116 LETTERS ON ENTOMOLOGY. 
The vibrations caused by the friction of the 
thighs and elytra striking upon this drum, are 
reverberated by it. The crickets make their 
intolerable chirping by rubbing the bases of 
their elytra against each other. I must describe 
their form to give you some idea how they do 
it. The elytra of both sexes are divided longi- 
tudinally into two portions ; a vertical or lateral 
one which covers the sides, and a horizontal or 
dorsal one which covers the back. 
In the female both these portions resemble 
each other in their nervures or veins, which 
running obliquely in two directions, by their in- 
tersection, form numerous small meshes of a 
lozenge shape : the elytra of these have no ele- 
vation at the base. In the males the vertical 
portion does not materially differ from that of 
the females; but in the horizontal, the base of 
each elytrum is elevated so as to form a cavity 
underneath. The nervures also, which are 
stronger and more prominent, run here and there 
into different kinds of forms ; particularly near 
the end of the wing you may observe a space 
nearly circular, with the vein running round it. 
The friction of the nervures of the upper or' 
convex surface of the base of the left-hand 
elytrum, which is undermost, against those of 
