PHYLUM ARTHROPOD A 119 



prothorax is greatly enlarged and extends back like a shield over the other two 

 segments of the thorax. It is also seen to be composed of four distinct sclerites, 

 one behind the other. The sternum of the prothorax has a sharp posteriorly 

 directed spine; the pleuron is rudimentary. The prothorax bears the first pair 

 of legs. Is the prothorax independently movable? 



Cut away the backward extension of the pronotum. In the middle of the 

 side, in the membrane between the prothorax and mesothorax, find an oval 

 opening. This is a spiracle, or stigma, one of the openings into the respiratory 

 system. In the mesothorax, identify the dorsal tergum, or notum, the lateral 

 pleuron, the ventral broad sternum. Both tergum and pleuron are composed 

 of more than one sclerite. The mesothorax bears dorsal outgrowths, the first 

 pair of wings, and a ventral pair of appendages, the second pair of legs. Cut 

 off the wings. 



The metathorax is similar in form and parts to the mesothorax and bears the 

 second pair of wings and third pair of legs. Cut off the wings. Locate another 

 spiracle between the ventral portions of the pleura of the mesothorax and 

 metathorax. 



Examine the wings. Compare them as to form, size, color, thickness. What 

 do you consider to be the functions of each pair? The wings arise as saclike 

 outgrowths of the body wall. During development the two walls of the sac 

 become pressed together, forming a thin membrane. The veins or nerves of the 

 wings are respiratory tubes, filled with air, each surrounded by a tubular blood 

 sinus. After the insect attains its adult size, blood ceases to flow in the wings, 

 and they become dry, hard, and lifeless. Examine a piece of the second pair of 

 wings under the microscope, and note the air tube, or tracheal tube, and the blood 

 sinus around it in each of the veins. 



Remove a leg from the body noting the depression in the body where it fits, 

 the arthropodial membrane which attaches it to the body, and the muscles at 

 its base. It is composed of five segments. The rounded segment which adjoins 

 the body is the coxa. It is succeeded by a quite small joint, the trochanter. The 

 next segment is the long powerful femur. Beyond this comes the more slender, 

 spiny tibia. The terminal part of the leg is the tarsus, consisting of three joints, 

 and with five ventral pads, the pulvilli, and a terminal pair of hooks. Compare 

 the three legs of the grasshopper with each other. Do all have the same parts? 

 Can you associate differences in relative proportions of parts with differences in 

 function? Are the legs well "adapted" for their functions? What are the uses 

 of the pulvilli and terminal hooks? 



c) The abdomen and its appendages: Each abdominal segment consists of a 

 tergum, a U-shaped piece forming the dorsal and lateral walls, and a sternum, 

 the convex ventral plate. The pleuron is practically lacking but is represented 

 by the membranous fold where tergum and sternum articulate. Just above this 

 line of junction, on the lower border of the tergum will be found a spiracle, or 



