8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I46 



coxa of the leg is articulated. This sulcus and its ridge differentiate 

 the pleuron into an anterior episteniiim and a posterior epimeron. 

 Usually a triangular plate below the episternum, termed the trochan- 

 tin, forms by its lower angle an anterior articular point for the coxa. 

 The episternum itself may be variously subdivided, and often periph- 

 eral parts of the pleural area remain membranous. In the wing- 

 bearing segments the pleural sulcus extends up to the wing base, 

 and its ridge forms the fulcral support of the wing. Before the wing 

 fulcrum there is a small plate, the hasalare, and behind it another, 

 the siihalare, that give attachment to the direct muscles of the wing. 

 The pattern of the pleural sclerotization differs on the two alate 

 segments according to the relative development of the wings and to 

 the presence or absence of one of the pairs of wings. 



It is clear that the thoracic pleura of the pterygote insects are adap- 

 tive developments, first for the support of the legs and then for the 

 support of the wings as the latter were evolved from paranotal lobes. 

 It has long been a popular theory that the pleura represent primitive 

 subcoxal segments of the legs that have been incorporated into the 

 thoracic wall. Yet a subcoxal segment is not present in any of the 

 other arthropod groups; the coxa is always the functional base of 

 the limb on which the principal motor muscles of the leg are attached. 

 Differences in the leg segmentation among the arthropods are due 

 principally to the presence of one or two segments in the trochanteral 

 region of the leg. Most of the arthropods have a 7-segmented leg; 

 the insect leg is 6-segmented by loss of the second trochanter (the 

 crustacean basipodite). 



Sternum: The word is derived from the Greek sternon, which 

 means the human chest or breast region. In the Latin languages the 

 name was taken as the basis for words meaning sneezing, as in 

 Latin sternuto and sternutatio, in Italian sfeniutare, in Spanish cs- 

 tornudar, and in Latin-English sternutation. In vertebrate anatomy, 

 however, the name sternum was given to the breast bone {os pectoris 

 in Latin). In arthropod anatomy it has been extended to any one 

 of the segmental ventral plates of the skeleton. It is thus a curious 

 coincidence that the word sternum as used in entomology is cognate 

 with words signifying sneezing. 



External grooves of skeleton: Grooves on the surface of the 

 integument, particularly those of the head and thorax, give the 

 skeleton the appearance of being composed of sclerites united along 

 these lines. The grooves, therefore, have long been called "sutures" 



