402 THE ALIMENTARY CAXAL OF THE IMAGO. 



occasionally the food is rendered frothy by the expulsion of this 

 air, which frequently passes through the pseudo-trachese. This 

 was also observed by Reaumur [1, p. 209]. 



It does not appear to me probable that the tubular mouth 

 exerts any very considerable power of suction, as the size of its 

 cavity is not apparently capable of any great variation. No 

 doubt it is slightly increased by the contraction of the labral 

 muscles and of the transverse muscles of the haustellum. 

 I believe the food material is forced into it by the oral sucker 

 rather than drawn into it by the expansion of its cavity. The 

 main suction is due to the contraction of the dilator pharyngis, 

 whilst the recoil of the elastic wall of the pharynx certainly 

 drives the food back into the abdominal crop. It is afterwards 

 regurgitated into the mouth and prestomum, or even forms a 

 drop between the partially-closed lobes of the oral sucker 

 before it is finally swallowed and transmitted by the proventri- 

 culus to the chyle stomach, a fact known to and recorded by 

 Reaumur. 



It is possible that all the food is not first transmitted to the 

 crop, and that some may pass at once into the proventriculus. 

 The crop may only serve to store food when the chyle stomach 

 is already actively digesting, but flies usually regurgitate their 

 food before it passes into the chyle stomach. 



e. Nerves and Nerve-end Organs. 



The Nerves and Nerve-end Organs of the Proboscis. The 

 pharyngeal nerves are a pair of slender nerves which lie one 

 on either side of the oesophagus ; they are chiefly distributed to 

 the muscles of the pharynx and labrum. The labial nerves 

 are much larger, and descend behind the fulcrum ; each gives 

 a large branch to the palpus. They also supply the retractor 

 muscles of the proboscis and traverse the haustellum, to the 

 muscles of which they give branches; they terminate in 

 numerous twigs, which end in special sensory terminals at 

 the bases of the seta: which fringe the oral sucker. 



These setae are grooved, and the nerve-end organs at their 

 bases so closely resemble glands that Kraepelin describes them 



