NSECTA 



391 



these and passes into an anterior aorta which opens into the general 

 body cavity in the head region. The haemocoelic body cavity is very 

 spacious and the blood bathes all the organs. There is a dorsal hori- 

 zontal diaphragm perforated by many holes, which separates off the 

 pericardium in which the heart lies, and attached to this are paired 

 alary muscles, the outer ends of which are inserted in the terga (Fig. 

 292). By their contraction the passage of blood from the body 

 cavity into the pericardium and heart is facilitated. The circulatory 

 system is primitive compared with that of the decapod Crustacea and 

 much closer to that of the phyllopods (see p. 311). It is doubtful 

 whether there is usually any respiratory pigment in the blood, which 

 may, however, contain pigments, such as chlorophyll, derived from 

 the food and, in the bloodworm {Chironomus), haemoglobin. It also 

 contains leucocytes and, associated with it, are various cellular tissues 

 such as the fat body^ the oenocytes and nephrocytes and in various 

 beetles the photogenic organs. 



Fig. 292. Transverse section through dorsal part of the abdomen of Apis 

 ynellifica to show attachment of heart to the body wall and to the diaphragm 

 by the alary muscles (al.m.). After Snodgrass. (The insertion of the alary 

 muscles in the tergum is not shown.) dg. diaphragm ; f.b. fat body; h. heart; 

 mg. mid gut; mt. Malpighian tubule; tra. trachea. 



In the insects the tracheal system characteristic of terrestrial Arthro- 

 poda attains its most complete development. The ectodermal tubes 

 of the system form a network of which every part is in communication 

 with every other part. Typically it communicates with the exterior 

 by two pairs of openings called stigmata or spiracles on the thorax and 

 eight pairs on the abdomen (Fig. 293). The main branches leading 

 from the stigmata not only divide into finer capillaries leading to the 

 adjacent organs but communicate by means of lateral trunks with each 

 other. The capillaries or tracheoles never end blindly in the blood but 

 always in the cells of the body, whether muscular or glandular or 

 connective tissue, so that the oxygen is conveyed directly to the latter 

 without the intervention of the blood. These end tubes, as may be 

 seen in Fig. 294, are of the smallest calibre. The chitinous lining, which 

 in the main tracheae is strengthened, forming the spiral threads which 

 prevent collapse of the tubes, in the tracheoles is thinned down so 



