EPILOGUE 205 



spiders are passing through a primitive lung-book stage from 

 which none has yet emerged. Two pairs of lung-books without 

 tracheae occur in the more primitive families while most others 

 have an anterior pair of lung-books and a posterior pair of tracheae. 

 Lung-books provide a localised respiratory area from which 

 oxygen is distributed by the respiratory pigment, haemocyanin, in 

 the blood. Insects possess no respiratory pigments because all their 

 tissues are supplied with oxygen directly by the tracheoles. 



The entrance to both lung-books and tracheae is guarded by 

 spiracles closed by means of special muscles. In this way the 

 minimum quantity of air necessary for respiratory purposes is 

 allowed to circulate and consequently water-loss is much reduced. 

 When an insect or Arachnid is placed in an atmosphere containing 

 an abnormally high proportion of carbon dioxide the spiracles are 

 kept open to their fullest extent and the rate of water-loss in dry 

 air is then considerably increased. 



In all organisms growth tends to be a cyclical process, periods of 

 rest alternating with activity. In no animals, however, is it more 

 marked than in the Arthropoda whose development is punctuated 

 by a series of moults or ecdyses, each of which is preceded by a 

 period of active growth and followed by one of apparent in- 

 activity. All parts of the integument are moulted together, inde- 

 pendently of nerve supply: the stimulus for this is hormonic in 

 nature, metamorphosis too being under hormone control. 



During moulting the old endocuticle is digested by enzymes in 

 the moulting fluid and withdrawn in solution through the 'pore 

 canals'. These are numerous fine tubes, sometimes numbering over 

 a million per sq. cm of surface, which penetrate the exo- and 

 endocuticle. A new epicuticle is always laid down beneath the old 

 one before moulting takes place, so that this process is accompanied 

 by little loss of water and a high degree of economy is achieved. 



Woodlice and millipedes are almost entirely vegetarian, while 

 centipedes and Arachnids, with the exception of certain mites, 

 are primarily carnivorous; but different species of insects seem to 

 be able to thrive on almost any kind of organic matter capable of 

 supporting metabolism. In most of these animals water conser- 

 vation is of prime importance; insufficient water is obtained with 

 the food to allow for much excess to be lost during excretion. 



