THE HARVEST-FLIES. 
205 
are alternately tiglitenecl and loosened, recovering their nat¬ 
ural convexity by their own elasticity. The effect of this 
rapid alternate tension and relaxation is the production of a 
rattling sound, like that caused by a succession of quick 
pressures upon a slightly convex and elastic piece of tin 
plate. Certain cavities within the body of the insect, which 
may he seen on raising two large valves beneath the belly, 
and which are separated from each other by thin partitions 
having the transparency and brilliancy of mica, or of thin 
and highly polished glass, tend to increase the vibrations of 
the sounds, and add ^reatlv to their intensitv. In most of 
our species of Cicada the dinims are not visible on the out¬ 
side of the body, but are covered by convex triangular 
pieces on each side of the first ring behind the thorax, which 
must be cut away in order to expose them. On raising the 
large valves of the belly, however, there is seen, close to 
each side of the body, a little opening, like a pocket, in 
which the dram is lodged, and from which the sound issues 
when the insect opens the valves. The hinder extremity 
of the body of the female is conical, and the under side 
has a longitudinal channel for the reception of the piercer, 
which is furthermore protected by four short grooved pieces 
fixed in the sides of the channel. The piercer itself consists 
of three parts in close contact with each other; namely, two 
outer ones grooved on the inside and enlarged at the tips, 
which exteraally are beset with small teeth like a saw, and a 
central, spear-pointed borer, which plays between the other 
two. Thus this instrument has the power and does the work 
both of an awl and of a double-edged saw, or rather of two 
key-hole saws cutting opposite to each other. Xo species of 
Cicada possesses the power of leaping. The legs are rather 
short, and the anterior thighs are armed beneath with two 
stout spines. 
The duration of life in winged insects is comparatively 
very short, seldom exceeding two or three weeks in extent, 
and in many is limited to the same number of days or hours. 
