114 THE PHYSIOLOGY OF INSECT SENSES 



honeybee possesses from 3,000 to 30,000 sensilla placodea (Schenk, 

 1903; Mclndoo, 1914b; Vogel, 1923 b; Melin, 1941 ; Kuwabara and 

 Takeda, 1956; Dostal, 1958). Although the behavioural experiments 

 of von Frisch (1921) and Dostal (1958) support the idea that these 

 sensilla are olfactory, it is still difficult to comprehend in what way 

 their structure is particularly suited to chemoreception (see also 

 Snodgrass, 1935; Slifer, 1961). 



Sensilla basiconica are so variable in size, shape, and number of 

 neurons that a general description is meaningless. Extensive electron- 



FiG. 71. Diagram of a thin- walled sensillum basiconicum from a grass- 

 hopper antenna. A, section through a perforation showing finger-like 

 processes of dendrite ; B, small portion of scolopoid sheath showing the 

 dendritic processes passing through; C, longitudinal section of peg 

 showing dendrite passing through scolopoid sheath at base ; D, external 

 surface showing permeable basal spot and numerous fine perforations 

 of peg. (Redrawn from Slifer et al., 1959.) 



microscope studies have been made only of grasshoppers on the an- 

 tennae of which two types occur (Slifer, 1961). The length of a typical 

 thick-walled peg (sensillum basiconicum) is 20-50 [i (Fig. 14) (Slifer, 

 Prestage, and Beams, 1957). The tip possesses an opening about 2 (x in 

 diameter. A tubular cuticular sheath extends from this opening down 

 the lumen of the peg, constricts in the region of its base, then again 

 widens. It is the scolopoid sheath. The dendrites of the neurons, usually 

 five, are conducted within this sheath to the tip of the peg, where, 

 according to Slifer, Prestage, and Beams (1957), they are exposed to 

 the air and bathed in a fluid derived from the vacuole of the tormogen 

 and trichogen cells. At moulting the cuticular sheath is drawn out 

 through the orifice at the tip of the peg. 



