EPILOGUE 



In the introduction to this volume the complexity of factors 

 influencing the ecology and distribution of animals was indicated, 

 and the interactions with their environments of the species men- 

 tioned have been stressed throughout. Ecology can be studied 

 from the point of view of the species (autecology) or from that of 

 the particular habitat in which numbers of species occur (syneco- 

 logy). The former is a simpler approach and more suited to the 

 activities of the individual natural historian: the latter usually 

 involves team work. Whichever is adopted, however, the other 

 should constantly be borne in mind or else the picture will become 

 unbalanced and distorted. 



Considerable uniformity is apparent throughout the groups that 

 have been considered here: this is not altogether surprising in 

 members of the same phylum. Arthropods have an exoskeleton 

 which, in all classes, seems to be basically similar although it may 

 vary greatly in complexity. An exceedingly thin outer epicuticular 

 layer of 'cuticulin', a condensed lipo-protein tanned with quin- 

 ones, is always present but only in insects and Arachnids does this 

 support the impervious layer of wax to which their success on land 

 is largely due. 



It would be mistaken however to regard the absence of a discrete 

 cuticular wax layer as a primitive characteristic, although the forms 

 that lack one are so restricted in their choice of environment that 

 they cannot be regarded as entirely successful land animals. Rather, 

 it seems that a particular method has been exploited for surviving 

 the conditions of life on land. The primitive respiratory organ is 

 the skin, but special respiratory structures have been evolved in all 

 but the smallest and simplest of the Arthropoda. In insects a 

 system of minute air tubes or 'tracheae' lead into even finer 

 'tracheoles' that carry oxygen directly to the tissues where meta- 

 bolic processes take place, while the Arachnida possess both tubu- 

 lar tracheae and lung-books. Scorpions have lung-books, while 



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