PROBOSCIS OF PHORMIA REGINA — DETHIER 1 59 



may also be initiated by stimulation of interpseudotracheal papillae, 

 and it is probable that here also stimulation of a single cell may suffice. 



Feeding may be terminated as a result of : ( i ) cessation of sensory 

 input from the cells mentioned either by removal of the stimulus or 

 by adaptation; (2) central adaptation; (3) inhibitory impulses into 

 the brain from the gut, signaling satiation (Dethier and Bodenstein, 

 1958) j (4) stimulation of rejection neurons in the chemoreceptive 

 hairs of the tarsi or labellum. 



The fact that many phases of the complicated feeding response 

 may be triggered by a single cell raises the interesting question of 

 how the afferent impulses are routed once they reach the brain. Fur- 

 thermore, it is of interest that the sequence of events is ordered, that 

 the number of steps, the simultaneity of steps, and even the omission 

 of steps under certain experimental conditions all follow from stimu- 

 lation of a single neuron and are controlled apparently by the fre- 

 quency of afferent impulses which in turn are regulated by the in- 

 tensity of the stimulus. 



In addition to the simple situation as just outlined, there are more 

 complicated experimental situations where antagonistic stimuli may 

 be balanced, with the result that the feeding response which is elicited 

 may be complete acceptance, complete rejection, or some intermedi- 

 ate action. 



Eventually the analysis of neural pathways will have to be made 

 electrophysiologically ; nevertheless, as a preliminary step, it is de- 

 sirable to learn which effectors are employed in the above-mentioned 

 situation and what their innervation may be. 



MUSCLES OF THE PROBOSCIS 



The proboscis of the blow fly is a tubular erectile organ evolved 

 from the mouth parts and clypeal regions of the head (fig. i). In 

 repose it lies withdrawn into the ventral regions of the head capsule. 

 It consists of three well-defined sections ; the proximal section or ros- 

 trum, the middle section or haustellum, and the distal oral disc. The 

 principal supporting element is a large internal skeletal piece termed 

 the fulcrum. 



There are 16 muscles in the proboscis of the blow fly ; 5 have their 

 origins in the head capsule, 7 originate in the rostrum, and 4 originate 

 in the haustellum. 



The muscles of the proboscis of Phormia are nearly identical to 

 those of Calliphora as described by Graham-Smith (1930) ; accord- 

 ij^gly* his nomenclature and scheme of presentation will be employed 

 in the following detailed descriptions. 



