THE ORGANS OP RESPIRATION. 159 



Some which are never free, but lie concealed beneath portions of the 

 horny integument, have no exterior horny ring, but a double-lipped 

 incision, the lips of which are formed by a thickened margin fringed with 

 short hair. This structure is very apparent in the large spiracle which 

 lies in the uniting membrane of the pro- and mesothorax, and parti- 

 cularly in Gryllotalpa (PI. XI., No. 1, f. 2, a. .), where, by reason of its 

 length, it is very distinct. The horny lips are connected at their corners 

 by a kind of joint, but in Gryllotalpa the lower corner of this incision, 

 which lies near the anterior coxae, is broader and more prominent ; and the 

 corner of the exterior lip projects beyond the opposite interior one, form- 

 ing a kind of covering, thus preventing the influx of improper substances. 

 The entire spiracle is closed by means of a small muscle, which, origin- 

 ating from an inner horny projection of the lower corner of the lip, 

 inserts itself in two horny half-rings, which surround the commence- 

 ment of the tracheae. The orifice is opened or shut by the contraction 

 or dilatation of this muscle. 



Other spiracles, which besides the lips possess an oval horny margin, 

 present a somewhat more complicated structure. The horny ridge 

 (PI. XXIII, f. 1 3,a,) is no distinct part, but merely the raised edge of 

 the integument surrounding the spiracle ; it thus forms an exterior 

 ring, to which the lips of the incision are attached. These lips (the 

 same b. 6.) stand at the base of the ring, and are frequently covered 

 upon their external surface like it upon its internal circumference, 

 with sculptured horny scales (Oryctes nasicornis). Where they meet 

 they again form a small projecting margin which, as in the former kind 

 of structure, is surrounded by a fringe of fine hair. The corners of 

 the lips lie close to the inner margin of the exterior ring, so that the 

 true opening, upon the lips being closed, appears as the diameter of the 

 oval ring. The closing apparatus of these spiracles is very complicated. 

 The ends of the incisions, namely, or the corners of both lips, are pro- 

 longed inwardly into a point (the same, c. c.), to which two triangular 

 horny plates are so attached, that one angle of the triangle with the 

 projecting point, and the second with the opposite one of the other horny 

 plate, form a joint, but the third remains free. From the last, as well 

 from the sides of the triangle which are applied to each other, a flat 

 muscle originates (the same, e.) which, when it contracts, brings the 

 free points of both triangles together, but those which stand in connec- 

 tion with the inner points of the corners of the lips, it separates from 

 each other ; thus is the incision closed : but when the muscle again 



