714 Mr. Gilbert J. Arrow on 



the abdomen and sometimes by working the abdomen 

 against tlie legs while these are held at rest. 



This instrument is not at all of a widespread type, being 

 confined to about fifty species at present known, all of 

 which inhabit Tropical America, while no trace of it is 

 found in others very closely related to them. In a few 

 species belonging to the neighbouring genus Lagochilc 

 the same organ appears in a slightly modified form, the 

 file upon the hind-leg being less narrow and not rising 

 abruptly from the surface of the femur, while the ridges 

 upon the sides of the body are confined to definite patches 

 upon two or three of the segments. In a third genus, 

 Geniatcs, of this group, less closely related, yet another 

 variant has been discovered by my friend.* The micro- 

 scopic ridges here form a compact mass placed entirely upon 

 the knees, either of all four posterior legs or of the third 

 pair only (PI. XXXVI, fig. 11). Geniates is an insect of 

 quite a different build to Macraspis and Lagochile, with 

 more slender legs and a less hard and shelly exterior. The 

 knees do not lie close to the sides and no ridges are found 

 in that situation. The posterior femora, unlike those of the 

 genera just mentioned, extend beyond the lateral edges of 

 the elytra, which form on each side a flange flattened above 

 and much thickened below. Crossing the outer half of this 

 thickened part at right angles are numerous strong, sharp- 

 edged ridges, well adapted for setting up vibrations in the 

 instrument occupying the corresponding position upon the 

 leg when this is drawn across the elytron. Above each of 

 these ridges is a very stout spine, the growth of which 

 seems to have produced the elevation which has become 

 modified for this special purpose. 



These three genera of Rutelida? are all inhabitants of 

 the same part of the world, viz. Tropical America, and 

 no other members of the family are as yet known to pro- 

 duce similar sounds. They are of great interest as showing 

 the apparently erratic occurrence of sound organs of the 

 same essential type (a type peculiar to this family 

 among the Lamellicorns) in small groups of a family, not 

 immediately related one with another but living in the 

 same environment. The natural inference is that the 

 organ has in furmer times been common to at least a 

 large part of the Rutelidse, but owing to some local causes 

 has only been retained in these isolated cases. 

 * Ohaus, Bed. Ent. Zeits., 1903, p. 237. 



