PHYLUM ARTHROPOD A 117 



tion for particular functions? What system of the lobster has made the least 

 progress? Do both the lobster and the frog seemed to have attained a fairly 

 high degree of adjustment (adaptation) to the conditions under which each 

 lives? 



B. THE ANATOMY OF THE GRASSHOPPER 



The grasshopper is selected as a representative of the great group of insects 

 because it is relatively large, easily obtained, and a rather generalized form. The 

 following description is based upon the large Florida grasshoppers. 



i. External anatomy. Obtain a preserved specimen. Compare with the 

 earthworm and especially with the lobster. Is the animal bilaterally sym- 

 metrical? Is it segmented throughout? In what part of the body is the segmen- 

 tation most apparent? least apparent? Does it have jointed appendages like 

 the lobster? Do they occur on every segment? What part of the body lacks 

 them? Is this in accordance with the principle of specialization of anterior 

 regions, which we observed in the other animals studied? Does the animal have 

 a definite color pattern ? 



The body is covered by a chitinous exoskeleton, similar to that of the earth- 

 worm and the lobster. It is secreted by the ectoderm, or epidermis, which lies 

 beneath it. As in the lobster it consists of hard regions, or sclerites, joined 

 together by thin membranes, the arthropodial membranes, at lines known as 

 sutures. 



The body is divided into head, thorax, and abdomen. Are these regions 

 more distinctly separated than in the lobster? Each part consists of a definite 

 number of segments. Each segment as in' the lobster is typically composed of a 

 dorsal sclerite, the tergum (commonly called notum in insects), a lateral sclerite, 

 the pleuron, and a ventral sclerite, the sternum. 



a) The head and its appendages: Is the head readily movable upon the body? 

 The head shows no segmentation. The larger part of it is inclosed in one hard 

 sclerite, the epicranium, in which may be distinguished a top (vertex), sides 

 (genae], and a front (frons}. Looking at the head from in front, the lower 

 limit of the frons is marked by a distinct transverse suture. Below this suture 

 is a rectangular sclerite, the clypeus, and below this and attached to it another 

 sclerite, the bilobed upper, lip or labrum. Observe that the labrum is movable; 

 it is not, however, an appendage. 



The head bears eyes, antennae, and three pairs of mouth parts. There is a 

 pair of large compound eyes situated in the upper parts of the genae. Examine 

 them with the hand lens and note the minute hexagonal areas into which the 

 surface is divided. Each of these is the outer end of an ommatidium, and the 

 structure of the compound eye is the same as in the lobster. The grasshopper 

 also has three simple eyes, or ocelli, one anterior to the dorsal portion of each 

 compound eye on the ridge which separates frons and gena, and the third in 



