722 Mr. Gilbert J. Arrow on 



In Gopris there is a provision, found in many other 

 beetles, to secure rigidity in the closed elytra. The two 

 edges which meet down the middle line of the back are each 

 provided with a groove, formed by a fold the lower edge of 

 which is more prominent than the upper one. The two 

 edges interlock so that the elytra can be held firmly 

 together. Near the junction each elytron has on its inside 

 a projecting rib, and there is a deep groove along the 

 middle of the propygidium which fits over the two ribs and 

 so secures still further rigidity. Just in front of this groove, 

 upon the preceding segment, are a few short transverse 

 ridges, and the ribs just described are also cut into trans- 

 verse ridges, but the latter are exceedingly fine and 

 numerous. Different writers have described one or other 

 of these series of ridges as the source of the sound, but it 

 does not seem to have been realized that both are essential 

 parts of the apparatus, the longitudinal movements of the 

 abdomen causing the delicate chords composing the elytral 

 ribs to be plucked by the corresponding ridges of the back, 

 while the propygidial groove guides these moven)ents and 

 secures the close approximation of all the parts. 



This type of apparatus has not been found in any other 

 genus of Coprid®, but it occurs again in almost the same 

 form in the genus Trox* belonging to another family, 

 the TrogidiB. Here again, as in Copris, the elytra fit 

 tightly together in the resting position, and in certain 

 species which are without wings are inseparably fused 

 at the junction. The finely-ridged bars at the suture 

 are scraped by one or more sharp-edged plectra placed 

 transversely upon the penultimate abdominal segment. It 

 is stated in Darwin's " Descent of Man" that Trox sahtdosus, 

 a British species and one of the smallest representatives of 

 the genus, squeaks quite loudly when handled. The faculty 

 seems to be general in this large and world-wide genus, 

 but I have found no indication of it in any other Trogidaj. 



The method of stridulation in Helioropris, the second 

 genus of Copridse in which it has been discovered, was 

 described by Mr. Gahan in the paper referred to. The 

 sound is here produced by pressing the abdomen not up- 

 wards but downwards. The insects of this genus are 

 amongst the bulkiest of all beetles, but the stridulating 

 plate is minute and occurs near the inner margin of each 

 hind coxa, while the inner half of the inturned part of the 

 * Sharp, Entom. Mon. Mag., 1897, p. 206, 



