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Show -380 SEXUAL SELECTION. PAP.T II. for the 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 Dytiscidre 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 altogether differently situated, the rasp being on the mesothorax, which is rubbed against the pro-thorax; Landois counted 238 very fine ribs on the rasp of Cerambyx .heros. Many Lamellicorns have the power of stridulating, .and the organs differ greatly in position. Some species 8 stridulate very loudly, so that when Mr. F. Smith caught a Trox sabulosus, 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 Typhrous a narrow ridge runs obliquely across (r, fig. ~5) the coxa of each hind-leg, having in G. stercO?'arius 84 ribs, which are scraped by a speciallyprojecting part of one of the abdo- Jltg. 25. Hind-leg of Geotrupcs · 1 I th 1 stercornrius (from Landols). m1na segments. n e near Y J'. Rasp. c. Coxa • .f. Femur. allied Copris lunaris, an excessively t. Tibia. t1·. Tarsi. narrow fine rasp runs along the sutural margin of the elytra, with another short rasp near the basal outer margin; Lut in some other Coprini CIIAl'. X. COLEOPTERA. 381 the rasp is seated, according to Leconte/3 on the dorsal surface of the abdomen. In Oryctes it is seated on the· pro-pygidium, and in some other Dynastini, according to the same entomologist, on the under surface of the elytra. Lastly, vVestring states that in Omaloplia brunnea the rasp is placed on the pro-sternum, and the scraper on the meta-sternum, the parts thus occupying the under surface of the body, instead of the upper surface as in the Longicorns. We thus ~ee that the stridulating organs in the different coleopterous families are wonderfully diversified in position, but not much in structure. '\Vithin the same family some species are provided with these· organs, and some are quite destitute of them. This diversity is intelligible, if we suppose that originally various species made a shuffiing or hissing noise by the· rubbing together of the hard and rough parts ~ their bodies whieh were in contact; and that from the noisethus produced being in some way useful, the rough surfaces were gradually developed into regular stridulating organs. Some beetles as they move, now produce, either intentionally or unintentionally, a shuffling noise, without possessing any proper organs for the purpose. Mr. Vlallace informs me that the Euchirus longimanus (a Lamellicorn, with the anterior legs wonderfully elongated in the male) "makes, whilst moving, " a low hissing sound by the protrusion and contraction. ''of the abdomen ; and when seized it produces a ''grating sound by rubbing its hind-legs against the· ''edges of the elytra." The hissing sound is clearly clue to a narrow rasp running along the sutural margin of each elytron; and I could likewise make the grating. 73 I am indebted to :M:r. Walsh, of Illinois, for having sent meextracts from Leconte's 'Introduction to Entomology,' p. 101, 143. |