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 tracheae. 
Ramifications of the tracheae extend along the legs, the antennae, 
which diverge from the head, and along the three-forked tail; 
small oblong corpuscles of blood may he seen passing rapidly 
around the tracheae with every pulsation of the dorsal vessel. 
This vessel, says Mr. Bowerbank, extends nearly along the whole 
bngth 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 
Fig. 30. 
lower valve is seen closely compressed within the upper one. At 
the commencement of the expansion of the artery, the blood is seen 
flowing 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 flow 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 
