EXTERNAL ANATOMY OP THE COLEOPTEKA. 
35 
four dorsal pieces, and bears one of the wings on 
each side : these wings are membranaceous, often very 
large, and lie in a small compass under the elytra 
when not required for flight, the membrane being 
thickened where contact takes place on the wing 
being folded. 
The metasternum (Fig. 10) is usually composed of 
the sternum (a), the episterna 
(b b), and the epimera ; but 
in Dytiscus and some ( Geode - 
phaga) the latter are want- 
ing, being replaced by the 
largely developed coxce (d d) 
of the hinder legs, of one of 
which e is the trochanter. 
The parapleura, or paraptera 
(c c), are side-pieces, which 
in some beetles are very con- 
spicuous. 
The remaining part is the 
abdomen, the upper side of 
which, being covered by the 
elytra, is softer than the 
lower, or ventral surface ; the 
apical segments on the upper 
side, however, where not 
covered by the wing-cases, are much harder than the 
protected portion of the abdomen, and are called the 
pygidium and propygidium. The entire abdomen is 
divided into segments, fitting like those of a telescope, 
aud usually nine in number: in Fig. 11 only six are 
visible, the extreme apical one, comprising the gene- 
rative organs, being retracted within the sixth (which, 
Metnstornum of thorax of 
D. marginalia. 
