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Hi Dave: argh, you are a lot of work.
Quoting David & Alison Webster <dwebster@glinx.com>:
> Hi Steve & All, Aug 28, 2013
> I don't see any connection between more sensitive hearing and more
> massive tibia.
Actually, I was thinking of a more sensitive GROUND vibration
detector, but it might possibly translate to better 'hearing' too, as
follows below.
> The larger tibia should if anything dampen the vibrations.
Easy to say that breezily but not so, depends on the layout. In
the roach, the physical situation is that the SGO membrane that
carries the 25 or so stretch sensitive cell endings spans most but not
all of a second channel in the leg, that returns blood (haemolymph)
back to the body. (A first, parallel channel conducts blood slowly
down into the leg). The membrane is attached at two extended sites on
the leg's stiff wall, solid anchor points; the remaining, third
attachment is to a large coffee bean-shaped expansion of the tracheal
system that is filled with air and is therefore compressible. (Insects
get oxygen and remove CO2 by sending branched air tubes, tracheae,
from holes on the sides of the body, spiracles, directly into the
muscles and other organs, rather than making extensive use of blood
pigments like haemoglobin; two large tracheae descend each leg). When
the tarsus (foot) is deflected by a ground vibration, a pressure wave
moves up the blood channel and is able to transiently compress the air
in the coffee bean, which allows the membrane to deflect further than
if it was simply anchored to the almost incompressible leg wall.
Because the two main tracheal trunks of each leg open to the outside,
sound (airborne pressure variations) of certain frequencies can get
into the system, and this would pre-adapt the SGO to detect sound
effectively in the future, provided that it later developed a couple
of extra modifications. This has happened: the ears of crickets and
bushcrickets developed as outgrowths of the prothoracic SGO by
incorporating a couple of tympana and a large internal trachea that
crosses the midline, and are sensitive, directionally selective ears,
as came up earlier. The SGO itself is also directional but less so
and less sensitive.
If you have a much bigger SGO support membrane in an inflated wasp
tibia (not known, but that would be have been something to have looked
for), you might get better amplification of the ground vibration than
a roach has, but no-one has yet looked at this in a pelicinid.
Sensory detection processes are now regarded as signal-to-noise (S/N)
discriminations. That is, at threshold, the central nervous system
(CNS) is looking to detect a tiny neural signal over background noise
&/or intrinsic noise in the system. The latter is usually set by
membrane ion channels that go off randomly and generate the same kind
of response that that the stimulus itself does. If the noise and the
signal occur in the same frequency band, this noise cannot be removed
by frequency filtering. It can be reduced, though, by averaging the
responses of many similarly-responding cells in parallel, by
converging the signals on to one CNS cell: the stimulus signals remain
the same so that result stays the same, but the noise pulses sometimes
move positive and sometimes negative, so tend to cancel out, so the
S/N ratio improves. This is how the roach SGO works (it might be more
familiar in the case of your eye's rods, about 500 of which contribute
by averaging their responses to the input on a single retinal ganglion
cell: consequently you can see passably in very dim light but your
acuity is way down, because light from several directions is combined
by the averaging). The major down side is that the S/N ratio only
improves ideally as the square root of the number of nerve cells
averaged. So if you had a huge wasp SGO with 630 cells, the S/N
improvement would be twice that of a roach with only ~25 cells
[sqrt(630) = 25]. Though a big 'expense' to make, a S/N doubling would
be a big deal: a few percent advantage for a useful feature is usually
enough to spread a new phenotype through a population. Even the
selective advantage of industrial melanism in a moth was pegged at
less than this (~50%), and yet the melanic form took only a few
decades to reach >90 percent penetration, and was similarly fast to
reverse when soot free air came along.
So an extremely large hind tibia might (1) be physically more
sensitive to ground displacement, (2) be a better neural performer
because of its overall S/N improvement.
> If hearing were critical and if enlarged tibia really did help, then
> why are the other four tibia not enlarged ?
This is just daft. As a counter-example, then, why did not the
cricket develop ears on all of its 6 legs, not just on the front two
only? Answer: this is not a problem of optimizing an engineering
design because there was no designer, unless you are a
Creationist/Intelligent Design advocate (I hope not). It's a
non-question because this is not how evolution is (mostly) thought to
work. Some spontaneous genetic change takes place (gene duplication is
a favorite explanation, sometimes verified) and in this case
hypothetically would result in an enlarged structure. Because the
master segmental control genes operate to some extent
segment-independently (witness the antennipedia mutant in Drosophila),
this might occur only in one pair of legs. The increased sensitivity
that this conferred alone would be advantage enough for the
modification to spread through the population. Evolutionary change
usually proceeds in small increments upon an existing base, that give
a small advantage over what went before. You can't argue for optimal
engineering design (unless you are an Creationist/ID-er).
> In addition, the target larvae eat grass roots and the process of
> moving forward in the soil and munching grass roots must make a good
> deal of noise.[do you mean underground?]
Likely true underground but I don't think that's the question, which
is how you covert significant motion/work inside in an amorphous,
damping solid like soil into pressure changes in the adjacent air.
I'd think that the conversion efficiency would be so low that even we
couldn't detect it with our superior hearing around 1000-4000 Hertz,
but this could be wrong. You could test it with a sound level meter
whose tip was acoustically shielded inside a plastic horn placed over
the chosen spot; tiny $3 lapel mikes are quite good at frequencies
down to 1000 Hz and fairly sensitive -- maybe an experiment to try,
Dave?
>
> Thinking about this reminded me of an experience ~1942. I was
> weeding our garden in the evening and when it got too dark to
> distinguish weeds from crop I just sat quietly for a few minutes
> and started hearing popping sounds in the direction of the Radish
> row. This was a light sandy-loam (as I learned much later) but