124 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



circum-esophageal commissures pass forward around the esophagus, and unite at 

 the brain, or supra-esophageal ganglion, located between the eyes. Does the 

 nervous system of the grasshopper resemble that of the lobster? 



Draw in the nervous system on your lateral view of the grasshopper. 



g) The sense organs: The chief sense organs have already been noted. The 

 following additional statements may be made. Read also Hegner (pp. 245-48). 

 Insects are provided with a great variety and abundance of sense organs. Much 

 work still remains to be done upon the structure and functions of these. Organs 

 of touch in the form of tactile hairs are present all over the body, but particularly 

 on the antennae, mouth parts, and cerci. Organs of taste are present as sensory 

 pits on the epipharynx, hypopharynx, and probably on the maxillary and labial 

 palps. The sense of smell is incredibly keen in insects, and is located mainly 

 upon the antennae in the form of sensory pits; it is probable that olfactory 

 organs are present elsewhere also. Hearing is localized in the grasshopper in 

 the chordo tonal organ; in other insects hearing may be served by auditory 

 hairs and auditory pits. The compound and simple eyes are the organs of vision; 

 there is no doubt that insects see objects, and they may even perceive colors, 

 but they are especially quick in detecting movements of external objects, a 

 capacity undoubtedly due to the compound structure of the eyes. 



3. General considerations on the grasshopper. What are the chief differ- 

 ences between the external anatomy of the grasshopper and the lobster? What 

 internal systems of the two animals show the most differences? What systems 

 of the grasshopper are segmented? What particularly effective and highly 

 specialized systems does the grasshopper possess? From your study of the 

 anatomy of the grasshopper, can you give reasons why insects are the most 

 successful and dominant animals on the earth, excepting man? 



C. GENERAL SURVEY OF ARTHROPODS 



Examine the specimens and compare them with each other and the lobster 

 and grasshopper as to degree of segmentation, specialization and number of 

 appendages, general form, symmetry, and divisions of the body. The common 

 groups of arthropods are: 



i. Crustacea, forms with two pairs of antennae, many pairs of appendages, 

 hard crustaceous exoskeleton, and gills as respiratory organs. Examine the 

 following representatives (not arranged by classification). 



a) Entomostraca, small to microscopic Crustacea, common in fresh water, 

 often inclosed in a bivalve carapace. Preserved forms are of little value; living 

 forms if available will be demonstrated. 



b) Barnacles, sessile forms, inclosed in calcareous plates. Marine forms, 

 covering the rocks along the seashores. 



