GENERAL CHARACTERISTICS OF THE SENSORY SYSTEM 19 



detail in Chapter III. The plate organs (sensilla placodea) are marked 

 externally by an oval or elliptical plate surrounded by a narrow 

 membranous ring. Each is innervated by a number of neurons (Fig. 1 1). 

 One type of sensillum (sensillum scolopophorum, scolopoid 

 sensillum, peg organs, chordotonal organs, stiftfUhrcnder Sinnesor- 



FiG. 12. Different forms of scolopoid sensilla. A. Terminal peg from a 

 grasshopper sensillum. (From Eggers.) B. Terminal peg from the 

 tracheal organ of Gryllus. (From Schwabe.) C. Terminal peg from the 

 femoral chordotonal organ oiPediculus. (From Graber.) D. Terminal 

 peg from a chordotonal organ in the abdomen of a cerambycid larva. 

 (From Hess.) E. A simple chordotonal organ with one sensillum. 

 (From Eggers.) F. A. chordotonal sensillum from the haltere of a 

 muscid fly. (From Pflugstaedt.) G and H. Scolopoid sensilla from the 

 tympanic organ of Cicadetta coriaria S- (From Vogel.) I. Sensillum 

 from the tibial organ of Decticus. (From Schwabe.) J. Sensillum from 

 the subgenual organ of Decticus. (From Schwabe.) 



gane) does not fit conveniently into any category. The sense cell is a 

 bipolar neuron, but its dendrite is not always associated with a 

 particular cuticular structure. These sensilla usually do not occur 

 singly. They are generally gathered together in bundles having a 

 common point of attachment to an undifferentiated part of the body 

 wall. Sometimes a small external pit, thickened disk, or nodule 



