VII. THE DIVERSITY OF INSECTS 



{Adaptation; convergence; mimicry) 



Naturalists have always marvelled at the diversity of animal life, and in 

 particular at the way in which so many animals are beautifully con- 

 structed for the hfe they lead. 



When it was suggested that the species as we now know them had 

 arisen by evolution from simpler forms, it was necessary to provide 

 some explanation of how they could have become so closely adapted to 

 their way of life. The two principal theories were the Inheritance of 

 Acquired Characters — generally associated with the name of Lamarck — 

 and Natural Selection, the theory of Darwin and Wallace. 



Whatever the theory, it is a fact that animals, including insects, tend 

 all the time to grow more 'speciaHsed', i.e. to say, more closely adapted 

 to a particular mode of life. If a species is distributed over a wide area, 

 with different conditions in different parts, then this process of special- 

 isation is incUned to produce a number of local forms, each suited to 

 local conditions, and shghtly different from the others. Eventually these 

 forms grow so far apart that they cease to interbreed, and become 

 distinct species, which are said to radiate from a common ancestor, or 

 stem form. 



Since animals in the course of evolution become adapted to their way 

 of Ufe, distantly related animals often come to look superficially aUke, 

 as a result of living in the same way. This is called convergence. Finally, 

 it sometimes appears that one species gets an advantage from looking 

 like another, a phenomenon known as mimicry. 



Adaptation 

 There is space to mention only a few examples of adaptation in insects. 

 We have already (Chapter III) spoken of the mouthparts (Fig. 25), 

 which in a cockroach or a grasshopper are 'primitive', or 'generalised', 

 and have three pairs of general-purpose organs : mandibles, maxillae 

 and labium, the last consisting of a pair of appendages merged together. 

 Bugs, butterflies and many flies have one pair or more of these drawn 

 out into a tongue-like proboscis, with which they suck up Uquid foods. 



no 



