The diversity of insects 



already mentioned some of these in Chapter III. Mayflies (Ephemer- 

 optera) have a series of leaf-Hke or blade-Hke gills along each side of the 

 abdomen. Stoneflies (Plecoptera) and alderflies (Megaloptera) have gills 

 in the form of segmented appendages fringed on both sides. The 

 dragonflies (Odonata), on the other hand, have two methods: either 

 internal gills in the rectal chamber of the intestine, or three external 

 gills at the tail. 



The nymphs of dragonflies have another strongly adaptive feature 

 in the mask, a double-jointed labium that can be shot forward to seize 

 another small animal, and feed upon it. 



The bodily shape, and the arrangement of the legs of larvae is, of 

 course, closely adapted to their habits. The very active larvae, such as 

 those of beetles (Fig. 33a) have three pairs of thoracic legs, and usually 

 lift the abdomen clear of the surface. This type of larve is generally 

 carnivorous. Larvae like caterpillars (Fig. 33c), which have prolegs on 

 the abdomen as well as the segmented legs on the thorax, can progress 

 only slowly, since they have to make muscular movements in the 

 abdomen as well as in the thorax (see Chapter V). Caterpillars browse 

 on leaves, and move steadily about, like a herd of cattle in a meadow; 

 they do not run. Legless larvae live where food is abundant, and easily 

 obtained, but they can move quickly to get out of a place that does not 

 suit them. We have seen in Chapter V how they may rely on their 

 prolegs to get them away. 



A peculiar adaptation is shown by larvae of flies of the genus Fannia, 

 which breed in rotting materials rich in nitrogen. The segments of the 

 thorax and abdomen are drawn out into fringed processes, which are 

 beUeved to serve as a flotation device: they increase the surface area, 

 and so make surface tension more eff"ective in supporting the weight of 

 the body. The larva has to keep its hind spiracles clear of the water in 

 order to breathe, and it is beheved that increasing the surface area in 

 this way enables the larva to survive in cesspools and other organic 

 liquids. 



Although it often seems that a particular structure, such as the ones 

 we have mentioned, is adapted to some obvious purpose, we must not 

 think of insects as being always built for the job, like a carefully de- 

 signed machine. Many peculiar structures that are found in insects have 

 no obvious function. 



Everybody knows the 'pincers' of earwigs, which look so big and 

 threatening, but no one knows for certain what they are used for. Three 

 possibiUties have been suggested: they might be used for attack and 

 defence; or to grasp the other member of a pair during copulation (they 

 are present in both sexes, but are more strongly curved in the males); 



113 



