NO. 6 INSECT ABDOMEN — SNODGRASS II 



reason to believe that lateral sclerites are secondary subdivisions of 

 earlier formed pleurosternal plates, they may be given the non- 

 committal name of parastcrnitcs. When true pleural plates or lobes 

 of the abdomen are subdivided longitudinally, the upper and lower 

 parts may appropriately be termed epipleurites and hypopleurites, 

 respectively ; but such a division seldom occurs in the abdominal 

 pleura, and the term " epipleurite " is commonly misapplied by students 

 of insect larvae to paratergal lobes, or sclerites of the dorsum. 



The fact that the dorso-pleural groove forms a conspicuous line 

 of infolding along the side of the abdomen in many insect larvae 

 (fig. 3, a-a) is probably the reason for its frequently having been 

 termed the "pleural suture." Hopkins (1909) thus named it in his 

 study of the larva of Dcndroctonus, and he designated the lateral 

 lobes above the groove " epipleural " and those belov^r it " hypopleural." 

 The former he believed represented the epimeron of a thoracic 

 pleuron, and the latter the episternum. No such homology as this, 

 however, is possible, since the pleural suture of a thoracic segment 

 is morphologically a vertical groove in the subcoxal sclerotization of 

 the leg bases, taking only secondarily a horizontal position in the meta- 

 thorax of adult beetles. The so-called " pleural suture " of the larval 

 abdomen, moreover, as we have seen (fig. 3 A, B, C. a-a), extends 

 into the thorax above the subcoxal sclerotizations {Sex), and thus 

 throughout the body separates the dorsum from the true pleural 

 region. Lateral lobes or sclerites of the abdomen lying above the 

 dorso-pleural groove are, therefore, paratergal (fig. 3 B, patg), and 

 not " epipleural." Only the so-called " hypopleural " areas lying 

 ventrad of the dorso-pleural groove, that is, between it and the true 

 sternal region, are properly pleural in the sense that they correspond 

 with the subcoxal areas of the thorax (B, Scx^) containing the scler- 

 ites of the thoracic pleura. The ventro-lateral lobes or plates of the 

 larval abdomen may, then, be termed the abdominal pleura inasmuch 

 as they appear to represent the subcoxae of the thorax. The abdommal 

 pleura are never divided vertically in a way to suggest a true homology 

 with the division of a thoracic pleuron into episternum and epimeron. 



The relation of the muscles to the lateral lobes of the abdomen 

 in the larvae of Coleoptera has been studied by Boving (1914) and 

 by Craighead (1916). Boving, here following Hopkins (1909), calls 

 the lateral groove of the abdomen the " pleural suture," but in all his 

 subsequent work he terms it the " ventro-lateral suture." Craighead 

 identifies the lateral areas of the abdomen with the corresponding 

 areas of the thorax in cerambycid larvae, but since he regards the 



