26 THE MOUTH-PARTS OF THE THY8ANOPTERA. 



a well-developed chitinous organ, with a swollen base 

 and a spine-like distal portion, which latter, when 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 chitinous struct- 

 ure under the edge of the clypeus is a rudiment of the 

 organ for this side. 



The asymmetry extends also to the chitinous endocra- 

 nium of the head. Through 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 first-mentioned is not present, and the chiti- 

 nous rim of the epicranium, even, appears to be here im- 

 perfect. The radiating part of the endocranium 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 sleiiderness. 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 supplied with muscles. It consists of a 

 single piece. 



