532 EXTERNAL ANATOMY OF INSERTS. 



segments into which the alitrunk is resolvable, the first 

 mav be the meditrunk {meditruncus), and the other the 

 potrunk (potruncus). 



I. Substance. — With regard to its substance, the trunk 

 in general is softer than the head, and harder than the 

 abdomen, especially as to its upper surface ; but in some 

 cases, where it is not protected by the elytra, as in the 

 rove- beetles {Staphylinus L.), the abdomen appears as 

 hard as the trunk. Though usually not very different 

 from the elytra in this respect, in Meloe, Lytta, and other 

 vesicatory beetles, it is of a firmer consistence. 



II. General Form. — In the Coleoptera Order the only 

 part of the trunk that is visible on its upper-side is the 

 prothorax : the mesothorax, with the exception of the scu- 

 tellwn, and the metathorax, being entirely concealed by 

 it and the elytra; so that, with regard to shape, it may 

 nearly be considered as merging in the prothorax. Be- 

 low it is more visible, and may be slated as more or less 

 quadrangular; in oblong beetles inclining to a parallel- 

 ogram, and in shorter or hemispherical ones to a square. 

 In the majority it is more convex below than above, except 

 in the case of the hemispherical or gibbous beetles (Coc- 

 cinella, Erotylus, &c), in which the under-side is flat and 

 the upper-side very convex. In the Diurnal Lepido- 

 ptera the trunk approaches to a cubical shape, in the 

 Nocturnal it is more spherical. A similar difference ob- 

 tains in the Hymenoptera and Diptera : in the bees, 

 wasps and flies, the trunk approaching to the figure of 

 a sphere; in the ants, Scolice, crane-flies, &c. to that of 

 a cube. The upper part of it in many Ichneumonida, 



