34 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 52 



THE MESOTHORAX 



The mesothorax is the shortest of the three thoracic segments. The 

 form is that of a ring with the anterior diameter smaller than the 

 posterior one. The meso- and metathorax are very closely connected, 

 giving the appearance of one single unit. Seen from above (dorsal) 

 the scutellum only is visible, while in the lateral and ventral aspect 

 the mesopleura and the mesosterna can be distinguished. The two 



Fig. i6. — Gnatholrichus retusus Lee: Mesothorax, view from anterior foramen. 

 The parts are disconnected by stretching under the coverglass. 



A, external aspect; B. internal aspect; Co, coxa; El, elytra; Ep, epimeron; Pr, preepi- 

 sternum; Psc, prescutum; Eps, episternum; Pst, presternum; Pstr, poststernellum; Set, 

 scutellum; St, sternum; Stl. sternellum; Stn, scutum; Tr, trochanter; o, articulation of 

 elytra; h, clavicola; c, pleural hook of scutum; d, pleural clavicola. 



pairs of appendages are implanted between the pleura and tergum 

 and the pleura and sternum respectively. 



Mcsonotuni. — From the mesonotum or tergum only the scutellum 

 is exposed dorsally. The rest of the notum is covered by the elytra 

 laterally and is hidden by the pronotum anteriorly. 



The scutellum. — The scutellum (fig. i6. Set) is a triangular plate, 

 heavily chitinized, covered with short bristles, and externo-anteriorly 

 fused with the prescutum. There occur slight differences in shape in 

 the different species but they are of no imix)rtance. 



