15 



The tracheae from the stig- 

 mata connect with main lines, 

 which extend the length of 

 the body ; from these the 

 branches and smaller tubes com- 

 municate with all parts. In 

 Fig. 12, the cross-shaded por- 

 tions are the tracheae, the very 

 fine cross lines representing the 

 thickenings mentioned above. 



In large insects which fly 

 long distances, such as pond-flies, 

 butterflies and moths, some beet- 

 les, flies, bees, etc., there often 

 occur large distensions of cer- 

 tain tracheae to form air sacs, 

 which are probably of use to 

 the insect in making the body 

 more buoyant, and in increas- 

 ing the rapidity of breathing. 



Insects are air-breathers, 

 with few exceptions ; most cf 

 those which live in the water 

 must come to the surface from 

 time to time for a supply of air, 

 and in many cases the habit of 

 living in water, in decaying or- 

 ganic matter, in plant tissues, or 

 as animal parasites, necessitates 

 some modification, or special 

 of adaptation, in the manner of 



obtaining the necessary air. 

 (Redrawn after Miall & Denny.} Mosquito larvae (Fig. 13) 

 breathe by means of a special organ at the posterior end of 

 the body, which is brought into contact with the air. 

 Maggots of flies which live in plant or animal tissues often 

 have spiracles at that end of the body, which communicate 

 with the air. In the accompanying illustration (Fig. 13) the 

 tracheal tube, at the posterior end of the body, is shown at 

 right angles to the body. The main tracheal trunks are 

 indicated by the faint lines which may be traced through the 

 segmented abdomen into the broad thorax, where they 

 diverge. 



Water beetles come to the surface and carry down, 

 when they dive again, a film of air held by the fine hairs of 



Fig. 12. 



Tracheal system 

 cockroach. 



