EXTERNAL ANATOMY OF INSECTS. 709 
hypopygium*. Though usually a single small piece, in 
Edessa and many Pentatome it consists of several plates ; 
and in Trichius it is very large: it is mostly inéire, but 
in the male Dytiscz it is cleft ; in Monochamus subocella- 
tus trilobed; in dessa tripartite; in Centrotus Taurus 
it is boat-shaped and hollowed out to receive the stalk 
of the ovipositor. It is also generally in the same line 
with the body, but in Xenos it is turned up and bent in- 
wards®. 
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 trans- 
verse 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 Arad. In the hive-bee the transverse sec- 
tion is almost an equilateral triangle; in Belostoma gran- 
dis the disk of the under side of the part in question is 
longitudinally elevated into a trigonal ridge, the sec- 
tion of which is an equilateral triangle, the sides being 
quite flat. In general, in the vertical section of an ab- 
domen, the vertex of the triangle points downwards, but 
in Libellula it points upwards. In Blatta this section 
is nearly lanceolate; in Goertus olens it is a segment of 
a circle with the convex side downwards; in “shna with 
that side upwards; and in Agrion the section is circular. 
In Copris, Ateuchus, &c., the abdomen is very short and 
thick; in Staphylinus slender and long; in Aradus, Nepa, 
&c., depressed and flat; compressed in Opfzion and 
*"Prate VII. Frc: 6.2’: ® Linn, Trans. xi. t. ix. f. 15. 6, 
