74 THE STUDY OF INSECTS. 



The number of spiracles varies greatly in different insects. There 

 is, however, never more than one pair on a single segment of the 

 body. They do not occur on the head, but are borne by each of the 

 thoracic segments, and by the first eight abdominal segments. Thus 

 there are eleven segments that may bear spiracles, but they are 

 always lacking on some one or more of these. 



These spiracles are either simple openings into the respiratory 

 system, or are provided with valves, sieves, or fringes of hair for the 

 exclusion of dirt. They lead into a system of air-tubes termed tra- 

 chece (tra'che-ae). The accompanying figures will indicate the distri- 

 bution of the main trunks of these tracheae in a cockroach (Figs. 86 and 



FIG. 86. Tracheal system of Cockroach. FIG. 87. Tracheal system of Cockroach. 



The dorsal integument removed and the The alimentary canal removed to show 



alimentary canal in place. (After Miall the ventral tracheal communications, 



and Denny.) (After Miall and Denny.) 



87). There is a short trunk arising from each spiracle ; these are all 

 connected together by a large longitudinal trunk on each side of the 

 body, and by numerous transverse trunks. From these large tracheae 

 there arise a great number of smaller ones, not shown in the figures, 

 which branch and subdivide, and extend to all parts of the body. When 

 one dissects an insect the viscera are found to be connected together 

 by the ramifications of these tracheae, so that in order to remove any 

 organ it is necessary to cut some of them. The smaller branches of 

 the tracheae are exceedingly minute, and are intimately associated 

 with the various tissues. By means of these fine tracheal trunks the 

 air is carried to the tissues ; hence the blood plays a much smaller 

 part in respiration than it does in the Vertebrates. 



Although insects are, strictly speaking, air-breathing animals, 

 many of them, as is well known, live in the water. The study of the 

 ways in which aquatic insects breathe is a very interesting one ; it 



