OF THE TRACHE.E. 193 



they happen to be situated, as the right spiracle of the 

 prothorax]or prothoracis spiraculum dextrum : the fact 

 of a segment possessing or not possessing spiracles is of 

 importance in classification. In the figure of the caterpillar 

 of the privet moth, at page 82, a spiracle may be observed 

 under each of the oblique bands which ornament its sides ; 

 and in the pupa or chrysalis of the same insect, figured at 

 page 196, the same spiracles are still to be traced. These 

 breathing apertures are so very obvious in the majority of 

 caterpillars that they can scarcely have escaped the notice 

 of the most unobserving. 



From each spiracle a single trachea enters the body ; 

 this single trachea is invariable ; but the systems of trachea 

 within are on various plans ; the principal are the detached 

 tracheae, and connected trachea. They are called detached 

 when, after entering the body as a single cylindrical tube, 

 they separate, diverge, and ramify, throughout the region 

 in their immediate vicinity, without any obvious connexion 

 with the trachea of the adjoining spiracles ; and connected 

 when there is a grand longitudinal trachea traversing each 

 side of the body throughout its length, into which each 

 spiracle sends its particular trachea, sometimes as a single 

 tube, sometimes after a slight ramification. 



Whether the trachea possess the detached or connected 

 form, their minute branches may be traced wandering over 

 to the opposite side of the insect, and uniting with similar 

 minute branches emanating from that side, so that the sys- 

 tem of respiration is always connected, and acts by uni- 

 formity of impulse. The ramifications of the trachea, to 

 our perception, appear infinite ; they pervade the skin, 

 muscles, nerves, stomach, intestines, legs, wings, and all the 

 organs of sensation ; the organs of respiration are therefore 

 more generally diffused in insects than in other animals 

 The blood of animals, in order to maintain its vital powers, 



o 



