594 EXTERNAL ANATOMY OF INSECTS. 
in soft ones they also are soft. The most impenetrable 
ones that occur to my recollection are those of Iliger’s 
genus Doryphora, and the softest and most flexile those 
of Telephorus, Meloe, and affinities. With regard to in- 
dividuals, they are mostly as hard as the prothorax, and 
harder than the back of the abdomen. Elytra also, as 
far as my observation goes, are never diaphanous. 
2. Articulation with the trunk. This is by means of 
a process of the base of the elytrum which I call the avis * 
or pivot, attached by elastic ligaments, and certain little 
bony pieces (osselets Chabr.) in the socket under the side 
of the anterior angle of the dorsolum®. You may easily 
remove the elytra attached to the mesothorax from Geo- 
trupes stercorarius, which will enable you to see the mode 
of articulation with little trouble ¢. 
3. Expansion. It is by means of the bony pieces just 
mentioned that the organs in question are opened and 
shut? under the action of the antagonist muscles. In 
opening for flight the two elytra recede from each other, 
and are elevated so as not to retain their horizontal po- 
sition, which would interfere probably with the play of 
the wings, but to form an angle with the body. When 
they return to a state of rest, the sutures usually meet 
and coincide longitudinally; but in some cases when 
closed, as in Necydalis, &c., they diverge from each other 
at the apex; and in Meloe, like the Orthoptera, to which 
that genus approaches, one laps over the other. 
4. Parts. The parts to be considered in an elytrum 
are the areas, the axis, the suture, the margin, the epi- 
* Prate XXVIII. Fic. 3—5. 0”. 
> Chabrier Sur le Vol des Ins. c.i. 439. 
° Prate XXVIII. Fre. 10. 4 Chabrier ubi supr. 
