PHYSIOLOGY OF INSECTS. 103 



sternellum) ; these divisions are but rarely dis- 

 tinguishable. 



341. The pleura are also divided by sutural 

 lines or indentations into four smaller portions, 

 called their first, second, third, and fourth sections 

 (prtepleura, pleura, pleurella, post-pleurella) ; 

 these divisions are rarely distinguishable on more 

 than one segment of an insect. 



342. The wings and legs being invariably 

 inserted in the pleura or sternum of the seg- 

 ments, generally force out of their places the 

 portions in the neighbourhood of their origin, so 

 that care is required in deciding on and applying 

 the names to each. 



343. The upper, or scutal portion of each 

 segment, has no such interruption to simple 

 development ; this portion, therefore, is employed 

 in description in preference to the others. 



344. When the scutum of either segment is 

 apparently undivided, it is called simply scutum ; 

 when, as in the mesothorax of the hornet, one 

 obvious line crosses it, the anterior portion is 

 the scutum, the posterior, the scutellum ; when, 

 as in the bluebottle-fly, two obvious lines cross 

 the mesothorax, the first section is the prce- 

 scutum, the second, the scutum, and the third, the 

 scutellum. 



345. The propodeon in the hornet, and a vast 

 number of other insects which resemble it in 

 structure, has a longitudinal line or indentation 



