4o2 © THE ALIMENTARY CANAL OF THE IMAGO. 
occasionally the food is rendered frothy by the expulsion of this 
air, which frequently passes through the pseudo-trachee. 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 
ctop, 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 cesophagus; 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 setz which fringe the oral sucker. 
These sete are grooved, and the nerve-end organs at their 
bases so closely resemble glands that Kraepelin describes them 
