•380 
SEXUAL SELECTION. 
Part II. 
for tlie rasps are seated on the inferior surface of the 
elytra, near their apices, or along their outer margins, 
and the edges of the abdominal segments serve as the 
scrapers. In Pelobius hermanni (one of Dytiscidae or 
water-beetles) a strong ridge runs parallel and near to 
the sutural margin of the elytra, and is crossed by ribs, 
coarse in the middle part, but becoming gradually finer 
at both ends, especially at the upper end ; when this 
insect is held under water or in the air, a stridulating 
noise is produced by scraping the extreme horny margin 
of the abdomen against the rasp. In a great number 
of long-horned beetles (Longicornia) the organs are alto- 
gether differently situated, the rasp being on the meso- 
dhorax, which is rubbed against the pro-thorax ; Landois 
counted 238 very fine ribs on the rasp of Cerambyx 
her os. 
Many Lamellicorns have the power of stridulating, 
and the organs differ greatly in position. Some species 
stridulate very loudly, so that when 
Mr. F. Smith caught a Trox scibii- 
losus, a gamekeeper who stood by 
thought that he had caught a 
mouse ; but I failed to discover the 
proper organs in this beetle. In 
Geotrupes and Typhaeus a narrow 
ridge runs obliquely across (r, fig. 
25) the coxa of each hind-leg, 
having in Gr. stercorarius 84 ribs, 
which are scraped by a specially- 
projecting part of one of the abdo- 
niinal segments. In tlie nearly 
*r. Rasp. c. Coxa. /. Femur, allied Copris lunar is , an excessively 
t. Tibia, tr. Tarsi. n ^ 
narrow fine rasp runs along the 
sutural margin of the elytra, with another short rasp 
near the basal outer margin ; but in some other Coprini 
