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 transvei'se 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 striae 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 tlic 

 integuments, having not yet acquired their deep colouring, arc 

 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 striae. The pre- 

 paration repays the trouble, as nothing can be more elegant 

 than the aspect of the strise, 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 Erichsou 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 



