How they perform the primary functions 



Beetles (Coleoptera) are an exception. Although they belong to 

 the Holometabola, larvae and adults often live in much the same 

 surroundings, and all have mouthparts of the biting and chewing type. 

 The adults are rather furtive insects, which crawl and run more often 

 than they fly. They have not evolved into a distinct way of life of their 

 own, Hke the more aerial adults of Diptera and Hymenoptera. This is 

 the more surprising since beetles are a phenomenally successful group, 

 and there are nearly as many different species of beetle as of all the other 

 insects put together. 



Fig. 25 shows some examples of the way in which mouthparts may 

 be adapted to different feeding habits. 



Nutrition 



Insects need food for two main purposes : to repair worn tissues and 

 build new ones, i.e. to grow; and to burn as a source of energy. If 

 immature insects (nymphs or larvae) receive too little food, or the wrong 

 sort, they may cease to grow, but may continue their metamorphosis, 

 either at the normal speed, or at a slower rate than usual. The result 

 is an adult that is much smaller than the average. 



Adult insects of one species sometimes vary considerably in size, and 

 people are apt to think of the small ones as being 'young adults'. In 

 fact once an insect has moulted to the adult stage, and has fully inflated 

 and hardened its cuticle, it does not grow any more, however long it may 

 live; an exception to this rule being the mayflies (Ephemeroptera), 

 which have wings as a subimago, or 'dun', and then moult again to the 

 true adult, or 'spinner'. 



A growing insect needs proteins, carbohydrates and fats, but not 

 necessarily a balanced diet as we understand it. Many insects normally 

 have a restricted and monotonous diet, and most insects can survive on 

 one food if they have to. 



Biting and chewing insects tear off pieces of the food they eat, and 

 digest them in the intestine. A very large number of insects eat leaves 

 and other vegetation, but they are seldom able to digest the cell-walls 

 and other cellulose parts of the plant: Either the cells are broken open 

 by chewing, or the contents are dissolved out, leaving the cell-walls 

 intact. The only insects that can digest cellulose are termites, some 

 beedes, and a few cockroaches, and they do so mostiy by keeping a 

 colony of baaeria and protozoa (single-celled animals) in the intestine, 

 relying on them for assistance. A very few insects can themselves digest 

 cellulose. Many insects that bore in wood do not in fact digest the wood, 

 but feed only upon the starches and sugars that it contains. 



Biting and chewing insects that live on animal protein have an 



45 



