158 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



boscis, by stimulating single chemoreceptive hairs on the tarsus or 

 labellum of the blow fly. Following the development by Hodgson, 

 Lettvin, and Roeder (1955) of means of recording electrically from 

 single hairs, several workers have begun detailed correlative studies of 

 the behavioral and electrophysiological events incorporated in probos- 

 cis extension, the first component act of the feeding reaction. Great 

 interest attaches, therefore, to the precise route which the action po- 

 tentials generated in the chemoreceptor unit follow to and through 

 the cerebral ganglia and thence to the effectors which cause the pro- 

 boscis to extend. Similarly, there is interest in the pathways utilized 

 by action potentials generated by unacceptable compounds, such as 

 sodium chloride, which are rejected by the fly and result in withdrawal 

 of the proboscis. It becomes necessary, therefore, to map the course 

 of the nerves in the head and the origins and insertions of the various 

 muscles associated with movements of the proboscis. 



The most complete morphological studies of the proboscis of mus- 

 coid Diptera have been those of Lowne (1870, 1890-95) on Calliphora 

 vomitoria and C. erythrocephala, Hewitt (1914) on Miisca domestica, 

 Graham-Smith (1930) on Calliphora erythrocephala, and Snodgrass 

 on a variety of species. Although Phormia regina has been the sub- 

 ject of numerous physiological studies, it has not attracted the atten- 

 tion of morphologists. The present study was undertaken with the 

 views peculiar to physiology foremost in mind. 



METHODS 



Dissections were made on freshly killed flies which were from 2 to 

 5 days old and on specimens preserved in alcoholic Bouin's fixative. 

 When fresh specimens were employed, the tissues were first bathed in 

 a methylene blue solution which was then drained off and replaced 

 with water. In addition to gross dissection, serial sections were pre- 

 pared by the usual histological techniques. 



THE FEEDING REACTION 



The feeding reaction of the blow fly as described by Dethier (1955) 

 and Dethier, Evans, and Rhoades (1956) may be subdivided into the 

 following component actions: Extension of the proboscis, spreading 

 of the labellar lobes, and sucking. The complete sequence of events 

 may be brought about by stimulation of a single labellar or tarsal 

 chemoreceptive hair. Although each hair is provided with three bi- 

 polar neurons, evidence from electrophysiological studies has shown 

 that only one need be activated for proboscis extension. Sucking 



