82 
THOMPSON YATES LABORATORIES REPORT 
base, and turning sharply back, turns upwards for a short distance to terminate 
in a rounded knob, which articulates with the chitinous surface described at the 
distal end of the labium. To the outer bend of this rod the tendon of the 
muscle of the labella is attached. When these muscles contract, the labellae are 
drawn apart and rotated in such a way that their inner surfaces look downwards : 
it is probable that only the anterior distal portion of their inner sufaces is applied 
to the skin. 
The internal structure of the labium. In a section of the proboscis about its 
middle (plate XV, fig. i) it is seen that the chitinous exoskeleton of the labium is lined 
with a delicate spongy tissue containing very large rosette-shaped cells — a continuation 
ot a similar tissue lining the cranium. Beneath the chitinous envelope, here and there, 
is a row of low cubical epithelial cells (hypodermis). Situated about the centre of the 
section are the two labial tracheae, one on each side side, each surrounded by a 
delicate cellular sheath ; with each runs a comparatively large nerve-trunk — -the 
nerves to the proboscis. The tracheae are the terminal branches of the large tracheae 
to the head ; they join the nerves to the proboscis immediately after their origin from 
the suboesophageal ganglion : on their way down the labium they give off small lateral 
branches and becoming smaller, eventually break up into innumerable fine branches 
about the lower third of the labium to supply the labellae. The nerves, two in 
number, are the main anterior branches of the suboesophageal ganglion. Running on 
either side of the common salivary duct they enter the labium beneath the salivary 
receptacle on the under and outer side of the tracheae, being closely applied to them : 
after a straight course they split up in the labellae into many fine fibres which are 
distributed over their inner surfaces. 
Internal structure of the labellae. Applied to the outer wall and bulging into 
the interior of each labella, almost completely filling it is a mass of deeply staining 
tissue which, with a high power, is seen to be composed of numerous cells very 
similar in shape and size to the nerve cells of the supra- and infra-oesophageal ganglia 
of the head. Over the surface of this densely cellular mass, the nerve to the 
proboscis ends by splitting up into fine filaments (plate XIX, fig. 2). The close 
relation of the nerve to the proboscis to this structure, points to its being ganglionic 
in nature, probably supplying the numerous sensory hairs on the inner surface of the 
labellae with nerve fibres. These ganglia are well supplied with very fine tracheae, 
the terminal branches of the tracheae to the proboscis. 
Muscles of the labium are of two sets : — 
Those attached to the base of the labium. 
Those arising within the labium. 
The latter — the muscles of the labellae — are two long slender paired 
muscles, each arising by numerous separate bundles of fibres from the dorsal and 
ventral surfaces of the lateral chitinous ridges of the labium ; they are directed very 
