CHEMORECEPTION 115 



The typical thin-walled sensillum basiconicum of the grasshopper 

 antenna is 16 [j. long and 3 fx in diameter at its base (Fig. 13). The scol- 

 opoid sheath, instead of extending to the tip as in the thick-walled 

 pegs, makes a right-angle turn at the base of the peg and comes to the 

 surface. At moulting the old sheath is pulled out through this area. 

 The dendrites of the forty to sixty neurons enter the scolopoid sheath 

 in the usual manner, but when the latter turns, they perforate its side 

 and continue into the lumen of the peg (Fig. 71). Within this fluid-filled 

 cavity each one branches once or twice and each branch terminates at 

 one of about 150 openings on the peg's surface (Slifer, Prestage, and 

 Beams, 1959). At its termination each branch is composed of about 

 twenty-four parallel finger-like processes (Fig. 71). The long, hotly 

 debated question as to whether the neurons of chemoreceptors are 

 exposed to air, a question for which the conventional answer had been 

 negative (cf. Dethier, 1954 b) would now seem to have been answered 

 in the affirmative. 



Contact Chemoreceptors 



Receptors sensitive to stimulation by chemical solutions applied 

 directly have been identified positively in Diptera, Lepidoptera, 

 Hymenoptera, and Coleoptera. They are trichoid sensilla located on 

 the legs and mouth-parts. Small sensilla basiconica on the labella of 

 flies have also been identified as gustatory receptors. There is no doubt 

 that there are also other types of gustatory receptors, but their identity 

 still requires confirmation. For details regarding the location of con- 

 tact chemoreceptors in various insects the review of Frings and Frings 

 (1949) should be consulted. 



The contact chemoreceptive hairs, as exemplified by those on the 

 tarsi and labella of Diptera, are actually compound sensilla ; that is, 

 they are organs containing several kinds of receptors. In the black 

 blowfly, Phormia regina, the receptors occur as groups of three, four, 

 or five bipolar neurons associated with long (30-300 \l) hairs which 

 characteristically possess two lumina (PL II). The cell bodies of the 

 neurons lie in a subhypodermal position beneath the hair in association 

 with the tormogen and trichogen cells and one or more neurilemma 

 and tracheal cells. The entire group of cells is wrapped in a thin 

 membrane which is continuous with the basement membrane of the 

 surrounding hypodermis and with the sheath enveloping the nerve. 

 The nerve consists of the axons of the receptor cells, which are be- 

 lieved to extend, without synapsing, to the central nervous system. The 

 dendrites of the neurons extend into a scolopoid sheath and then up 



