



CH. Ill] THE COCKROACH 85 



All, except some aquatic insects, exhibit dorso- ventral 

 flattening of the abdomen in expiration, but in some species 

 other movements are equally, if not more, evident. Dragon- 

 flies for example diminish the transverse horizontal di- 

 mension of the abdomen strongly, while the Aculeate 

 Hymenoptera alternately shorten and lengthen the body 

 by telescoping the segments more or less. 



Besides the special respiratory movements, the normal 

 contraction of the muscles of the body and limbs in 

 locomotion must assist in the movement of the air within 

 the body; every muscular contraction causes a pressure 

 upon the tracheal tubes in that region, and forces the air 

 along the tube, perhaps both outwards and inwards, and 

 to some extent assists in the respiratory interchanges. 



The true respiratory movements are entirely reflex 

 actions, persisting in the decapitated insect and even 

 in the detached abdomen, as may often be noticed in 

 a wasp bisected on a dinner plate, and in other species 

 where the nervous system has not undergone concentration. 

 In such cases the movements are hastened or retarded 

 by the same causes (heat, cold, and so on) as hasten or 

 retard them in the intact insect. 



Circulation. The direct supply of air to the tissues 

 of the body relieves the blood from the duty of convey- 

 ing oxygen and carbon dioxide, and thus the relative 

 insignificance of the blood system receives an explanation. 

 The blood is colourless and is kept flowing through the 

 irregular channels and spaces among the tissues by the 

 contractions of the dorsally situated, chambered, tubular 

 heart, which receives the blood through a pair of valved 



