42 MEDICAL ENTOMOLOGY 



The lobelia are oval lobes, together about the same diameter as the end 

 of the labium when in the position of rest, but capable of great disten- 

 sion and change of form. They are completely sepa- 

 The Labella . J J . . , r . 



rated from one another in front by a fissure which is 



continuous with the groove on the anterior surface of the labium, and 

 which extends a little distance on to the posterior surface also, partially 

 separating them on that aspect. Each labellum has an outer and an 

 inner wall, which enclose a space continuous with the haematocoele of 

 the labium, and therefore with the rest of the body cavity. 



The outer wall is continuous with the outer wall of the labium. It 

 consists of membrane, reinforced, especially at the distal border, with a 

 deposit of chitin sufficient to impart to it a certain amount of rigidity. It 

 bears many long and short hairs scattered over its surface, and a promi- 

 nent fringe of macrochaetae at the distal margin of the proboscis. 



The inner walls of the labella in the resting state are in contact with 

 one another, but when in use are diverged so as to come into the same 

 plane, lying perpendicular to the long axis of the proboscis and in con- 

 tact with the surface on which the fly is feeding ; they are very highly- 

 specialized to form the structure known, from the resemblance of its 

 channels to the tracheae or air tubes of the body, as the pseudotra- 

 cheal membrane. This important structure, by means of which the fly 

 absorbs its food, requires a detailed description. 



The groundwork of the inner wall consists of a thin, transparent, 

 and apparently structureless membrane, which is continuous at the dis- 

 tal margin of the proboscis with the outer wall of 



the labella, and is attached internally to the discal 

 Membrane - 



sclerite, a ring of chitin which surrounds the presto- 

 mum. This membrane is traversed by a number of grooves, which 

 commence at the periphery and converge to the prestomum in a 

 regular manner, the upper ones running inwards and downwards, and 

 the lower ones inwards and upwards. The details of their arrange- 

 ment differ a good deal in even closely allied species. Usually there 

 are between twenty and thirty grooves altogether, and of these the distal 

 six and the proximal six each unite to form a larger channel, which 

 opens at the prestomum, while the middle channels open directly. Those 

 formed by the union of several separate channels are of a slightly 

 different nature to the rest, and are termed collecting channels. 



The minute structure of these channels is very remarkable. Each 

 is an actual depression in the homogenous membrane which forms the 

 inner wall of the labellum. The lumen of the groove, which in section 



