CHAPTER XXII. 



INSECTS AT LARGE 



It is well first to note that insects (often wrongly called Hexapoda, 

 on account of their having three pairs of legs), are winged six-legged 

 arthropods (Pterygogenea), ( ), (Fig. 213). The 



body is divided into three distinct regions the head, the thorax, and 

 the abdomen. The head has the following appendages : a single pair of 

 antennae ( ) ; usually two compound eyes ; three 



simple eyes called ocelli ( ) ; and four different 



kinds of mouth-parts. These mouth-parts consist of a labrum (single, 

 and not one of the series of metameric appendages), mandibles, maxillae, 

 and labium; these last three being paired. 



The thorax is composed of three segments prothorax, mesothorax, 

 and metathorax. Each segment is protected by four exoskeleton plates 

 a dorsal tergum, a ventral sternum, and two lateral pleura. There is a 

 pair of walking legs on each thoracic metamere, while the last two usu- 

 ally also have a pair of wings attached. 



The abdomen usually consists of eleven segments, on which there 

 are no appendages except accessory reproductive organs and sometimes 

 a sting at the posterior end. 



In general there are two types of mouth-parts. These may vary 

 considerably. Grasshoppers and beetles have biting mouths, while the 

 true bugs have mouths arranged for sucking, and some insects such as 

 the bee have specialized mouth-parts which may be used for either biting 

 or sucking. 



The walking legs have five parts: a proximal coxa ( ), 



often fixed immovably to the sternum to which it is attached ; a short 

 trochanter ( ) ; a long femur ; a slender tibia ; and a 



jointed tarsus, which is usually provided with little hooks or pads, at its 

 free ends. As insects have varying modes of life, such as swimming, 

 flying, digging, and leaping, the legs of each type of insect are adapted 

 to the particular functions of each. 



It is from the last two thoracic segments that the wings arise. The 

 wings are of two types. Broad ones, such as the butterfly possesses, are 

 used for sailing, while smaller ones like those on flies can be moved 

 quickly, thus causing more rapid movement of the animal. There may 

 be scales or hairs on the wings. Likewise, wings may be thick or thin, 

 light or heavy, and vary in many other ways. The so-called "veins" in 

 insect wings are not veins at all, but thickenings supporting the wings. 



As insects are complex organisms, all the interior structures nor- 



