ANATOMY AND PHYSIOLOGY 71 



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 in- 

 crease in strength lags behind that of weight more and more; consequently 

 more and more strength is required simply to move the insect itself, and 

 less and less surplus strength remains for carrying additional weight. 

 Thus the larger insect is apparently the weaker, though it is actually the 

 stronger, in that its total muscular force is greater. 



The writer uses this explanation to account also for the inability of 

 certain large beetles and other insects to use their wings, though these 

 organs are well developed. Increasing weight (due to a larger supply of 

 reserve food accumulated by the larva) has made such demands upon the 

 muscular power that insufficient strength remains for the purpose of flight. 



Statements such as this are often seen a flea can jump a meter, or 

 six hundred times its own length. Almost needless to say, the length of 

 the'body is no criterion of the muscular power of an animal. 



4. NERVOUS SYSTEM 



The central nervous system extends along the median line of the floor 

 of the body as a series of ganglia connected by nerve cords. Typically, 

 there is a ganglion (double in origin) for each primary segment, and the 

 connecting cords, or commissures, are paired; these conditions are most 

 nearly realized in embryos and in the most generalized insects Thysa- 

 nura (Fig. in). In all adult insects, however, the originally separate 

 ganglia consolidate more or less (Fig. 112) and the commissures frequently 

 unite to form single cords. Thus in Tabanus (Fig. 112, C) the three 

 thoracic ganglia have united into a single compound ganglion and the 

 abdominal ganglia are concentrated in the anterior part of the abdomen; 

 in the grasshopper, the nerve cord, double in the thorax, is single in the 

 abdomen. Various other modifications of the same nature occur. 



Cephalic Ganglia. In the head the primitive ganglia always unite 

 to form two compound ganglia, namely, the brain and the suboesophageal 

 ganglion (disregarding a few anomalous cases in which the latter is said 

 to be absent). 



The brain, or supracesophagcal ganglion (Fig. 113), is formed by the 

 union of three primitive ganglia, or neuromeres (Fig. 55), namely, (i) 

 the protocerebrum, which gives off the pair of optic nerves; (2) the deuto- 



