2 INVERTEBRATE ZOOLOGY 



which is composed largely of a very hard and resistant substance 

 called chitin, and serves the double purpose of a protection for 

 the internal soft parts and a surface for the attachment of 

 muscles. It is, in fact, the skeleton of the animal, and is called 

 an exoskeleton, in contradistinction to an internal supporting 

 structure which would be called an endoskeleton. All inverte- 

 brate animals, except some of the lowest, are provided with a 

 cuticular exoskeleton, but it is only the arthropods in which it 

 is composed largely of chitin. In fact, the possession of such 

 a hard and resistant external covering is one of the reasons why 

 insects have so successfully maintained themselves in the uni- 

 versal struggle for existence. 



Observe that the body of the animal is composed of a number 

 of serially arranged segments. These are called somites or meta- 

 meres, and the segmented type of structure presented by the 

 insect body is called a metameric type of structure. Observe 

 that the body is sharply divided into three divisions — the head, 

 thorax, and abdomen. 



The head is unsegmented and bears on its anterior and dorsal 

 surface a pair of long, jointed feelers or antennae, which are impor- 

 tant sense-organs, a pair of large compound eyes, and three small, 

 dot-like eyes, called ocelli, which it may be necessary to look for 

 with a hand lens ; on its ventral side are the mouth-parts, the 

 organs which taste, grasp, and masticate the food. Examine 

 these mouth-parts carefully with a hand lens ; notice that there 

 is a short overhanging upper lip, beneath which is a pair of 

 powerful jaws having a lateral or side position instead of a 

 dorso-ventral one like the jaws of vertebrates. Beneath the 

 jaws are two other pairs of mouth-parts, the maxillae and the 

 under lip, which, however, will not be studied at present ; notice 

 the two pairs of elongated and segmented palps, which are 

 probably organs of taste. 



The thorax is composed of three somites or metameres, which 

 are called, respectively, the pro-, meso-, and metathorax. Each 



