THE LOCUST 157 



exoskeleton, composed of a horny substance called chitin, protects the 

 internal organs and serves as a place of attachment for the muscles. 

 Bend the animal and observe that the exoskeleton is not absent but 

 only thinner at the joints of both body and appendages, so that the 

 entire outer surface is covered by a continuous armor. Recall the 

 skeleton of the crayfish. Observe that on each abdominal somite the 

 exoskeleton consists of a dorsal portion, the tergum, and a ventral 

 portion, the sternum, joined by a thinner region below the line of the 

 spiracles. Compare this sort of skeleton with that of the frog or man, 

 which is an internal skeleton, or endoskeleton. 



(c) The head is made up of several somites, as indicated by the 

 fact that it bears several pairs of appendages, but these somites are so 

 closely united that they cannot be distinguished. On the head are two 

 large compound eyes. Examine the surface of one of them with a 

 handlens and see that it is divided into a large number of small areas, 

 each of wliich is the surface of one of the small independent visual 

 units, or ommatidia, which make up the compound eye. There are 

 also three simple eyes, or ocelli, which can be seen with the handlens. 

 Two of these lie in front of and near the top of the two compound 

 eyes; the third one lies in the median line somewhat ventral to the 

 antennae, or feelers. The skeleton of the top and front of the head 

 is known as the epicranium. Below this is the clypeus, from which is 

 suspended the labrum, or upper lip. The side of the head below the 

 compound eye is the gena. The head bears four pairs of appendages, 

 which will be identified and studied in greater detail later. 



(d) The thorax consists of three somites: the prothorax, meso- 

 thorax, and metathorax, each of which bears a pair of legs. The 

 dorsal surface of the prothorax has the form of a hood consisting of 

 several fused plates and extending backward some distance over the 

 mesothorax. Each leg of the grasshopper has five divisions: the coxa, 

 a short segment by which the leg articulates with the body; the tro- 

 chanter, also a short segment (not distinct in the large jumping leg) ; 

 the femur, a long segment; the tibia, a long segment bearing spines; 

 the tarsus, which is divided into several parts bearing pads below and 

 ending in a pair of hooks and a little pad. The thorax bears two pairs 

 of wings, a pair of tough, thick ones, and a pair of thin, membranous 

 ones. To which somites do they belong? 



(e) The abdomen includes ten somites and is without appendages. 

 Somites 2-7 in the female, and 2-8 in the male, are complete rings and 

 essentially alike. The first somite is interrupted at the sides by a 

 backward extension of the metathorax, which bears the third pair of 

 legs. The dorsal portion of the first somite bears the tympanic mem- 



