140 OUR HOUSEHOLD INSECTS 



situated in the abdomen, and the other two in the 

 thorax. Those belonging to the abdomen are not very 

 easy to detect, as they are small, and all but one pair 

 obscurely situated. The chitinous integument which 

 bounds and determines the form of each segment of the 

 abdomen is not a complete ring round the body, but 

 consists of two distinct parts, a band across the back, 

 called the tergum, and another underneath, the sternum. 

 These are united towards their edges by a membranous 

 junction; and it is in this, at the junction of the 

 segments,, that the stigmata lie, concealed by the over- 

 lapping edges of the terga and 

 sterna. Bach is an oval aperture 

 situated on the summit of a small 

 conical eminence (Fig. 42), and 

 capable of being closed by an 

 internal valve, whereby dust and 

 other foreign matters are excluded. 

 FIG. 42. Abdominal Spir- A large tracheal tube proceeds from 



acle of Cockroach. -, -, , -, 



each, and very soon begins to sub- 

 divide into smaller ones, the ultimate ramifications of 

 which pass to the remotest parts of the body, and even 

 into the jaws, wings, legs, and antennae. By means of 

 this system of tubes, air is conveyed to all parts of the 

 organism, so that the blood is aerated, not, as in 

 most kinds of animals, by being brought from the body 

 at large and collected in some special organ, such as a 

 lung or a gill, there to come into contact with the air, 

 but by having the air conveyed to it in all parts of the 

 body at once. 



The introduction and expulsion of air is, of course, 

 accompanied by movements of the body walls ; but these 

 are not very easy to see, as they are but slight in 

 amount. Plateau succeeded in demonstrating their 



