BY INSECTS IN FLYING. 381 
ture of the stigma. Their presence however is not general: in the Ta- 
banus bovinus they do not appear, but are found in Hristalis tenax. I 
have given an engraving of the hinder air-hole, magnified forty times, in 
which it is shown on the inner side. We find an oval band-like claudent 
muscle or sphincter, to both ends of which other muscular bands are 
attached: perpendicular to the inner surface of this claudent muscle stand 
sixteen to eighteen small horny lamellz, which are of the same breadth 
as the muscle, and connected in the middle by another longitudinal 
horny band. On the other side, which is directed outwards, the claudent 
muscle is clothed with skin, upon which are feathery hairs, which cover 
the entrance of the air-pipe like a sieve and exclude foreign bodies. _I 
am at present inclined to consider these small horny lamellz rather as 
a mere scaffolding serving to support the claudent muscle, but leave it 
to naturalists to decide whether, and how far, they contribute to the for- 
mation of the sound. In either case however this can be but of little 
consequence, as many insects possess no such plates. 
I have further to combat some objections which have already been 
publicly urged against the correctness of my theory of the production 
of the sound. M. Silbermann has given publicity to this in France by 
a translation of the chapter of my work which treats on this subject in 
his Revue Entomologique, by which also M. Goureau was led to perform 
similar experiments. The latter finds* all that I have said perfectly exact, 
except that the sound is suspended when the stigma is closed, and this 
is precisely the main point of the question. On the contrary, however, 
I must maintain that, although a sound may be heard as long as the 
gum employed to close the stigma is moist, and the air can make its way 
through it, none is audible when the gum is perfectly dry. The animal 
indeed dies of suffocation soon after. M.Goureau has not taken this cir- 
cumstance into consideration, but has even hazarded the opinion that the 
sound originates through the friction of the edges of those plates which 
compose the thorax. This can however be the case in those insects only 
in which we find on the thorax really distinct plates connected by fibres; 
and as the diptera are not in possession of such, his theory cannot be 
applied to them. But even in respect to the other orders with skeleton 
hips his theory is untenable ; first, because the mobility of these hips, 
on account of their intimate connection, must be very trifling; and se- 
condly, because the sound is far too strong to be produced by the friction 
of such minute surfaces. That the feeble sound of the chirping Capri- 
corn beetle ( Cerambycina), when the body is in a quiet position, is really 
produced by such friction is well known; but it is also obvious, from the 
feebleness of this sound, that the loud buzzing of the flying insects, 
which besides sounds quite differently, cannot arise from such friction. 
* Revue Entomologique, by G. Silbermann, Strasb. 1835, vol. iii. p. 107. 
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