Reapiration in Invertebrate Animals. 135 



traced irrefutably the physiological circumstances which neces- 

 sitate at this limit in the scale a new order of respiratory organs. 

 The object is inimitably accomplished ; for the Jirst time in the 

 serial history of animal life, an air-breathing being is intj-oduced 

 on the stage. Nature sm-mounts all difficulties by adroitly 

 resorting to an unexpected but matchless variation of her former 

 plan ; all at once, and without apparent reason, a new and 

 extraordinary system of organs is contrived ; an exquisite appa- 

 ratus of aeriferous vessels is so skilfully blended with all the 

 other and normal constituents of the living body, that an air- 

 breathing animal results without deformity of exterior contour : 

 in a small space a large result is realized. An insect is a dimi- 

 nutive animal ; its muscular and nervous systems are intensely 

 active ; its fluids are highly corpusculated and fibrinized ; a con- 

 siderable proportion of oxygen is absolutely essential. Could it 

 by any other expedient have been adequately supplied ? But the 

 simple distribution of patulous trachea throughout all the struc- 

 tures of the body, by which air is rather brought to the blood 

 than blood to the air, would most imperfectly accomplish the 

 great function of breathing. It was not enough to provide an 

 elastic inimitable spii-al, by which the passive patency of each 

 tube is maintained. Such property as that of physical ehisticity 

 in a structure so singularly beautiful answers another end; it 

 recoils on the contracting of the tube. The contracting of the 

 tracheae is in the insect the act of ea-spiration ; by this act the 

 diameter of the tubes of the universal tracheary system is dimi- 

 nished, and the air is driven out through the spiracular orifices ; 

 this act is rythmicaUtj followed by that of mspiration, in which 

 the physical elasticity of the spiral, by rebound, restores the tube 

 to its former diameter*. No part of the circulating system but 

 the dorsal vessel is capable of contracting and dilating. This 



• I have ililigently sought for the announcement of this fact amongst the 

 varied and excellent writings of Mr. Newjx)rt, M. E. Blanchard, and M. 

 Leon Dufour. No allusion whatever is anywhere made to this property of 

 rythmic contraction and dilatation, which I have proved, by repeated obser- 

 ?itions on larva; and adult insects and M\Tiapods, the tracheje to possess. 

 The omission is the more surprising, since, without such a property-, the tra- 

 chearj- system woidd be mechanically imperfect as an apparatus of respiration. 

 As the vessels do not contract, there would be no provision for renewing 

 the air in the extremes of the system. The working of the general muscles 

 of the body external to the system would obNnously prove a most imperfect 

 substitute. What is denied indeed to the vessels is conferred on the 

 trachea;. I cannot prove that the parietes of the trachese are capable of 

 originating this movement. I cannot <lemoustrate them to be muscular. 

 It is possible that the opening and shutting of the air-tubes may only follow 

 from those alternate acts of contraction and dilatation in the abdominal 

 segments by which the dorsal and ventral arches of the abdomen are alter- 

 nately elevated and depressed like the ribsi of the vcrtcbratcd animal. 



