22 FIELD ZOOLOGY. 



body. From this main trunk there arise in each segment 

 three sets of branches: the branch going upward to 

 supply the muscles of the back and the wings ; the visceral 

 branch, running interiorly and supplying the organs of 

 the alimentary tract, the kidneys, and the reproductive 

 organs; and the ventral branch, which carries air to the 

 ventral ganglia, the ventral muscles, and the legs. In 



some insects these spiracles 

 are protected from dust and 

 other foreign substances 

 much as our noses are, by 

 fine hairs; in others a tiny 

 flap of chitin closes the open- 

 ing; in some others the wing 

 covers bend down over the 

 line of spiracles; but in other 

 insects, as in the order of the 

 bugs, there seems to be no 

 protection whatever. 



In insects which are very 



FIG. 12. Lateral gill from abdomen r . n- .1 



. A , u XT swift fliers there are, lust 



of a May fly nymph, Hexagema J 



variabiiis. Enlarged. (Fohom.) under the chitin body cover- 

 ing, tracheal pockets, or en- 

 largements of these air conductors; these are supposed 

 to be for the storing of a reserve supply of air which may 

 be drawn upon by the insect during the long, swift flights 

 which use up so much oxygen. 



In aquatic insects, or nymphs of insects which later 

 become air breathers, the spiracles are not present at 

 first, and the animal breathes while in the water by 

 means of tracheal gills, developing the spiracular system 

 much later. These gills present to the water an extensive 

 surface with a thin tegument, and under which the tracheae 



