Chap. XIII.] THE COCKROACH. 269 



the abdomen, though the thoracic region in the cockroach also 

 undergoes a slight change of form. In expiration the terga 

 are flattened and depressed, the sterna being at the same time 

 slightly raised. Opposite movements effect inspiration. The 

 movements of expiration and inspiration may readily be watched 

 in a tired bee. It must not be supposed that the tracheae are 

 alternately filled and completely emptied during this process. 

 But little air enters or leaves the body ; in the ultimate ramifi- 

 cations of the vessels carbonic acid gas is exchanged for oxygen 

 by a process of diffusion. 



The Nervous System. The central nervous system (Fig. 82, 

 D.) is, in the cockroach as in the crayfish, ventral, and consists 

 of a double- chain, the coalesced ganglia of which are united by 

 longitudinal connectives. 



The coalescent cerebral ganglia (brain) are shown in Fig. 82. 

 There are two large hemispheres (h. s.) from which the optic 

 nerves (op.) pass off to the eyes. Small nerves proceed from this 

 part of the brain to the white spot (n. w. s.). Below the hemi- 

 spheres are the antennary lobes (an. /.), from which large anten- 

 nary nerves (an.) are given off. Below these again a nerve is given 

 off from the commencement of the commissure on each side, and 

 these unite in a frontal ganglion (/. g.) which lies on the oesopha- 

 gus ; thence the median recurrent nerve passes upwards and back- 

 wards under the hemispheres, and is seen in the dorsal view 

 shown at C. (r. n.) 



Large commissures pass on either side of the gullet from the 

 cerebral ganglia to the sub-oesophageal ganglia, which also lie in 

 the head. From their ventral aspect nerves are given off to 

 the mandible, maxillae, and labium. From these ganglia com- 

 missures (con.) pass backward through the neck to the first 

 of the three coalescent pairs of the thoracic region, which inner- 

 vate respectively the pro- meso- and metathorax and their ap- 

 pendages. The nerves of the wings have a Y-shaped junction 

 with the nerve-chain, as shown in Fig. 82, D. In the abdomen 

 there are six pairs of closely-united ganglia, the last being large. 

 The abdominal commissures of the male are looped. 



