PROBOSCIS OF PHORMIA REGINA — DETHIER I7I 



cles controlling extension of the rostrum. On the other hand, under 

 certain conditions, a weak stimulus can cause extension of the haustel- 

 lum and spreading of the lobes of the labellum even though the 

 proboscis as a whole is in the retracted position. Under these condi- 

 tions, three different sets of muscles are receiving adequate stimula- 

 tion; namely, the retractors of the paraphyses, the transverse muscles 

 of the haustellum, and the retractors of the furca. 



It would appear from these observations that the quantitative re- 

 cruitment of motor fibers is regulated in part by the intensity of 

 sensory input but that information from another source recruits the 

 association neurons responsible for selecting which sets of muscles 

 are to be placed in operation. 



Still another type of proboscis response characteristic of a weak 

 stimulus is a leisurely extension which is in marked contrast to the 

 violent extension occasioned by a strong stimulus. It is clear, there- 

 fore, that the intensity of stimulus controls not only the recruitment 

 of motor fibers but also the intensity of motor response. 



Sucking is carried out in its entirety by the dilators of the cibarial 

 pump. It is initiated by stimulation of either the labellar hairs or the 

 interpseudotracheal papillae and is undoubtedly monitored by sensilla 

 situated in the labrum-epipharynx hypopharynx. The sensilla in the 

 labial and pharyngeal regions send fibers into the labral nerve which, 

 as the labrofrontal nerve, enters the brain. The fibers of the inter- 

 pseudotracheal papillae and labellar hairs ascend in the labial nerve. 



The muscles responsible for sucking are innervated by fibers of the 

 labral nerve. Accordingly, while it is possible that all the fiber tracts 

 utilized for proboscis extension lie within the suboesophageal ganglion, 

 the act of sucking requires that afferent impulses pass via the cir- 

 cumoesophageal connectives to the brain. One might speculate that 

 there is tighter central control over sucking than that exercised over 

 extension. From an adaptive point of view tighter control is obvi- 

 ously desirable. Furthermore, unacceptable compounds which may 

 escape detection by the labellar hairs (e.g., 1-arabinose, which is ac- 

 ceptable as far as the labellar hairs are concerned but unacceptable as 

 far as the interpseudotracheal papillae are concerned) stimulate rejec- 

 tion neurons in the interpseudotracheal papillae and are routed to 

 the brain to halt sucking. Recent experiments by Arab (in press) show 

 that sensilla in the labrum-epipharynx also monitor sucking. These 

 sensilla send fibers directly to the brain; consequently, at least spa- 

 tially, very fine control should be possible. 



In the absence of sensory input sucking ceases, and the proboscis 



