102 CICADAS; DOG-DAY OR HARVEST-FLIES. 
of two large parchment sacs, ribbed and gathered into numerous 
plates, furnished with powerful muscles, and situated in large 
cavities at the base of the abdomen. When in action the air is 
driven with great force against the ribbed surfaces, and vibra- 
tions are set up which produce the sound in accordance with the 
number and form of the fluted spaces and ribs.” (Uhler.) 
Each of the above cavities is covered with a thin plate which 
can be lifted, and which acts as a sounding-board; by — or 
lowering it the sounds are modified. 
Fic. 97d. Twig showing scars from Fic. 97e.—Newly hatched larva—greatly 
punctures after second year—nat- enlarged. After Riley, Div. of Ento- 
uralsize. After Riley, Div. of En- mology, Dept. of Agriculture. 
tomology, Dept. of Agriculture. 
Tibicen septendecim Linn. (The Periodical Cicada or The 
17-Year Locust). 
This species is remarkable as requiring seventeen years to 
develop. As it is not found in Minnesota, but close to our 
borders in Wisconsin, it is not necessary to say much about it. 
Wherever it occurs it is well known owing to the great numbers 
in which it appears at intervals of seventeen years. This peri- 
odical appearance is due to the enormously long time required 
for the early stages to obtain their growth, and all the members 
of one generation appear as Cicadas at about the same time. 
The winged females deposit their eggs in slits made in the twigs 
of trees, as shown in Fig. 97, and small fruit-trees are some- 
