88 THOMPSON YATES LABORATORIES REPORT 
us also that the two halves of the olive and the little tongue being in the above- 
mentioned position have an importance in directing the movements of the fllariae 
towards the wound made by the stylets. Perhaps the gases emitted in the first 
moment of the bite help the entry of the filariae into the body of the definitive host.' 
One cannot read this paragraph without being struck with the remarkable 
ingenuity displayed in its account of how the filariae leave the proboscis of the 
mosquito. But a very careful and exhaustive study of the structure and relations of 
the parts forming the proboscis has convinced us of the utmost difficulty the most 
inquiring of observers would experience in deciding the occurrence of any such slit in 
the upper surface of the labium, as the authors believe they have seen. Furthermore, 
the upper surface of the labium is composed of chitin almost as thick as that on the 
outer surface (plate XV, fig. 3). Moreover, from the illustration of the longitudinal 
section of the proboscis accompanying Low's article, it appears to us that the head of 
the filaria in the labium is considerably beyond the middle of the labium, in fact 
appears to reach the distal end of the labium proper — as Manson 1 says, ' to the 
tip of the proboscis.' Such a position, if the filariae escape in the manner 
Grassi and Noe imply, would necessitate their exit, middle part first, at the 
bottom of the very acute angle formed by the two almost completely folded parts of 
the labium. The difficulties involved in such a method of exit appear to us 
insurmountable. 
Referring again to the structure of the extreme tip of the labium (page 80), 
we have stated that at about 0*16 mm. from the tip of the proboscis the labium 
proper appears to end bluntly but its upper surface is found to continue on, gradually 
tapering to a blunt point covered with fine hairs. And again (page 81), above and 
resting between these areas (lateral areas on the end of the labium proper) is the 
concave tip of the labium (seen as a concave band of chitin in section) on which the 
stylets rest ; these three parts (the two areas and the concave chitinous band) enclose 
a roughly triangular area covered by a delicate membrane thrown into folds ; above 
it extends along the under surface of the tip of the labium, and on each side is in 
connection with the bases of the labellae. 
When the tip of the proboscis is applied to the surface of the skin, it has 
been seen that the two labellae swing apart and are rotated so that their inner surfaces 
are in contact with the skin, and that the piercing stylets are directed in their course 
by the concave upper surface of the extreme end of the labium. By the swinging of 
the labellae the delicate folded membrane is somewhat stretched and is close to the 
surface of the skin. This membrane is exceedingly delicate so that in transverse 
section even with the high powers of the microscope (i O.E.) the sections of its folded 
edges are represented by thin fine lines. It will thus be seen that this is the most 
delicate part of the labium ; and as both Low indicates in his illustration and 
1. Manson, Tropical Diseases. London, 1901. P. 496. 
