I METAMORPHOSES OF INSECTS 25 



the perfect insect, so also portions of the nervous 

 structures, the disposition of which is in remarkable 

 relation to the segments, become concentrated to 

 perform the perfected functions of this wondrous 

 system. Instead of the nervous matter being almost 

 equally distributed to every segment, the ganglia 

 of the different segments approach and become 

 aggregated, the cords are shortened and enlarge into 

 fresh trunks, and nerves alter in position and unite 

 into one bundle, changes that are brought about and 

 rendered necessary by the other changes that take 

 place in the body. This concentration is most 

 marked in insects that undergo complete metamor- 

 phoses. The greater proportion of the nervous 

 masses is removed to the head and thorax, appar- 

 ently as a means of concentrating the energies of 

 these particular regions. In some insects especially, 

 the ganglia are usually aggregated together in certain 

 segments, in the part of the body that is very actively 

 employed. A most curious union and complexity 

 occurs in the distribution of nerves to the wings. 

 These, the most powerful and constantly employed 

 organs, must act not only with energy, but in per- 

 fect unison with each other, and hence must be 

 supplied with power from the same centre. The 

 union exists in many insects of rapid or long- 

 continued flight, while in heavy fliers, or those 

 accustomed to exert themselves for but short dis- 

 tances, such combination is absent. The ganglia 

 which form the brain are small in the larva in 

 relation to the size of the head. They increase in 

 the pupa, and attain their full development near the 



