ol 



FIG. in. Central 

 nervous system of a thy- 

 sanuran, Machilis. The 

 thoracic and abdominal 

 ganglia are numbered in 

 succession. a, antennal 

 nerve; b, brain; e, com- 

 pound eye; I, labial nerve; 

 m, mandibular nerve; 

 mx, maxillary nerve; o, 

 oesophagus; ol, optic lobe; 

 s, subcesophageal gang- 

 lion ; sy, sympathetic 

 nerve. After OUDEMANS. 



ENTOMOLOGY 



cerebrum, which innervates the antennae; and (3) 

 the Iritocerebrum, which in Apterygota bears a pair 

 of rudimentary appendages that are regarded as 

 traces of a second pair of antennae. 



The subcesophageal ganglion (Fig. 113) is 

 always connected with the brain by a pair of 

 nerve cords (cesophageal commissures) between 

 which the oesophagus passes. This compound 

 ganglion represents at most four neuromeres: (i) 

 mandibular, innervating the mandibles; (2) super - 

 lingual, found by the author in Collembola, but 

 not yet reported in the less generalized insects; 

 (3) maxillary, innervating the maxillae; (4) labial, 

 which sends a pair of nerves to the labium. 



The minute structure of the brain, though 

 /highly complex, has received considerable study, 

 but will not be described here for the reason that 

 the anatomical facts are of no general interest so 

 long as their physiological interpretation remains 

 obscure. 



Sympathetic System. Lying along the me- 

 dian dorsal line of the oesophagus is a recurrent, 

 or stomatogaslric, nerve (Fig. 114), which arises 

 anteriorly in a frontal ganglion and terminates 

 posteriorly in a stomachic ganglion situated at the 

 anterior end of the mid intestine. Connected 

 with the recurrent nerve are two pairs of lateral 

 ganglia, the anterior of which innervate the dorsal 

 vessel and the posterior, the tracheae of the head. 

 The ventral nerve cord may include also a median 

 nerve thread (Fig. in) which gives off paired 

 transverse nerves to the muscles of the spiracles. 



Structure of Ganglia and Nerves. A gang- 

 lion consists of (i) a dense cortex, composed of 

 ganglion cells (Fig. 115), each of which has a 

 large rounded nucleus 'and gives off usually a 

 single nerve fiber; and (2) a clear medullary 

 portion (Punktsubstanz) derived from the pro- 

 cesses of the cortical ganglion cells and serving as 

 the place of origin of nerve fibrillae. There are, 



