80 ENTOMOLOGY 



increase in strength lags behind that of weight more and more; conse- 

 quently 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. 113). In all adult insects, however, the originally separate 

 ganglia consolidate more or less (Fig. 114) and the commissures fre- 

 quently unite to form single cords. Thus in Tabanus (Fig. 1 14, 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 subcesophageal 

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

 to be absent). 



The brain, or supra&sophageal ganglion (Fig. 115), is formed by the 

 union of three primitive ganglia, or neuromeres (Fig. 57), namely, (i) 

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

 cerebrum, which innervates the antennae; and (3) the tritocerebrum, 

 which in Apterygota bears a pair of rudimentary appendages that are 

 regarded as traces of a second pair of antennae. 



