710 EXTERNAL ANATOMY OF INSECTS. 



remarkable peculiarities, I shall pass them without fur- 

 ther notice, and call your attention to the last, which is 

 opposed to the podex, and which I have named the hy- 

 popygium*. Though usually a single small piece, in 

 Edessa and many Pentatomce it consists of several plates; 

 and in Trichius it is very large : it is mostly intire, but 

 in the male Dytisci it is cleft; in Lamia ocellata trilobed ; 

 in Edessa tripartite; in Centrotus Taurus it is boat- 

 shaped and hollowed out to receive the stalk of the ovi- 

 positor. It is also generally in the same line with the 

 body, but in Xenos it is turned up and bent inwards 5 . 



iv. Shape. With regard to shape, in some Orders the 

 abdomen varies considerably; but the most general form 

 is one that approaches to trigonal, so that a transverse 

 section will be a triangle, with the vertex more or less 

 obtuse, and the base more or less convex; some tendency 

 to this form will often be found even in those insects whose 

 abdomen appears almost as flat as a leaf, as in many 

 Aradi. In the hive-bee the transverse section is almost 

 an equilateral triangle ; in Belostoma grandis the disk of 

 the under side of the part in question is longitudinally 

 elevated into a trigonal ridge, the section of which is an 

 equilateral triangle, the sides being quite flat. In gene- 

 ral, in the vertical section of an abdomen, the vertex of 

 the triangle points downwards, but in Libellula F. it 

 points upwards. In Blatta this section is nearly lanceo- 

 late ; in Staphylinas olens it is a segment of a circle with 

 the convex side downwards ; in JEshna F. with that side 

 upwards; and in Agrion the section is circular. In 

 Copris, Ateuchus, &c, the abdomen is very short and 



* Plate VIII. L'. *> Linn. Trans, xi. t. ix.f. 15. b. 



