THE XEBVOU5 STSTE3I, 



33 



sternal, epistemal, and epimeral pieces, freely articulated 

 together, correspond in function with the sternum, the ribs, 

 and the clavicles of birds.* The thorax is expanded and C':>n- 

 tracted at each motion of the 

 wings, as in birds and other ani- 

 mals, and becomes fixe<l at each 

 increased eflbrt as a fulcrum or 

 point of resistance upon which 

 the great muscles of the wings 

 are to act, thus identifying this 

 part of the body in function as 

 in structure with that of other ani- 

 mals." (Newport.) 



The Xeevous Systeji. In its 

 simplest fonn the nervous system 

 consists of two longitudinal cords, 

 each with a swelling (nerve-knot, 

 or ganglion.) correspnjnding to 

 each segment (Fig. 43). This 

 cord lies on the ventral side of the 

 b«xly. but in the head it passes 

 upwards, sending a filament from 

 each side to siuround the oesoph- 

 agus.! -A^s in the Vertebrates, 

 the nervous cord of insects is 

 composed of two distinct columns 

 of fibres placed one upon the other, 

 column, which is nearest to the exterior of the body, is that in 

 which the ganglia, or enlargements, are situated. The upper 

 one, or that which is internal and nearest to the viscera, is 

 entirely without ganglia, and passes directly over the ganglia 

 of the under coliman without forming part of them, but in ver\- 



* Bennet on the Anatomy of the Thorax in Insects, and its Fnnction during 

 Flight. Zoological Joamal. vol. i. i>. 3»4. 



t The brain of inse^'ts is fomieil of several pairs of ganglia, corresponding, 

 probably, to the number of primitive segments composing the head. The nc rroas 

 conl is thus, in the head, massed together and eompacteil to form a brain. 



Fig. 43, Servous System of Co-rydalHS eontutus. a, ■'cerebrum;'' b, "cere- 

 brelhim ;^ c. thoracic ganglia, which distribute a nerve to each leg: d. eight pairs 

 of abdominal ganglia. The dotted lines represent the wrings. — Frcn Lfi. 'y. 



3 



Fig.ti. 



The under or eternal 



