The South Australian Naturalist. 51 



ing our subject further. Ramifyino: everywhere through the 

 body are thousands of little branching tubes known as tracheae ; 

 usually additional ])ouclies are present at intervals, and these, 

 by contracting and expanding, further assist in pumping air 

 along the tracheae. The general mechanism of the apparatus 

 is very simple : the greater part of the system is situated in the 

 abdomen, which is itself capable of considerable contraction 

 and expansion. It is by alternate movements of the abdomen 

 that an insect breathes, for when the body is contracted the 

 tubes and pouches are compressed and the vitiated air is ex- 

 pelled. The tracheae are lined throughout their length with a 

 coiled, spring-like strip of chitin, so that when the abdominal 

 pressure is released they automatically resume their normal 

 tubular form and fresh air is drawn into the system. The air 

 enters the tubes through a number of external orifices, known 

 as spiracles, which can be opened and closed at will. When 

 they are open the system receives and expels air in response to 

 the respiratory movements of the body; when closed all com- 

 munication with the surrounding medium is stopped, and the 

 supply is thus regulated according to the wants of the indi- 

 vidual. To give an analogy, the spiracles are the nostrils and 

 the tracheae the lungs. 



Most insects rapidly succumb when deprived of oxygen,* 

 and aciuatic species must therefore come to the surface for air, 

 or whilst remaining under water have some adaptation to 

 enable them at the same time to obtain atmospheric air; or 

 again they may utilise the small amount of air dissolved in the 

 water, or the oxygen given oft' by aquatic plants. Certain 

 insects which are obliged to ascend to the surface collect a 

 globule of air and carry it about with them beneath the water 

 in a similar ^vay to a diver bearing a cylinder of oxygen. In 

 place of the cylinder, the gas is trapped and held by specially 

 modified hairs, or is stored beneath horny wing cases; when 

 the globule is exhausted the insect must ascend for a fresh 

 supply. Others have adaptations in the form of a tube or 

 tubes, the orifices of which ma\^ be thrust above the water, 

 and through which the owner breathes whilst remaining sub- 

 merged. Species which have no means of communication with 

 the atmosphere are provided with gills. 



*Althougli apparently asphyxiated, some terrestrial insects possess the 

 faculty of returning to life after having been to all appearances dead for 

 somj? days. This was demonstrated by Mr. A. M. Lea during his recent 

 observations of the wheat -weevil in this State. 



