38 Prof. J.C. Schiddte on the Tunnelling Coleopterous 
in others it is entirely smooth all over in both sexes; and this 
is really the appearance presented when the parts are observed 
through an ordinary pocket magnifier. But whilst, on the one 
hand, it seems impossible that the friction of the two ridges 
against one another could produce a sound in those species 
where they are described as entirely smooth (supposing always 
the description to be correct), a careful examination shows, on 
the other hand, that the lateral part of the ridge on the abdo- 
men, which Erichson evidently looks upon as the source of the 
sound, cannot by any means be concerned in its production. It 
lacks two essential conditions, being neither in a favourable po- 
sition nor furnished with transverse grooves sufficiently fine. 
The creaking sound produced by many insects depends on a very 
rapid and powerful friction of a very thin edge against a grooved 
surface, the fine transverse strie of which catch hold of and again 
let go the edge. The thinner the edge, the finer the strize, and 
the greater the velocity of the movement, the higher is the note; 
and if the velocity and strength of the movement are small and 
the grooves coarse, no’ sound, or a mere low rattling noise, can 
be produced. But that lateral part of the abdominal ridge 
which, in some species, under a moderate power, shows transverse 
grooves is placed so far forward that the ridge on the femur 
could touch it only when the leg is stretched out, moved by its 
tensors, when the movements would not by any means be strong 
or quick enough; and its direction is, moreover, such that the 
grooves could not alternately catch and let go the ridge on the 
femur. Besides, these grooves are so distant from each other, 
so coarse, and so deficient in sharpness, in comparison with the 
strize on the creaking-apparatus of other Coleoptera, that even 
on that account they cannot be regarded as sources of sound. 
Even in animals so large as Necrophori and Cerambyces, the 
striz on the surface of the creaking-apparatus are so extremely 
close and minute that they show interferential colours*, and are 
distinctly observable only by the assistance of a very strong mag- 
nifier. The structure does not come out clearly till the parts are 
examined under the microscope by strong side light and a mag- 
nifying-power of 50-100 times. If the creaking-apparatus of 
Heterocerus deserves that appellation, the strie must be expected 
to be still more minute, and the surface would appear smooth 
‘part of the ridge is described as grooved in both sexes of H. marginatus, 
mtermedius, and levigatus, grooved in the male but smooth in the female 
of H. fossor, femoralis, fusculus, and hispidatus, smooth in both sexes of 
H. parallelus, obsoletus, and sericans. 
* On the creaking-apparatus of Necrophori, vy. Naturhistorisk Tids- 
skrift, ser. 2. vol. i. (1844), pp. 61, 69; and on that of Cerambyces, Nat. © 
Tid. ser. 3. vol. ii. p. 494 [Aun, & Mag. Nat. Hist. vol. xv. pp. 191, 192]. 
