CIRCULATION OF BLOOD. 89 



SPIEACLE OF TIBULA. 



Hiis has a solid disc in the centre, and radii proceed from 

 thence to the margin. 



SPIRACLES OF WATER. LARVJ2. 



These are best examined in the living larvse of the Gnat, 

 where the last segment of the abdomen is prolonged into a 

 tube, the mouth of which remains 'at the- surface of the 

 water whilst the animal breathes. 



There are, however, other and quite different modes of 

 respiration in aquatic larvse, which form beautiful micro- 

 scopic objects. 



AERATING' LEAFLET OF LIBELLULA, 



the larva of one of the Dragon-flies, those- slender; beau- 

 tiful blue and scarlet flies, which glance like living sun- 

 beams across our path on a Summer's day, and may be 

 found in thousands resting on the reeds and bushes at the 

 river's side. When these atfe in the larval state, they have 

 three leaf-like plates at the extremity of the abdomen, over 

 which innumerable tracheae -ramify and draw from the water 

 that supply of air which is needful for their life. 



ABDOMEN OF EPHEMERA, OR SPIRACLES. 



The larva of the pretty May-fly, or Ephemera, has on 

 either side of its body a row of little leaflets, each of which 

 is an external spiracle, and when alive it is most interesting 

 to watch its palpitations, the play of those tiny organs 

 drawing oxygen from the water to aerate the blood. It is 

 best thus to see it, because we are able to observe the cir- 

 culation of the blood through the transparent skin. 



CIRCULATION OF- 



All creatures that have life have blood : it is the nourish- 

 ing fluid which is needful for existence. In insects, it is 

 colourless, but composed of minute corpuscles, which are 

 propelled through the body, not by arteries and veins, but 



