INSECTA. 209 



The nervous system of the chaffer {Melolontha) has been dissected 

 and delineated by Strauss with a minuteness and accuracy second 

 only to those of Lyonnet. In these beautiful plates * are shown the 

 bilobed brain with its auxiliary ganglia for the eyes and antennae ; 

 the storaato-gastric nerves and their small lateral cephalic ganglia 

 are also clearly exhibited. The sub-oesophageal or maxillary ganglion 

 is of an oblong form ; the brachial ganglion is triangular ; the elytral 

 ganglion is of a circular, and the alar ganglion of a pyriform, figure ; 

 these two latter being concentrated into almost a single mass, and 

 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 »f concentration of the nervous system is pre- 

 sented in the insects of the Hemipterous order. 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, re- 

 ceiving 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 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 Lepi- 

 dopterous insect undergoes in its metamorphoses from the larval into 

 the perfect state, have been beautifully and accurately illustrated by 

 Heroldt in the cabbage butterfly, and by Mr. Newport in a species 

 of sphynx : but Lyonnet had anticipated both these observers, in re- 

 cognising as well the principle as the details of these changes, which 

 he briefly describes at the termination of the monograph already 

 quoted. 



The twelve ventral ganglions of the larva (^fig. 103.) are sub-equal 

 and, except the two last, at regular distances ; in the pupa the inter- 

 ganglionic columns are shorter, but the body, becoming still more 

 abbreviated and concentrated, throws those columns into curved lines. 

 The eleventh and twelfth ganglions coalesce ; the sixth and seventh 



* Here the work, entitled, " Considerations generales sur I'Anatomie Comparee 

 des Animaux Articules," &c., 4to. 1828. was shown. 



P 



