224 THE ORGANIZATION 



to have a more complicated system, above Insects. But the 

 truth is, the blood circulation in Spiders is not so intricate nor 

 so highly specialized, when compared with that of Insects, as 

 has been asserted. In Insects the only definitely circumscribed 

 canal of the system is the so-called dorsal vessel, or heart (//, A 1 , 

 A 2 ). It is a mere tube which lies close to the back, in the mid- 

 dle line, and extends from the head to the posterior end of the 

 abdomen. It is open at both ends, (A, A 2 ,) and at the successive 

 points where it is narrowed are a pair of apertures, one on each 

 side, which are guarded by a valve on the inside. There is also 

 a pair of internal valves at the posterior opening. The process 

 of circulation is a very simple one : the blood enters the poste- 

 rior and lateral apertures of the heart as it expands; then upon 

 its contraction all the valves are closed and the blood is forced 

 toward the head, and, passing out at the anterior opening, flows, 

 in numerous currents, and, at first appearances, in undetermined 

 channels, among the various organs, and into the legs and wings. 

 A careful examination of some of the more transparent insects, 

 such as the May-fly, (Ephemera,) Gall-fly, (Cynips,) Plant-louse, 

 (Aphis,) Lace-winged fly, (Chrysopa,) Dragon-fly, (JEshna, 

 Ag-rion, Libellula,) and the grub or worm of many more, has 

 convinced me, that, notwithstanding the apparent lack of walls 

 to the channels of circulation, the course of the blood is none 

 the less definite ; always passing in one set of channels going 

 from the heart, and returning toward it in another set. This 

 is particularly noticeable in the head, legs, and wings. 



The breathing- organs consist of numerous branching tubes, 

 trachea;, (fig. 132, br 1 ,} which spread out from certain fixed points 

 along the right and left sides of the body. The air enters the 

 body through minute apertures, spiracles, (br,} of which there 

 are two in each ring, one on each side, as in this beetle (fig. 130, 

 s to s 1 ). At each spiracle (fig. 132, br) a tube (br 1 ) arises and 

 branches into innumerable twigs, which spread themselves over 

 the various internal organs. The walls of the tracheae being 

 very thin, the contained air finds a ready absorbent in the circu- 

 lating fluid. 



