MALARIA EXPEDITION TO NIGERIA 83 
obliquely towards the tip of the proboscis. These bellies of muscle end in minute 
tendons which join a very long common tendon, running parallel to the chitinous 
ridges and extending the whole length of origin of the muscle. These long tendons 
do not quite reach the mid-line of the labium (plate XIX, fig. 2, l.m') ; becoming some- 
what thicker, they are eventually inserted into the bases of the labellae, chiefly at the 
chitinous angle mentioned above. These muscles do not appear to take origin from 
the basal third of the chitinous ridges of the labium. Dimmock 1 describes in Culex 
two muscles in relation to each labella, a flexor and an extensor, the flexor being to 
the inner, the extensor to the other side of the cavities of each lobe, and having 
oriarin within the head. 
Muscles attached to the base of the labium. One pair of muscles is attached 
directly to the base of the labium. These are a pair of spindle-shaped muscles, 
each of which arises from the under surface of the basal chitinous support of the 
maxilla and is inserted into a ridge of chitin projecting from the groove which 
separates the labium from the under surface of the head (plate XIX, fig. 1, and XVII, 
fig. 1, 1.m'). Dimmock describes these muscles in Culex as extending along the labium. 
The clypeus, or epistom, is the anterior projecting hood-shaped portion of 
the face from which the proboscis is suspended. It is limited above from the rest of the 
head by a deep groove; behind and to the right and left of this groove arise the antennae 
which are to some extent supported by the upper surface, this being slightly hollowed 
out for the reception of their basal joints ; at the sides and posterior are the genae or 
cheeks, separated from the clypeus by grooves. In transverse section the clypeus appears 
as a blunt, wedge-shaped piece, the thinner end of which is formed by the upper wall 
of the pharynx (plate XVI, fig. 2), surrounded below and its sides by a U-shaped 
area (plate XVII), which for the most part eventually breaks up into the parts forming 
the proboscis. 
From the anterior wall, and from that part of the under surface of the clypeus 
which forms the roof to the labrum at its origin, project two plates of chitin 
(endosternites) for some little distance (plate XVIII, fig. i 3 f) : these are. approximated 
below and have an upper and a posterior free edge and two surfaces, inner and 
outer ; the upper posterior angle is lengthened into a blunt process (plate XVI, fig. 2, /). 
These plates are homologous to the fulcrum of other Diptera — -for example, Muscii, 
Eristalis — -which have a proboscis capable of extension and retraction. The fulcrum 
of such Diptera is greatly developed, and moves around an axis at the anterior angle 
of the head, and encloses the pharyngeal muscles. In the Culicidae the proboscis is 
fixed in a more or less permanently extended position, and the fulcrum is ill-developed 
and firmly attached to the anterior wall of the head. 
The inner walls of the clypeus afford attachment to three sets of muscles :— 
1. Muscles in connection with the labrum. 
2. Muscles to the base of the epipharynx. 
3. Muscles in connection with the pharynx. 
1. Dimmock, The Anatomy of the Mouth parti, etc., of some Diptera. Boston, 1881. P. 18. 
