CHEMORECEPTION 113 



By observing the responses of insects (from which various append- 

 ages or parts thereof have been extirpated) to food, naturally occurring 

 attractants, and odours to which they have been conditioned, it has 

 been established that the principal sites of chemoreceptors are: 

 antennae, maxillary and labial palpi or their homologues, legs, and 

 ovipositors. Since all of these areas contain a large and heterogeneous 

 population of sensilla, the assignment of a chemoreceptive function to 

 specific ones has been an arduous and uncertain task (cf. Dostal, 1 958 ; 

 Schneider, 1961). Thus, despite extensive histological studies from 

 those of Hauser (1880), Schenk (1903), and Vogel (1923 b) to the 

 present time, the identity of chemoreceptors is known in fewer than a 

 score of cases. 



Olfactory Receptors 



An olfactory function has been assigned to the following receptor types 

 with a fair degree of certainty : in the honeybee, sensilla placodea (von 

 Frisch, 1921; Dostal, 1958), sensilla basiconica, and possibly sensilla 

 coeloconica (Dostal, 1958); in the dung beetles, Geotrupes sylvaticus, 

 G. vernalis, Necrophorus tomentosus, N. vespilloides, Silpha novebor- 

 acensis, and S. americana, sensilla basiconica (Warnke, 1931, 1934; 

 Dethier, 1947 a) ; in lepidopterous larvae, sensilla basiconica (Dethier, 

 1941; Morita and Yamashita, 1961); in the human louse, sensilla 

 basiconica (Wigglesworth, 1941); in housefly larvae, sensilla basi- 

 conica (Bolwig, 1946); in Drosophila, sensilla basiconica (Begg and 

 Hogben, 1946); in Phormia regina, sensilla basiconica (Dethier, 

 unpublished) ; in saturniid moths, probably sensilla trichodea, sensilla 

 basiconica, sensilla coeloconica (Schneider, 1961) ; in the grasshoppers 

 Melanoplus differentialis and Romalea microptera, sensilla basiconica 

 and sensilla coeloconica (Slifer, 1961). 



The saUent features of sensilla placodea as seen with the light 

 microscope are illustrated in Fig. 11. Details of structure have been 

 clarified by the electron-microscopic studies of Richards (1951) and 

 Slifer (1961). The sensillum is essentially a thin circular or oval plate 

 connected to the surrounding cuticle by a delicate circumferential 

 membrane and equipped with twelve to eighteen bipolar neurons 

 (Vogel, 1921, 1923 b). Each dendrite possesses one or two minute 

 refringent bodies of unknown function, the Riechstabschen (Vogel, 

 1923 b). Furthermore, each dendrite ends in a cilium-like structure 

 (Slifer, 1961) similar to that described in tympanic organs. From each 

 cilium-like structure there extends a fibrous strand. The strands termi- 

 nate on the extreme edge of the plate (Slifer, 1961). The antenna of a 



