STRUCTURE, GROWTH AND ECONOMICS OF INSECTS 



nymphs of Stone-flies and Caddice- 

 flies have filiform or cylindric tra- 

 cheal-gills, larvas of Culex, Corethra, 

 Simiilimn and Chironomus have 

 blood-gills, while nymphs of Dragon 

 FHes have internal tracheal-gills . 

 Tracheal gills are outgrowths of the 

 body wall with fine tracheal tubes, 

 in which there occurs during res- 

 piration an exchange of gases be- 

 tween the air in the tubes and the 

 water. They are usually external 

 but in the nymphs of Dragon Flies 

 they are internal, being arranged 

 in rows on the inner walls of a gill 

 chamber in the posterior portion 

 of the alimentary canal. 



Blood-gills are outgrowths of the 

 body wall in which the blood flows. 

 The exchange of gases in respira- 

 tion occurs between the blood in- 

 side and the water outside. 



Structurally a trachea consists 

 of a chitinous wall or intima as a 

 lining membrane spirally thickened 

 at regular intervals by elastic 

 threads called tcBnidia, and a cellu- 

 lar wall of hypodermal cells, the 

 pavement epithelium . 



(c) Circulatory System. — While 

 there is a blood circulation in insects 

 the only blood vessel is a dorsal 

 tube lying just beneath the notum. 

 The heart or posterior portion of 

 the dorsal tube contains a number 



Fig. 28. — Respiratory system of the 

 larva of the budmoth (Tmetocera ocel- 

 lana) . (Insect opened along the median 

 of ventricles or chambers, each with dorsal line.) C, tracheal commissure; 



a lateral valve which allows the ^- ^°"'^^ ^'■^"'=^= ^- supplying heart; 



blood to flow in but not out. There 



v., ventral branch; F5., visceral branch. 



