156 THE COCKROACH : 



Mechanism of Respiration. 



In animals with a complete circulation, aerated blood is dif- 

 fused throughout the body by means of arteries and capillaries, 

 which deliver it under pressure at all points. Such animals 

 usually possess a special aerating chamber (lung or gill), where 

 oxygen is made to combine with the haemoglobin of the blood. 

 It is otherwise with Insects. Their blood escapes into great 

 lacunre, where it stagnates, or flows and ebbs sluggishly, and 

 a diffuse form of the internal organs becomes necessary for 

 their free exposure to the nutritive fluid. The blood is not 

 injected into the tissues, but they are bathed by it, and the 

 compact kidney or salivary gland is represented in Insects by 

 tubules, or a thin sheet of finely divided lobules. By a 

 separate mechanism, air is carried along ramified passages to 

 all the tissues. Every organ is its own lung. 



We must now consider in more detail how air is made to 

 enter and leave the body of an Insect. The spiracles and the 

 air-tubes have been described, but these are not furnished with 

 any means of creating suction or pressure ; and the tubes 

 themselves, though highly elastic, are non-contractile, and 

 must be distended or emptied \>y some external force. Many 

 Insects, especially such as fly rapidly, exhibit rhythmical move- 

 ments of the abdomen. There is an alternate contraction and 

 dilatation, which may be supposed to be as capable of setting 

 up expirations and inspirations as the rise and fall of the 

 diaphragm of a Mammal. In many Insects, two sets of 

 muscles serve to contract the abdomen viz., muscles which 

 compress or flatten, and muscles which approximate or tele- 

 scope the segments.* In the Cockroach the second set is feebly 

 developed, but the first is more powerful, and causes the terga 

 and sterna alternately to approach and separate with a slow, 

 rhythmical movement ; in a Dragon-fly or Humble-bee the 

 action is much more conspicuous, and it is easy to see that the 

 abdomen is bent as well as depressed at each contraction. No 

 special muscles exist for dilating the abdomen, and this seems 

 to depend entirely upon the elasticity of the parts. It was 



* This subject is treated at greater length in Prof. Plateau's contribution on 

 Respiratory Movements of Insects. (Infra, p. 159.) 



