MALARIA EXPEDITION TO NIGERIA 
81 
the labellae are applied. Above and resting between these areas is the concave tip 
of the labium (seen as a concave band of chitin tin section), on which the stylets 
rest : these three parts enclose a roughly triangular area covered by a delicate 
membrane, thrown into folds, and extending above along the under surface of the 
tip of the labium, fusing with its sides and tip ; on either side being in connection 
with the bases of the labellae and with the joint. This membrane bears a few very 
fine hairs, and it probably allows of considerable play when the labellae are separated ; 
with them it touches the skin when the mosquito sucks blood, being then stretched 
to some extent. 
The labellae are conical and roughly crescent-shaped in section ; their apices 
form the extreme tip of the proboscis. They present two surfaces, an outer convex, 
an inner concave ; and two borders an upper and a lower, the former being in the 
same line as the edges of the upper surface of the labium. 
In some species of Culex and in Anopheles maculipennis they consist of two 
parts, a distal, and a basal upon which the distal half is jointed to allow of some 
outward movement ; the joint being represented by a narrow white line beginning 
near the apex on the outer surface at its upper border and curving sharply downwards 
and outwards to the lower border ; about the centre of this line is a sharp upward 
bend. This peculiar division of the labella is absent in Anopheles costalis. Smith' 
who points out the homology of the so-called labium of the Diptera with the galea 
of other insects, states with regard to its tip in five species of Culex he examined 
' no two agreed in structure.' We have found this to be the case also in a few 
species of mosquitoes we have examined, especially with regard to the structure of 
the joint at the base of the labella. 
The outer surfaces of the labellae are covered with fine hairs and here and 
there coarser ones. The inner concave surfaces are marked by longitudinally 
ridges and folds. There are no 'pseudo-tracheae.' In transverse section (plate XV, 
fig. i), three regions are distinguished ; an upper somewhat flattened, the 
chitin of which is very thin and thrown into numerous small folds from which 
arise a felt work of exceedingly fine long hairs, crossing one another in all 
directions ; a lower area of fairly thick chitin limited by the rounded 
inferior border of the labella : from it arises long thick bristle-like hairs projecting 
downwards in between the tips of the labellae. These two areas, well marked 
near the tips of the labellae, gradually fade away towards their bases. Between 
them, the central region is deeply hollowed out and ridged and folded, its 
chitin is much thicker and free from hairs. A little below its centre, running 
longitudinally down this surface is a stout ridge of chitin which can be traced 
in a cleared specimen of the labium, mounted whole, to the base of the labella 
(plate XIX, fig. 3) : here it makes an outward curve to about the centre of the 
1. Smith, Tram. American Entomol. Sac, vol. xvii, 1890, p. 330. 
L 
