Chemoreception 



451 



tures. In the aquatic species of insects the subject of two types of chemore- 

 ceptors is controversial, and the problem is well reviewed by Dethier and 

 Chadwick.-*' There is good evidence for the existence in insects of a third 

 type of chemoreceptor which is excited at high concentrations, and the re- 

 sulting response is usually an avoiding reaction. These receptors presumably 

 mediate the general chemical sense previously mentioned (page 447). 



Placode 



mmal strand 



Cap cell- 



Enveloping eel 



scicle of 



ense cell processes 



Vacuole 



Sense cells 



Basement membrane 



Fig. 144. Diagram of a sensillum placodeum of the bee, vertical section. 



From Snodgrass. '" 



Fig. 145. A, The erected antennae of a fly during flight, exposing olfactory pits to the 

 air stream. B, Diagram of an olfactory pit showing the rod-Uke endings of the sense cells. 

 (After Lieberman^" from Wigglesworth,""). 



STRUCTURE. Functionally, the chemoreceptors of the insects tend to simu- 

 late the chemoreceptors of the vertebrates, but structurally they are much 

 more complex. Olfaction among insects is mediated by three types of sense 

 organs: pore plates (sensilla placodea), thin-walled pegs or cones (sensilla 

 basiconica), and thin-walled pegs or cones sunken in pits (sensilla coeloco- 

 nica), illustrated in Figures 144 and 145. These olfactory receptors all pos- 



