Genera Bledius, Heterocerus, and Dyschirius. 39 
to the naked eye or even under an ordinary pocket magnifier. 
Now this is precisely the state of the case. The ridge on the 
femur is not rubbed by the action of the tensors against the 
outer lateral part of the ridge, which in some species shows a 
few coarse transverse grooves (a sort of introduction, as it were, 
to the structure of the true apparatus), but it is rubbed, by the 
powerful action of its flexors, against the inner part of the arched 
ridge, which forms exactly a segment of a circle, the point of 
the coxa being the centre and the femur the radius, and which, 
though apparently smooth in all species and both sexes, is co- 
vered with transverse strie as regular, close, and minute, in 
proportion to the size of the animals, as in any of the larger 
insects just mentioned. Of course this is not observable except 
by means of the microscope, by side light and a suitable mag- 
nifying-power: it is best seen by a power obtained by using a 
proportionally strong eye-piece, if the instrument allows it. It 
is still better to choose specimens for the examination which 
have just gone through their transformations, and in which the 
integuments, having not yet acquired their deep colouring, are 
semipellucid. The first ventral segment should be cut off, carefully 
separated from the soft parts, cleansed with solution of caustic 
potash, and examined, under a strong magnifying-power, by 
transmitted side light, which, of course, ought to be directed 
along the arched ridge, across the transverse striz. The pre- 
paration repays the trouble, as nothing can be more elegant 
than the aspect of the strive, which cover the whole arch in the 
cases where this, by a low power, appears entirely smooth all 
over, but only the inner larger portion of it in those cases where 
the pocket magnifier shows transverse grooves on the outer or 
lateral part of the arch. Whilst, according to the account given 
in ‘ Naturg. d. Ins. Deutschl.,’ these latter species would appear 
to have the most developed creaking-apparatus, the reverse is 
_ the case, as it is the apparently smooth part of the arch which 
produces the sound, not the coarsely grooved part. 
It follows that several of the characters for species and sexes 
which Erichson thought to find in this creaking-apparatus lose 
very much of their value ; but it presents one peculiarity, hitherto 
overlooked, which more than makes up for the loss, and is of 
great utility in distinguishing closely allied species. The fore 
end of the arch, which generally exhibits a few coarser trans- 
verse grooves, is the broader of the two; and these two circum- 
- stances indicate clearly enough that the friction is calculated to 
commence at that end and continue inwards, when the femur 
is inflected, towards the lower or posterior extremity of the 
arch, which is more and more attenuated, and generally ends at 
the posterior margin of the segment. But in some species 
