366 



LECTURE XVI. 



radiating the nerves to the abdomen, like the termination of the spinal 

 marrow called cauda equina. The two median nerves of this series 

 chiefly supply the organs of generation. 



The three thoracic ganglia are blended together into one mass in 

 the Diptera ; and only two ganglions are developed on the abdominal 

 portion of the ventral chords. 



The greatest degree of concentration of the nervous system is pre- 

 sented in the insects of the Strepsipterous and Hemipterous orders. 

 In the Nepa or water-scorpion, for example, only three ganglions are 

 present in its nervous system. The first, or brain, consists of two 

 pyriform lobes in contact by their base. The maxillary ganglion is 

 square-shaped, receiving the oesophageal chords at its anterior angles, 

 and sending back their continuations from its posterior angles ; these 

 continue parallel with each other to the thorax, there expanding 

 into a large rounded ganglion, much more voluminous than the brain, 

 and from which radiate the nerves supplying all the rest of the body. 

 In the water-bug {Ilanatra linearis) a like concentrated condition of 

 the nervous centres prevails : the super-cesophageal mass resembles 

 that in Nepa; the sub-oesophageal ganglion {Jig. 147, a) is rounded ; 

 two long and slender columns connect it with a large 147 



4-lobed ganglion (b) situated at the end of the thorax. w. 

 From each side of this ganglion are given off three nerves 

 which diverge as they proceed to the brachial hemelytral 

 and alar muscles ; from the lower part of the ganglion, 

 which is subfusiform in shape, are sent off two bundles 

 of delicate nerves (c), which pass down into the lengthened 

 abdomen to supply the parts there situated. 



In certain Coleoptera, Hymenoptera, and Diptera, the 

 principal changes which the nervous system undergoes 

 in the progress to the imago state are the acquisition of 

 ganglions not present in the larva. 



The progressive changes which the nervous system 

 of the Lepidopterous insect undergoes in its metamor- 

 phoses from the larval into the perfect state, have been 

 beautifully and accurately illustrated by Herold * in the 

 cabbage butterfly, and by Newport f in a species of 

 sphynx; but Lyonnet had anticipated both these ob- 

 servers, in recognising as well the principle as the de- 

 tails of these changes, which he briefly describes at the 

 termination of the monograph already quoted.^ Ranatra linearis 



The twelve ventral ganglions of the larva (Jig. 159) are sub-equal, 



* CCXLI. 



t CCXLIII. 



t CCXL. 



