120 ESSENTIALS OF ZOOLOGY 



to the exterior, spiracles. The spiracles are provided with 

 valves which render the opening crescentic. In most insects 

 the spiracles are protected by setae from the ingress of dust. 

 The ten spiracles of the cockroach lead into tubes which 

 anastomose at once to form on each side a long lateral tube. 

 From these other tubes emerge which divide, part going 

 dorsally and part ventrally, and splitting up into innumerable 

 branches which ultimately end blindly in every part of the 

 body. The tracheae are lined by a chitinous cuticle strength- 

 ened by a fine spiral thickening, but their ultimate branches, 

 the tracheoles, have no such lining. The air is thus carried to 

 all the organs and tissues, respiratory gases being exchanged 

 at the tracheoles, and the blood is therefore relieved of the 

 office of carrying the oxygen from a respiratory organ to the 

 distant parts of the body. The air contents of the tracheae 

 are renewed by respiratory movements. The contraction of 

 the muscles of the abdomen expels the air, and inspiration 

 takes place with their relaxation. The action may be seen in 

 the case of bees on the alighting board of a hive. 



Circulatory System. The blood is colourless and is provided 

 with leucocytes. It is spread in undefined spaces all over the 

 body and appendages. The blood is circulated by a heart, 

 which with the anterior aorta are the only vessels. The 

 heart is a long vessel extending under the dorsal skin of the 

 thorax and abdomen. It is provided with thirteen pairs of ostia 

 opening into as many chambers of the heart. Each ostium 

 is provided with valves. The blood is forced out of the heart 

 anteriorly into a vessel directly continuous with it, and this 

 vessel, the anterior aorta, opens at once into the cavity of the 

 head. The action of the heart in removing the blood around 

 it draws a further supply into the pericardial space, which is 

 somewhat defined from the rest of the blood spaces of the body 

 by a septum, and the blood of the body is thus attracted to 

 flow towards the pericardial sinus. During its circulation it 

 obtains food absorbed from the enteron, which it conveys to the 

 cells throughout the body, and it obtains from them the waste 

 material from which it is relieved by the Malpighian tubules. 



Nervous System. The brain, or cerebral ganglion, is highly 

 developed in the cockroach, as it is in insects in general. It 



