4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. T46 



segment, but the segmentation becomes confusing because the seg- 

 ments are often divided into non-musculated subsegments. A true 

 leg segment is thus best defined as a section of the limb provided 

 with muscles (see Legs). In the same way the apical segment, the 

 flagellum, of an antenna is commonly divided into subsegments (see 

 Antennae). 



Segment areas and sclerotization : In an adult insect the cuticle 

 of each segment is usually sclerotized in a defmite pattern of plates, 

 but the pattern may differ on difTerent segments or on the same 

 segment in different insects. There often results, therefore, some 

 nomenclatorial confusion on the identification of the plates. 



In an unsclerotized wormlike animal, such as Peripatus, having 

 a series of legs along each side of the under surface, the only dif- 

 ferentiation of the body wall is its division by the legs into a dorsum 

 above the legs and a venter between them. If the segmental body 

 wall, as in some crustaceans, is completely sclerotized, the dorsal 

 plate is a tergiim or notum, the ventral plate a sternum. In some 

 of the diplopods and crustaceans and in the prothorax of some in- 

 sects the upper part of the tergal arch is produced on each side into 

 a paranatal lobe. The sclerotized lateral parts of the segment are 

 then called the pleura (sing, pleuron), and the name tergum or 

 notum is restricted to the dorsal sclerotization above the lobes. In 

 the winged insects the paranotal lobes of the mesothorax and the 

 metathorax are extended as the wings. The pleura of these segments 

 have to serve as supports for the wings as well as supports of the 

 legs and are modified accordingly. Each is strengthened by a strong 

 internal ridge formed by an external groove or sulcus from the leg 

 base up to the wing base. The groove differentiates the pleuron 

 into an anterior area called the episternum and a posterior area called 

 the epimeron. At the wing base various small sclerites are formed 

 which control the movements of the wings. Other modifications of 

 the pleura are often present (see Pleuron), and the pleural area in 

 wingless insects may be largely unsclerotized. The prevalent theory 

 that a large part of the thoracic pleuron has been derived from a 

 primitive "subcoxal segment" of the leg seems quite unnecessary 

 from a comparative study. 



In the same way as the pleuron, the tergum and the sternum are 

 usually differentiated into areas or distinct parts for mechanical 

 reasons. 



On the abdominal segments, the terga and sterna are connected by 

 membranes that may be regarded as pleural. But the small sclerites 



