94 



INTRODUCTION TO ZOOLOGY. 



animals, by muscular exertion and excitement; and Mr. Newport 

 has counted as many as one hundred and forty-two pulsations 

 in a minute in a species of wild Bee so excited."* 



Respiration is effected by means of two great canals 

 (tracheae) running along the sides of the body, beneath the 

 outer surface, and communicating with the atmosphere by 

 means of numerous short tubes, tenninating 

 at or near the sides of the body in breath- 

 ing pores (spiracles) ; internally the tracheae 

 divide into innumerable branches, convey- 

 ing the air to every portion of the body, 

 and thus pervading its organs and tissues. 

 This structure will easily be understood by 

 referring to the accompanying figures. The 

 Water- Scorpion {Nepa, Fig. 67) is an in- 

 sect common in fresh water; and the re- 

 spiratory apparatus of the same insect, 

 as it appears when highly magnified, is 

 Fig. 67.— Nepa. shown in Fig. 69. 



" There is one cu-cumstance connected with the tracheae 

 which is specially deserving of admiration, whether we con- 

 sider the obvious design of the contrivance, or the remarkable 

 beauty of the structure employed- It is evident that the sides 

 of canals so slender and delicate as the tracheie of insects 

 would inevitably collapse and fall together, so as to obstruct 

 the passage of the air they are designed to convey; and the 

 only plan which would seem calculated to obviate this would 

 appear to be to make their walls stifi" and 

 inflexible. Inflexibility and stiflfness, how- 

 ever, would never do in this case, where the 

 vessels in question have to be distributed, in 

 countless ramifications, through so many 

 soft and distensible viscera ; and the problem 

 therefore, is, how to maintain them per- 

 manently open, in spite of external pressure, 

 and still maintain the perfect pUancy and 

 softness of their walls. The mode in which 

 this is effected is as follows : — Between the 

 two thin layers of which each air vessel 

 consists, an elastic spiral thread (Fig. 68) 



Fig. 68. — AlB-TUBE OF 



Ikbbct. 



• Owen's Lectures, page 223. 



