THE ORGANS OF CIRCULATION AND RESPIRATION. 143 



Respiratory Organs of Insects. 



The respiratory organs of Insects consist of ramified trachea! 

 tubes, which communicate with the external air by stigmata or 



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spiracles. Of these spiracles the Cockroach has ten pairs 

 eis:ht in the abdomen and two in the thorax. The first 



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thoracic spiracle lies in front of the rnesothorax, beneath the 

 edge of the tergum ; the second is similarly placed in front of 

 the metathorax. The eight abdominal spiracles belong to the 

 first eight somites ; each lies in the fore part of its segment, 

 and hence, apparently, in the interspace between two terga and 

 two sterna. The first abdominal spiracle is distinctly dorsal 

 in position. 



The disposition of the spiracles observed in the Cockroach 

 is common in Insects, and, of all the recorded arrangements, 

 this approaches nearest to the plan of the primitive respiratory 

 system of Tracheata, in which there may be supposed to be 

 as many spiracles as somites.* The head never carries spiracles 

 except in Smynthurm, one of the Collembola (Lubbock). Many 

 larvae possess only the first of the three possible thoracic 

 spiracles ; in perfect Insects this is rarely or never met with 

 (Pulicidce?), but either the second, or both the second and 

 third, are commonly developed. Of the abdominal somites, 

 only the first eight ever bear spiracles, and these may be 

 reduced in burrowing or aquatic larvae to one pair (the eighth), 

 while all disappear in the aquatic larva of Ephemera. 



From the spiracles, short, wide air- tubes pass inwards, and 

 break up into branches, which supply the walls of the bod} r 

 arid all the viscera. Dorsal branches ascend towards the heart 

 on the upper side of the alary muscles ; each bifurcates 

 above, and its divisions join those of the preceding and suc- 

 ceeding segments, thus forming loops or arches. The principal 

 ventral branches take a transverse direction, and are usually 

 connected by large longitudinal trunks, which pass along the 



: The oldest Tracheate actually known to bear spiracles is the Silurian Scorpion 

 of Gothland and Scotland (Scudder, in Zittel's PaL-eontologie, p. 738). We need 

 not say that this is very far removed from the primitive Tracheate which morpho- 

 logical theory requires. The existing Peripatus makes a nearer approach to the 

 ideal ancestor of all Tracheates, if we suppose that all Tracheates had a common 

 ancestor of any kind, which is not as yet beyond doubt. 



