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POPULAR SCIENCE REVIEW. 
those cavities in the body of the Cicada which are filled with 
air, and where this ethereal fluid is thrown into waves by the 
vibrations of the drum. They are five in number — one median 
and four lateral. 
1 . The thoraco-abdominal cavity — the 44 grande cavite ” of 
Reaumur, is formed by the junction of two (thoracic and ab- 
dominal) cavities. It comprises the internal cavity of the 
metathorax and the five or six first abdominal somites, and 
is roofed in by the entothoracic partition, which gives attach- 
ment to the muscles of the legs and wings of the metathorax, 
while below it terminates in a vast cul-de-sac, at the bottom 
of which the abdominal viscera are packed. 
It is constricted at its middle by the 44 oval ring,” mentioned 
above, which is stretched between the muscles of the drum ( x , 
fig. 1 ) ; and reminds one of an hour-glass, of which the upper 
bulb would be the thoracic cavity, and the lower that of the 
abdomen. 
2. The 44 second cavern” (Duges), termed by Reaumur the 
44 cellule” (C, figs. 1, 2, 3). Its entrance is concealed by the 
operculum , which must be lifted in order that it may be seen. 
The cavity itself is a cul-de-sac , of which the external wall is 
concave, and shows an opening in the shape of a narrow slit, 
which is covered by the scutellum * of the metathorax. 
3. The sub-opercular cavity is a vast chamber covered by the 
operculum , bounded above and below by the folded membrane 
and the mirror respectively (fig. 3). It is completely closed in 
C. plebeia when the abdomen is lowered, but when this part of 
the body is raised it is open below and on the two sides. The 
two cavities are separated one from the other in the middle line 
by the body of the entogastrium. 
E. The Stigmata .f — Their action, though but little studied as 
yet, is of the greatest importance. Leon Dufour £ states that 
the Hemiptera — to which order the Cicada belongs — have 
generally but one pair of thoracic stigmata, and that those of 
the abdomen of the Cicada are six pairs in number ; but M. 
Carlet has found that there are three pairs of thoracic stigmata 
(in both sexes) and seven pairs in the abdomen. The third and 
lowest pair in the thorax are to be found beneath the oper- 
culum , or epimeron of the metathorax ( s , figs. 1, 2, 3). The 
first abdominal stigma (s', fig. 3) will be seen on the internal 
* In English, u little shield.” It is one of the four elements of the 
tergum, or upper section, of a thoracic “ somite.” 
t The air-holes which communicate with the trachea , or air-tubes, in 
Insects. The word is the plural of the Greek a-riyga, “ a spot.” 
X Recherches anatomiques et physiologiques sur les Hemipteres.” p. 
257. 
