HA R D WICKE 'S S CIENCE- GOS SIP. 



207 



In animals with a complete circulation, aerated 

 blood is diffused 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 hemoglobin of 

 the blood. It is otherwise with insects. Their 

 blood escapes into great lacunae, 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 theyare bathed by it, 

 and the compact kidney or salivary gland is repre- 

 sented 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. 

 Eveiy 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 by some external force. I\Iany 

 insects, especially such as fly rapidly, exhibit 

 rhythmical movements 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 telescope the segments. In the cockroach the 

 second set is feebly developed, but the first are more 

 powerful, and cause 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 long supposed 

 that, when the abdomen contracted, air was expelled 

 from the body and the air-passages emptied ; that 

 when the abdomen expanded again by its own 

 elasticity, the air-passages were refilled, and that no 

 other mechanism was needed. Landois pointed out, 

 however, that this was not enough. Air must be 

 forced into the furthest recesses of the tracheal 

 system, where the exchange of oxygen and carbonic 

 acid is effected more readily than in tubes lined by a 

 dense intima. But in these fine and intricate 

 passages the resistance to the passage of air is 

 enormous, and the renewal of the air could, to all 

 appearance, hardly be effected at all, if the inlets 

 remained open. Landois accordingly searched for 

 some means of closing the outlets, and found an 

 elastic ring or spiral, which surrounds the tracheal 

 tube within the spiracle. By means of a special 



muscle, this can be made to compress the tube, like 

 a spring-clip upon a flexible gas-pipe. When the 

 muscle contracts, the passage is closed, and the 

 abdominal muscles can then, it is supposed, bring 

 any needful pressure to bear upon the tracheal tubes, 

 much in the same way as with ourselves, when we 

 close the mouth and nostrils, and then, by forcible 

 contraction of tlie diaphragm and abdominal walls, 

 distend the cheeks or pharynx. Landois describes 

 the occluding apparatus of the cockroach as com- 

 pletely united with the spiracle. It consists of two 

 curved rods, the "bow" and the "band," one of 

 which forms each lip of the orifice. From the 

 middle of the band projects a blunt process for the 

 attachment of the occlusor muscle, which passes 

 thence to the extremity of the bow. The concave 

 side of each rod is fringed with setae, and turned 

 towards the opening, which lies between the two.* 



The mechanism described by Landois undoubtedly 

 exists, and his explanation of its action has been 

 generally accepted. It may be doubted, neverthe- 

 less, whether he has taken sufficient account of the 

 difficulty of forcing air by mechanical means into 

 tortuous and incredibly minute tubules. Inexpert 

 dissectors know well the annoyance caused by the 

 presence of an air-bubble in the injection syringe. 

 The air will not pass along the vessels, though 

 collapsed and empty, but blocks them up, and, if 



Fig. 125.— Abdominal Spiracle of Cockroach (leftside). 

 X about 35. 



further pressure be applied, bursts them. It is 

 equally difficult to withdraw air from fine passages. 

 If the body of an insect be opened, and placed in 

 the receiver of an air-pump, the trachea cannot be 

 completely emptied of air without much time and 

 exertion. The tenacity with which air adheres to 

 closely approximated surfaces is so great as to con- 

 stitute an apparently insuperable difficulty in the way 

 of distension of the tracheal system by any muscular 

 arrangement whatsoever. If unlimited force could be 

 applied, the air could not be made to fill the delicate, 

 branched tubes ; it would either shrink to a small 

 portion of its original volume, or burst the tissues. 



It is worthy of remark that in the Chilopoda, which 

 breathe by tracheal tubes, and whose respiratory 



* Landois und Thelen, Zeitsch. f. wiss. Zool. Bd. xvii. (1867). 



