II. HOW THEY ARE CONSTRUCTED 



We have seen already that the body of an insect is divided into head^, 

 thorax and abdomen, and these divisions are clearly shown in the insects 

 illustrated in Figs. 14, 16, both in the adult and in the young stages. 

 Note that insects have no 'tail' in the true sense, because this word is 

 used only for that part of the body lying beyond the anus, or posterior 

 opening of the intestine. In insects this opening comes at the tip of the 

 abdomen. 



External Structure of Adult Insects 

 The head of an insect has lost almost all trace of the original segments 

 from which it was built up. The plates that can now be seen in the head 

 are what are called 'secondary structures', and the lines, or sutures, 

 which mark off these plates are mostly to do with strengthening ridges 

 inside the head. 



We have already spoken of the appendages of a segment, which may 

 be made to perform one of several different functions. In the head, the 

 appendages of one segment have become the antennae, or feelers (see 

 Chapter IV), of which insects have only one pair, and the appendages 

 of three other segments have become the mouthparts: one pair the 

 mandibles; one pair the maxillae, with their feeler-like /)a/pz; and a third 

 pair the single labium. These mouthparts have taken up a variety of 

 different shapes in different groups of insects, according to the kind of 

 food and the method of feeding. This is discussed in Chapter III, 

 and several different ways in which mouthparts have been developed 

 are illustrated in Fig. 25. 



The majority of insects have eyes of some kind clearly visible on the 

 head. These may be quite small and simply constructed, or they may be 

 elaborate organs composed of hundreds of separate units. In many 

 insects, especially some which spend a great deal of lime on the wing, 

 such as many flies and dragonflies, the compound eyes are so huge that 

 hardly anything can be seen of the rest of the head (Figs. 69, 70). The 

 working of these eyes is discussed in Chapter IV. 



The head is joined on to the body by what is called a cervical region, 

 or neck. Some insects are stiff-necked, or have the head sunk into the 

 thorax like a round-shouldered man; but most insects can move the 

 head from side to side, and up and down, and also twist it round on a 

 horizontal axis. 



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