172 ELEMENTS OP COMPARATIVE ZOOLOGY. 



" neck." Separate the prothorax from the head and from 

 the mesothorax, and draw it from the side. 



Study meso- and metathorax together. Notice that on 

 the back the line between these somites is very distinct; 

 trace this line upon the side, and thence to the ventral 

 surface. Do you notice any other lines which seem to 

 divide meso- and metathorax ? Can you trace them on all 

 surfaces ? Do you find any spiracles in this region ? How 

 are the legs related to the somites ? Can you recognize in 

 each the same parts found in the prothoracic legs ? Where 

 are the wings ? Are they alike ? What is the prevailing 

 direction of the ribs or " veins " in them ? Can either pair 

 be folded like a fan ? Is there anything to protect the 

 hinder pair when at rest ? 



Draw a side view of meso- and metathorax, inserting ex- 

 panded wings, legs, etc. 



Eemembering what was found out about the crayfish 

 and sow-bug, and considering the thorax and tip of the 

 abdomen of the grasshopper, do you find anywhere a seg- 

 ment bearing more than a pair of jointed appendages ? * 

 So far as your present knowledge goes, would you be justi- 

 fied in saying that a pair of jointed appendages indicates 

 a somite of the body ? (Savigny's law.) 



Notice that the head is made up of a large solid piece 

 (epicranium), to which are attached various movable por- 

 tions. On either side of the head is a large compound eye. 

 With a sharp knife slice off one of these eyes and examine it 

 under a low power of the microscope. Why is it called 

 compound ? What is the shape of the parts (facets) of 

 which it is composed ? 



Look on the front of the head for the smaller bead-like 



* For reasons which cannot be discussed here, the wings of grass- 

 hoppers, etc., are not considered as jointed appendages. 



