170 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



The labellar lobes are opened by contraction of the retractors of the 

 furca. Thus, six sets of muscles (if the muscles of the air sacs be 

 included) contract to give full extension of the proboscis. All are 

 innervated by fibers from the labial nerve. 



Sucking is accomplished by the action of the dilators of the cibarial 

 pump. These receive their innervation from the labral nerve. 



RETRACTION OF THE PROBOSCIS 



Retraction of the proboscis within the head capsule is accomplished 

 entirely by direct muscular action. The haustellum is flexed by the 

 flexors of the haustellum and the flexors of the labrum ; the fulcrum 

 is brought into a horizontal position between the genae by contraction 

 of the retractors of the fulcrum ; the entire proboscis is drawn up into 

 the head by contraction of the retractors of the rostrum and the 

 accessory retractors of the rostrum. Thus, five sets of muscles are 

 concerned with retraction of the proboscis. All but two of these 

 receive their motor fibers from the labial nerve. The retractors of 

 the fulcrum and the flexors of the labrum receive their motor fibers 

 from the labral nerve. 



Closing of the labellar lobes is brought about by elastic recoil when 

 the muscles responsible for extension are relaxed. Cessation of 

 sucking is brought about by relaxation of the dilators of the cibarial 

 pump. 



THE FEEDING RESPONSE 



A sufficiently intense stimulus, as, for example, 2M sucrose, applied 

 to a single sensory neuron of the labellum can cause vigorous and 

 complete extension of the proboscis. This means that impulses 

 ascend afferent fibers of the labial nerve to the suboesophageal ganglion 

 where they are distributed to a minimum of six different sets of ipsi- 

 lateral motor fibers and also cross over to six different sets of contra- 

 lateral fibers. Similarly, stimulation of a single sensory neuron on 

 any of the legs causes impulses to ascend from the thoracic ganglion 

 to the suboesophageal ganglion where the same motor fibers as before 

 are stimulated. 



When the intensity of the stimulus is low, there may be only partial 

 extension of the proboscis. This may take the form of extension of 

 the rostrum and partial extension of the haustellum, with the labellar 

 lobes remaining appressed. This reaction would suggest that only 

 fibers to three sets of muscles are affected ; namely, the extensors of 

 the haustellum, the adductors of the apodemes, and the indirect mus- 



