960 



INSECTA. 



Lamellicornes, as shown in Swammerdam's 

 drawing of Oryctes,* and Burmeister has 

 delineated a like condition in the larva of 

 Calandra Sommeri.\ We have before seen 

 (fig- 405) that such is also the case in the brain 

 of Timarcha. From this it would appear that 

 the cerebral ganglia are the parts of the nervous 

 system last perfected in the larva, while it is 

 interesting to observe that the reverse is the case 

 as respects the terminal ganglion; that part, 

 as correctly remarked by Professor Grant,} 

 being the first to advance forwards and become 

 united to the penultimate ganglion, to form 

 the great caudal mass, at an early period of the 

 larva. This is evident in the Timarcha, in 

 which the eleventh and twelfth ganglia have 

 coalesced into a single mass, while the cerebral 

 ganglia only just meet above the oesophagus. 

 But this is not the case in the perfect insect, 

 (jig. 408 ; ) in which the cerebral ganglia (A) 



Fig. 408. 



Nervous system of perfect state of Timarcha tetie- 

 bricosa. 



A, cerebral mass or brain j B, optic nerves; C, 

 origin of sympathetic. 



have become greatly enlarged, united together, 

 and represent a distinct brain, from which pro- 

 ceed the nerves of sense, and united by long 

 crura to the medulla oblongata (1), from which 

 are given off as before the nerves of the organs 



* Biblia Nat. tab. xviii. fig. 1. 



t Zur Naturgeschichtc der Gattung Calandra, 

 fig. 13, Berlin, 1837. 



t Phil. Trans. 1832, part ii. p. 384. Outlines of 

 Comparative Anatomy, p. 193. 



of manducatlon. The cords are enlarged, as 

 also are the three ganglia of the thorax and 

 their nerves, and a coalescence has taken place 

 between the fourth, fifth, and sixth ganglia, 

 their intervening cords being entirely obliterated, 

 and the nerves aggregated together are now 

 derived from one mass, the great meta-thoracic 

 ganglion, the last part of the nervous system in 

 the thorax. A similar change haS also taken 

 place in the remaining part of the cord and 

 ganglia, which now forms the abdominal por- 

 tion. The cord between each of the ganglia 

 has been shortened, and the tenth, eleventh, 

 and twelfth are united and form the caudal 

 mass, which is situated about half way across 

 the abdomen. Such are the changes that take 

 place in the nervous system of an inferior type 

 of Coleoptera; in the higher forms, as in the 

 Melolontliida, the series of approximated 

 ganglia does not extend beyond the middle of 

 the meta-thorax, the cords being terminated by 

 a kind of cauda equina, all the nerves that go 

 to the abdomen are aggregated together and 

 extended into that region over the post-furca. 

 A similar structure, but in a less complete form, 

 exists in the Dytiscida, in which, as in the 

 Hydaticus cinereus, a short cord is found with 

 seven constrictions upon it, corresponding to 

 that number of ganglia which probably 

 existed in the larva state, but have nearly dis- 

 appeared during the metamorphoses. But that 

 this concentrated form of the nervous system is 

 not necessarily connected with high develop- 

 ment of other parts of the body is further 

 shown in the common Earwig, Forficula auri- 

 cularia, in which there are ten distinct sub- 



Fig. 409. 



Nervous system of the Earwig ( Forficula auricularia). 



oesophageal ganglia. The first, or medulla, is 

 large and closely connected by very short crura 

 with the brain, there being only a narrow 



