrease in size. While, therefore, it is quite true 

 that, considering their size, insects are very much stronger 



