GENERAL ZOOLOGY 



PHYLUM ARTHROPODA 



Class : Tracheata. Division : Insecta. Order : Hymenopkra 



A WASP 



Observe the size, shape, color, and external anatomy of the 

 animal. The body is small in size ; it is bilaterally symmetrical, 

 that is, it has a right and a left side which are alike ; it has a dor- 

 sal and a ventral side which are unlike, and also a forward and 

 a hinder end which are unlike, the forward or anterior end being 

 distinguished by the possession of important organs of special 

 sense and the mouth. All of these features are characteristic of 

 rapidly moving animals. Can you explain why ? On the ventral 

 side are the legs, which are also called appendages or extremities ; 

 on the dorsal side of the insect are the wings. 



The external surface of the animal is very smooth. This feature 

 is also correlated with rapid motion. Do you know how? The 

 animal is encased in a hard shell, called the cuticula, which is 

 composed largely of a very hard and resistant substance called 

 chitin, and serves the double purpose of a protection for the in- 

 ternal 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 invertebrate animals, ex- 

 cept 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 universal struggle for existence. 



