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 

 usually 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, 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 required. 



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, 

 used for sailing; and smaller ones like those on flies, which can be moved 

 quickly to cause a 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- 



