INTRODUCTION 7 



2. Ears in the form of tympanal organs, found only in a comparatively 

 small number of insects, on the base of the abdomen in grasshoppers and 

 on the base of the front tibiae in katydids and crickets. 



3. Modified hairs with a great variety of functions which may be grouped 

 into two classes, (a.) long hairs chiefly serving as organs of touch, certain 

 of them vibrating with definite tones are believed to be organs of hearing, 

 (b.) very much shortened hairs, often almost completely suppressed, retaining 

 and specializing however, the basal structures. These respond to chemical 

 stimuli and may be roughly classed into organs of taste and smell, according 

 as they respond to the contact of liquids or gasses. Many different kinds 

 of these modified sense hairs or pits may occur on the same insect indicating 

 perhaps the possession of senses unknown to us. 



Besides the cells which took part in forming the ventral plate there were 

 other cells in the embryo which remained in the yolk or only attached 

 themselves to the inner surface of the ventral plate. From these the blood- 

 cells, connective tissue, fat bodies and muscles arise. The blood cells 

 correspond with the white blood corpuscles, nothing existing in insects com- 

 parable iWith red blood cells. Connective tissue forms a thin investment 

 over air, the organs. When invaded by a nerve it develops into a muscle, 

 and when storing an inordinate amount of food material it is a fat body. 



Since the skeleton of an insect is external it is necessary to cast off the 

 skin from time to time to provide for the increasing size. This molting pro- 

 cess is quite complicated. The following steps may be recognized: — 

 Preliminary. 



1. Storing food to repletion. 



2. Growth of skin epithelium, increase in both number and size of cells. 



3. Emptying of digestive tract. 

 Somnus. 



4. Detachment of epithelium and muscles from cuticle thru the secretion 

 of a fluid as in a blister involving the withdrawal of the contents of the 

 harder parts, (claws, head of caterpillars, etc.) 



5. Completion of epithelium over ends of muscles. 



6. Secretion of expansion layer which becomes much wrinkled thus provid- 

 ing for enlargement of skin surface. 



7. Deposition of new cuticle against expansion layer, largely exhausting 

 the epithelial cells. 



8. Regaining of muscle attachment to skin. 

 Molt. 



9. Muscle action pulling legs and abdominal parts into thorax. 



10. Bursting of old cuticle from internal pressure, along back of thorax. 



11. Emergence of the insect. 



