SPECIES OF BRITISH HYDROPHILUS. 77 



proper orifices situated at its tail; and the creature is 

 obliged to ascend to the surface of the water, and elevate 

 its tail out of it, at every inspiration. When the air thus 

 inhaled has become unfit for respiration, it is expelled 

 at the same orifices, and a small bubble may frequently 

 be seen issuing from the tail, and ascending in the water. 

 On a careful examination of the skin, which by the by 

 affords an excellent object for the microscope, there will 

 not appear any spiraculas along the sides, as in perfect 

 insects. 



The respiratory system of this larva, being, when dis- 

 sected out, more opaque than that of most similar crea- 

 tures, is seen to great advantage when mounted in 

 Canada balsam ; and if it be accompanied with the cor- 

 responding organs of a caterpillar, the comparison 

 affords a beautiful example of adaptation to the different 

 elements suited to life. In the terrestrial animal, every 

 part being exposed to the atmosphere, mouths or orifices 

 for inhaling the air are arranged along both sides of the 

 body ; in the aquatic larva, this system could not be made 

 available, without the creature being compelled to quit 

 its natural element, at every moment it had occasion to 

 respire. To obviate this, it is furnished with two large 

 apertures at the tail, instead of those at the sides. 



I may mention that a preparation of this organ illus- 

 trates very beautifully one of the physical properties of 

 matter, namely, the colours produced by interference, 

 when light is made to pass between a series of fine lines. 



After this creature has remained for a considerable 



