The force exerted by Insects has long been remarked with surprise, and it is a fact familiar, not only to naturalists, but to all observant persons, that, making allowance for their small size, Insects are the most powerful of common animals. Popular books of natural history give striking and sometimes exaggerated accounts of the prodigious strength put forth by captive Insects in their efforts to escape. Thus we are told that the flea can draw 70 or 80 times its own weight.[87] The Cockchafer is said to be six times as strong as a horse, making allowance for size. A caterpillar of the Goat Moth, imprisoned beneath a bell-glass, weighing half a pound, which was loaded with a book weighing four pounds, nevertheless raised the glass and made its escape.
This interesting subject has been investigated by Plateau,[88] who devised the following experiment. The Insect to be tested was confined within a narrow horizontal channel, which was laid with cloth. A thread attached to its body was passed over a light pulley, and fastened to a small pan, into which sand was poured until the Insect could no longer raise it. Some of the results are given in the following table:—
Table of Relative Muscular Force of Insects (Plateau).
| Weight of body in grammes. | Ratio of weight lifted to weight of body. | |
| Carabus auratus | 0·703 | 17·4 |
| Nebria brevicollis | 0·046 | 25·3 |
| Melolontha vulgaris | 0·940 | 14·3 |
| Anomala Frischii | 0·153 | 24·3 |
| Bombus terrestris | 0·381 | 14·9 |
| Apis mellifica | 0·090 | 23·5 |
One obvious result is that within the class of Insects the relative muscular force (as commonly understood) is approximately in the inverse proportion of the weight—that is, the strength of the Insect is (by this mode of calculation) most conspicuous in the smaller species.
In a later memoir[89] Plateau gives examples from different Vertebrate and Invertebrate animals, which lead to the same general conclusion.
Ratio of weight drawn to weight of body (Plateau).
| Horse | ·5 to ·83 | |
| Man | ·86 | |
| Crab | 5 | ·37 |
| Insects | 14 | ·3 to 23·5 |
The inference commonly drawn from such data is that the muscles of small animals possess a force which greatly exceeds that of large quadrupeds or man, allowance being made for size, and that the explanation of this superior force is to be looked for in some peculiarity of composition or texture. Gerstaecker,[90] for example, suggests that the higher muscular force of Arthropoda may be due to the tender and yielding nature of their muscles. An explanation so desperate as this may well lead us to inquire whether we have understood the facts aright. Plateau’s figures give us the ratio of the weight drawn or raised to the weight of the animal. This we may, with him, take as a measure of the relative muscular force. In reality, it is a datum of very little physiological value. By general reasoning of a quite simple kind it can be shown that, for muscles possessing the same physical properties, the relative muscular force necessarily increases very rapidly as the size of the animal decreases. For the contractile force of muscles of the same kind depends simply upon the number and thickness of the fibres, i.e., upon the sectional area of the muscles. If the size of the animal and of its muscles be increased according to any uniform scale, the sectional area of a given muscle will increase as the square of any linear dimension. But the weight increases in a higher proportion, according to the increase in length, breadth, and depth jointly, or as the cube of any linear dimension.[91] The ratio of contractile force to weight must therefore become rapidly smaller as the size of the animal increases. Plateau’s second table (see above) actually gives a value for the relative muscular force of the Bee, in comparison with the Horse, which is only one-fourteenth of what it ought to turn out, supposing that both animals were of similar construction, and that the muscular fibres in both were equal in contractile force per unit of sectional area.[92]