126 THE MICROSCOPICAL NEWS. 
which first presents itself is the /adrum, the homologue of the 
upper lip of mandibulate insects, marked lbr. in figure. This is a 
straight, trough-like organ, rather longer than the head of the insect 
—hollow throughout, and having the slides turned over along their 
margin so as partly to enclose the interior. The base is consider- 
ably broader than that of the older setae, but like them the labrum 
tapers gradually. It terminates in a blunt point, closing in the end. 
This blunt tip, when examined under one of the higher powers, 
presents a peculiar structure, which at once suggests the presence 
of a sense organ. On each side of the upper part of a slightly 
rounded surface are placed two discoidal plates, studded with 
papilla, and having below them a convex body also papillated, but 
with much more minute papilla. What is the special function of 
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Fig. 37. 
these bodies I have been unable to find out; but, taking into con- 
sideration their position and structure, it seems very probably that 
it is sensorial. Running along the middle of each side of the 
labrum from base to tip is an extremely delicate, flattened ridge, 
minutely fringed at the edge. 
The hollow structure of the labrum naturally fits it to be the 
sheath of the other setze. 
Lying immediately below and on each side of the labrum are 
the two mandibles (mn. Fig. 37). These are long, broad, flattened 
blades, tapering to a point which is very slightly curved. One of 
these enlarged to 262 diameters is shown as Fig. 39. The outer 
edge is seen to be cleanly incisive, and from this the blade appears 
to become somewhat thicker as the inner edge is approached. This, 
throughout almost its entire length, is armed with extremely small, 
