246 HOUSE, GARDEN, AND FIELD 



THE AIR-TUBES OF INSECTS. 



All insects are air-breathers. It is true that some are 

 so entirely aquatic during their early stages as to possess 

 gills (the blood- worm is a common example), but every 

 adult insect breathes by taking in gaseous air. The chief 

 purpose of the winged stage in an insect is the dispersal 

 of the eggs, and this purpose would usually be defeated 

 altogether if the egg-laying insect could not range through 

 the air. No insect breathes by taking in air through its 

 mouth. The same thing is true of the greater part of 

 animals ; it is only vertebrates which breathe through 

 their mouths. An insect has a row of holes along the 

 sides of its body, through which air is admitted or ex- 

 pelled. The holes (spiracles) are defended by valves, 

 and sometimes by an elaborate fringe of branched hairs, 

 which not only exclude dust, but water. You may have 

 observed that when an insect falls into water, it does not 

 speedily drown. Its spiracles exclude the air sufficiently 

 long to give it fair time to wriggle out. Have you ever 

 seen an insect breathing ? A bee or wasp moves the 

 joints of its abdomen in and out, bending or straighten- 

 ing them at the same time. Some other insects raise or 

 depress the upper surface of the abdomen. Whatever 

 the action, it has the effect of alternately enlarging and 

 contracting the cavity of the body. It is not enough to 

 provide a series of holes ; the air must be forcibly driven 

 along through them, and along the air-tubes into which 

 they lead. For this purpose it is necessary that the 

 insect should be able to close the inlets tightly. Unless 

 the air is put under pressure, it cannot be forced along 

 narrow passages, and it cannot be put under pressure 

 so long as it is free to escape. Just within the spiracle, 

 the air- tube leading inwards is made to pass through a 

 clip, and by means of the clip the air-tube can be throttled 

 at pleasure. This is always done before the body-cavity 

 contracts. The blood which fills the cavity transmits 



