128 RESPIRATION 



Respiratory Organs. 



The respiration of Insects is carried on by means of a system 

 of vessels for the conveyance of air to all parts of the body ; this 

 system is most remarkably developed and elaborate, and contrasts 

 strongly with the mechanism for the circulation of the blood, 

 which is as much reduced as the air system is highly developed, 

 as well as with the arrangement that exists in the Vertebrates. 

 There are in Insects no lungs, but air is carried to every part of 

 the body directly by means of tracheae. These tracheae con- 

 nect with the spiracles the orifices at the sides of the body 

 we have already mentioned when describing the external struc- 

 tures and the air thus finds its way into the most remote 

 recesses of the Insect's body. The tracheae are all intimately 

 connected. Large tubes connect the spiracles longitudinally, 

 others pass from side to side of the body, and a set of tracheae 

 for the lower part of the body is connected with another 

 set on the upper surface by means of several descending 

 tubes. From these main channels smaller branches extend in all 

 directions, forking and giving off twigs, so that all the organs 

 inside the body can be supplied with air in the most liberal 

 manner. On opening a freshly deceased Insect the abundance of 

 the tracheae is one of the peculiarities that most attracts the 

 attention ; and as these tubes ' have a peculiar white glistening- 

 appearance, they are recognised without difficulty. In Insects 

 of active flight, possibly in some that are more passive, though 

 never in larvae, there are air-sacs, of more than one kind, con- 

 nected with the tracheae, and these are sufficiently capacious to 

 have a considerable effect in diminishing the specific gravity of 

 the Insect. The most usual situation for these sacs is the basal 

 portion of the abdominal cavity, on the great lateral tracheal 

 conduits. In speaking of the external structure we have remarked 

 that the stigmata, or spiracles, by which the air is admitted 

 are very various in their size and in the manner in which they 

 open and close. Some spiracles have no power of opening ; while 

 others are provided with a muscular and valvular apparatus for 

 the purpose of opening and closing effectually. 



The structure of the tracheae is remarkable : they are elastic 

 and consist of an , outer cellular, and an inner chitinous layer ; 

 this latter is strengthened by a peculiar spiral fibre, which gives 



