46 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. IO3 



least, both the episternum and the epimeron, though a separating 

 sulcus is not present. The anterior pleural margin is inflected in a 

 high apodemal ridge (figs. i6 H, 21 C, k) ; posteriorly the upper 

 pleural plate is limited by a deep groove (fig. 15, m) separating it 

 from the propodeum, which forms a strong internal ridge (fig. 16 H, 

 o) with which the endosternal arm of the metathorax (SA3) is fused. 

 From the lower part of the groove m (fig. 15) a weak impression (n) 

 goes posteriorly to the coxa! articulation. It is evident, therefore, 

 that the epimeron of the metathorax, as that of the mesothorax, has 

 been mostly eliminated, and that the apparent intersegmental groove 

 (m, n) before the propodeum includes the lower part of the pleural 

 sulcus extending to the coxal articulation. The position of the 

 posterior pleural muscles behind this groove and its internal ridge 

 confirms the above interpretation. 



The thoracic spiracles. — Of the three pairs of spiracles on the 

 thorax those of the first pair are of particular interest because of 

 their size, position, and structure. They are contained in the mem- 

 branous inner walls of the spiracular lobes of the pronotum (fig. 17 C, 

 iSp), and therefore open toward the body into the depressions of 

 the mesopleura (A, c) beneath the lobes, the entrances to which are 

 guarded by dense fringes of plumose hairs on the lobe margins. The 

 reflected lower margin of each pronotal lobe is supported on a small, 

 rectangular cartilagelike plate (D, a) arising from a lobe of the 

 anterior margin of the mesopleuron (A, B, b). The plate is a thicken- 

 ing of the intersegmental membrane, and serves to prevent movement 

 of the spiracular lobe. The orifice of each spiracle (C, F) is a simple 

 opening in the spiracular membrane leading into a large tracheal trunk 

 (Tra) going forward into the prothorax. Above the aperture is an 

 oval plate (F, Op), the ventral edge of which forms the upper lip 

 of the aperture ; the lower lip is merely the soft edge of the surround- 

 ing membrane. The plate can be pulled down over the aperture by a 

 slender muscle (/j) and thus serves as a closing lid, or operculum, 

 of the spiracle. The occlusor muscle arises from a small knob on 

 the anterior margin of the mesopleuron (C, /j), and is inserted by a 

 tendon in the membrane close to the anterior margin of the spiracular 

 orifice (F), not directly on the operculum. The simple closing ap- 

 paratus of this spiracle would appear to be more effective in blocking 

 the entrance of air into the trachea than for preventing its egress, 

 but the soft lower lip of the closed aperture is easily pushed inward, 

 and probably gives little resistance to parasitic mites seeking an 

 entrance. 



