MICROSCOPIC DRAWING AND ENGRAVING. 



tion, are situated in the membraneous paddles, or swimmers, pro- 

 jecting on either side of the body ; they imbibe the air from the 

 circumambient fluid which passes from them into the trachea?. 



Ramifications of the trachea extend along the legs, the antennae, 

 which diverge from the head, and along the three-forked tail ; 

 small oblong corpuscles of blood may be seen passing rapidly 

 around the tracheae with every pulsation of the dorsal vessel. 

 This vessel, says Mr. Bowerbank, extends nearly along the whole 

 length of the body, and is of great comparative magnitude ; it is 

 furnished at regular intervals with double valves, one pair for 

 each section of the body. 



A portion of this vessel, with its valves, is represented as seen 

 under a higher magnifying power in fig. 30. 



The action of these valves is a most interesting and beautiful 

 spectacle. While in the greatest state of collapse the point of the 



lower valve is seen closely compressed within the upper one. At 

 the commencement of the expansion of the artery, the blood is seen 

 Howing in from the lateral apertures, as shown by the arrows in 

 the figure, and at the same time the stream in the artery com- 

 mences its ascent ; when it has nearly attained its greatest state 

 of expansion, the sides of the lower valve are forced upwards by 

 the increasing now of the blood from the section below the valve, 

 the lateral openings are closed, and the main current of the blood 

 forces its way through the two valves. 



40. The three-pronged tail is beautifully fringed with bunches 

 of fine hair, as shown in the figure. As the time approaches at 

 which the insect is destined to pass into its next stage of existence, 

 the central prong of the tail becomes more transparent, and as- 

 sumes the appearance of a jointed tube or sheath ; the two external 

 prongs, at the same time, exhibit within them parts which are des- 

 tined to become the tail of the insect in the third stage of its life. 



The rapidity with which this creature moves is truly surprising ; 

 besides its six legs, it is furnished with the six double paddles at- 

 tached diagonally to the serpentine vessels on each side of its 

 78 



