26 THE MOUTH-PARTS OF THE THYSANOPTERA. 
a well-developed ehitinous organ, with a swollen base 
and a spine-like distal portion, which latter, wdien the 
organ is in position, passes through a sort of loop in the 
distal extremity of the one-sided labrum. Nothing corre- 
sponding to this conspicuous organ is apparent on the 
right side of the head, unless a very small ehitinous struct- 
ure under the edge of the clypeus is a rudiment of the 
organ for this side. 
The asymmetry extends also to the ehitinous endocra- 
nium of the head. Throngh the epicranium, when this 
has been rendered transparent, a dark bar of chitin may 
be seen on the left side extending from the thickened rim 
of the epicranium, at the outer edge of the clypeus, ante- 
riorly nearly to the base of the compound eye of this side 
where it meets several converging bars, a slender one of 
which extends from the base of the left antenna along the 
inside of the eye. On the right side, the bar correspond- 
ing to that lii'st-mentioned is not present, and the chiti- 
nons rim of the epicranium, even, appears to be here im- 
perfect. The radiating part of the endocraninm near the 
base of the right eye is, however, as easily made out as 
that on the left side, the bar alone, connecting it with the 
edge of the epicranium, being wanting. 
What is this jaw-like organ, and why should it be de- 
veloped on one side and not on the other? To the second 
of these questions I have no reply to make further than to 
suggest that it may be a case of asymmetry like that of 
the lungs and ovaries of serpents, — a sacrifice of one of a 
pair of organs for a gain in slenderness. To the first, I 
am compelled to reply that the organ has every appear- 
ance of being a mandible. Its form and its relations to 
the other mouth-parts, and to the epicranium, all indicate 
this. It is well su[)plicd with nuiscles. It consists of a 
single piece. 
