§ 329, 330. THE INSECTA. 409 



The ventral ganglia are usually small, coequal, and give off no nerves 

 except to the segments of the abdomen. The last ventral ganglion, alone, 

 is larger, for it furnishes, in addition, nervous filaments to the rectum, and 

 to the excretory ducts of the genital organs. 



Usually, the nerves arise in the ganglia by two or three principal roots. 

 Some nerves, however, arise from the interganglionic cord. In those 

 species where the ventral ganglia are entirely fused together, the nerves 

 arise close together, but immediately diverge in different directions. 



§ 329. 



As to the Intimate Structure of the nervous system of the Insecta, 

 both the nerves and the ganglia are always surrounded by a fibrous neu- 

 rolemma, and, according to carefully-made researches, are never wanting in 

 the two usual anatomical components. Between the extremely tenuous 

 primitive fibres, are interposed, in the ganglia, very small globules. These 

 last, nucleolated, usually contain also a finely-granular substance, colorless, 

 though sometimes reddish or brownish. ^^' 



There are, with the Insecta, as with the Crustacea, two modes of the dis- 

 position of the fibres in the ganglia. The first, which form nervous, infe- 

 rior cords, are disseminated in the ganglia ; while the others, which belong 

 to the superior nerves, simply pass through or over these ganglia. These 

 two kinds of fibres give off, laterally, nervous filaments, which, uniting, 

 form peripheric nerves of a mixed character. The superior cords corre- 

 spond, probably, to the motor nerves, and the inferior to the sensitive nerves 

 of- the Vertebrata.'-' 



§ 330. 



The Disposition of the nervous system in the various orders of Insecta 

 presents the following differences :^^> 



Among the Aptera, the ventral cord of the Pediculidae is composed of 

 three contiguous ganglia situated in the thorax. The prothoracic gan- 

 glion connects with the brain, and the metathoracic sends nerves to the ab- 

 domen.^-' The nervous system of the Poduridae differs from this, in their 

 three thoracic ganglia being separated, and their interganglionic longitudi- 

 nal commissures being wide apart. ^^^ More widely difterent still, is the 

 ventral cord of the Lepismidae ; it is composed of eleven ganglia connected 

 by double longitudinal commissures.'^' 



With the Hemiptera, this system is limited to two thoracic ganglia, of 

 which the ajnterior is the smaller. With Pentatoma, and Cicada, these 

 two ganglia are not separated except by a constriction ; while, with Nepa, 



1 It was undoubtedly from their extreme deli- p. 389, PI. XIII.-XVU. and Cyclopaed. loc. cit. p. 



cateness that these ganglionic globules were over- 946). Hagen (Entom. Zeit. 1844, ]). 364) has since 



looked by Treviranus (Beitriig. zur AufklSr. d. observed it with Aeschna grandis and Gryllo- 



Erscheiuung. u. Gesetze d. organisch. Lebens. I. talpavulsaris. 



Hft. 2, p. 62). They have been distinctly seen by 1 Various and special accounts of the general 



Elirenberg (Unerli. Struct. &c. p. 56, Taf. VI. fig. disposition of the nervous system of insects may 



&(fientrupes)), Pappenheim (Die specielle Gewebe- be found in Cuvier, Lecons, &c., III. 1845, p. 334. 



lehre d. Gehororg. p. 51), Helmholtz (De fabric. 2 Swammerdamm, IJib. der Natur. p. 36, Taf. 



Syst. nat. &c. p. 21), Hannover (Recherch. II. fig. 7. 



microscop. &c. p. 71, PI. VI. fig. 81, 82 {Aeschna)) s Nicolet, loc. cit. p. 44, PI. IV. fig. 1 (Smyn- 



and Will {Mailer's Arch. 1844, p. 81). thurus). 



'i Ttiis difference of the nervous cords was first 4 Treviranus, Verm. Sclirift. II. lift. 1, p. 17, 



pointed out by i\ewport Willi the pupa and imago Taf. IV. fig. 3. 

 of Sphinx ligusiri (Philos. Trans. 1834, part 11. 



35 



