MALARIA EXPEDITION TO NIGERIA 85 
portion. Anteriorly and laterally it is curved upwards, and unites with the sides of 
the clypeus (plate XVII, fig. 1). From its under surface near its posterior edge it 
gives off on each side a hook-like ridge of chitin (plate XVI II, fig. 1, x) from which the 
muscles of the salivary receptacle have origin. Dim mock describing the pharynx 
states 'the channel for the passage of food turns upwards and then backwards again, 
passing in its course a place where its wall approximate dorsally and ventrally ; this 
narrowing of the walls is probably a valve to prevent the return of fluids to the 
mouth during the pumping process.' 
In Anopheles costalis, situated in this position and attached to the upper surface 
of the slightly horizontally bent posterior end of the ventral chitinous plate, is a 
peculiar ridge of chitinous stout hair-like processes, which curve forwards so that 
their tips lie in the angle between the upper surface of the first and second parts 
of the pharynx. The hairs are of two kinds, an anterior large set — probably a single 
row — and a posterior, small, fine set situated in a clump immediately behind the 
former. The larger hairs consist of a short stout shaft firmly embedded in the 
chitinous pharyngeal wall ; this shaft supports a cup with a free rim curved outwards ; 
within the cup lies the oval-shaped bulbous extremity of the base of the hair ; this 
bulbous extremity contains a single large cell. The remaining free portion of the 
hair curves forwards and tapers to a fine point, and appears to have a central shaft 
enclosed within a chitinous cuticle from which barb-like processes project. The hairs 
of the posterior set are much finer and shorter, and are more numerous ; they appear 
to be simple in character. In transverse section (plate XVIII, fig. 2) this structure 
presents to some extent the appearance of ' rods and cones.' The suboesophageal 
ganglion lies in close proximity to this structure, but no nerve fibres have been traced 
to communicate with these specialised hairs, although such probably exist. That in 
the first place these hairs act in conjunction with the general conformity of this part 
of the pharynx as a valve to prevent the regurgitation of blood back into the mouth 
during the action of the pumping organ seems to admit of no doubt ; on the other 
hand such specialisation in structure would lead one to suppose that they possess also 
a sensory function. 
The mechanism of the proboscis. The mosquito, when alighting on the surface 
of the skin for the purpose of sucking blood, immediately raises the palpi almost at 
right angles to the proboscis. After probing about with the labellae for a suitable 
spot to pierce the skin, it plants them firmly on the surface, the proboscis being directed 
a little forwards. A moment later the labium is seen to bend backwards near its 
junction with the head, the stylets, remaining straight, becoming thus uncovered. 
The bending of the labium becomes more marked as the stylets sink into the skin, 
the angle of the bend travelling towards the middle of the length of the proboscis, 
so that when the stylets have entered the skin to nearly their full extent, the labium 
is bent double beneath the head of the insect. Reaumur was the first, probably, 
