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Hi Steve & All, Aug 28, 2013
I don't see any connection between more sensitive hearing and more
massive tibia. The larger tibia should if anything dampen the vibrations. If
hearing were critical and if enlarged tibia really did help, then why are
the other four tibia not enlarged ?
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.
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 there was still enough crust to resist seedling
emergence until the crust suddenly broke with a pop.
We hear a lot about light pollution but we seldom see anything about
noise pollution; the aircraft grounding after 911 excepted. It would not be
possible to hear this slight noise now or detect it with a sensitive mike,
unless one were in a sound-proof room, because there is too much background
noise.
So I wonder. In situations where hearing minute sounds is critical for
survival of some animal is it possible that excessive background noise puts
them at a disadvantage ?
Turning now to the enlarged hind tibia, this looks to me like a
mechanism to avoid doing a slow-motion somersault when the very long abdomen
is thrust into (usually uncultivated) soil. Someone must have put one of
these in a cage over sod and observed how they drill. Meanwhile I will
assume that the enlarged hind tibia, with tarsal claws hooked to some grass
crown, enables effective downward thrust of the abdomen.
Yt, Dave Webster, Kentville
----- Original Message -----
From: "Stephen R. Shaw" <srshaw@Dal.Ca>
To: <naturens@chebucto.ns.ca>
Sent: Wednesday, August 28, 2013 2:47 PM
Subject: Re: [NatureNS] Another parasitic wasp- Pelecinid
> Hi Nancy:
> Each of the six legs of most insects investigated carries a sensitive
> mechanoreceptor organ in the tibia, just below the 'knee', aptly called
> the sub-genual organ (SGO). It contains ~25 chordotonal sense cells
> (mentioned earlier re insect 'ears') strung around a membrane that spans
> a blood channel in the leg, a bit like a stretched trampoline. It's
> primarily a ground vibration detector. In cockroaches, the leg's
> displacement threshold to elicit a response is about the same as that in
> the basilar membrane at the threshold of hearing in the cochlea, ~1
> nanometer, so it's exquisitely sensitive. A German scientist that I'd
> met briefly earlier had observed pelecinids while vacationing in Canada a
> few years ago. He wrote suggesting that the extremely enlarged tibia of
> the hind legs, clearly visible in your photo, might contain a mega-SGO,
> that might be what the wasp used to detect vibrations emanating from June
> beetle larvae underground. This could be worth investigating and he was
> interested to do so.
>
> So I collected a couple of the wasps (apparently not available in
> Germany), and painstakingly fixed and embedded the swollen tibiae for
> electron-microscopy and sent the blocks to him to cut sections and
> examine, since that's what he seemed to want to do. I later learned from
> a junior colleague there that he had given up doing things himself and
> had become a lab manager-bureaucrat, so the blocks were sitting on his
> shelf and the project never went any further. In fact he probably really
> wanted me to do it so that we could 'collaborate' (X actually does the
> work while Y gains partial credit, the way of the science world these
> days).
>
> If this project had gone forward, it still would not have solved how the
> wasp decides to land on which bit of the lawn to check for vibrations.
> It would be interesting to see if the pelicinid SGO responds also to
> airborne sound, as does the cockroach SGO, though it is hard to imagine
> how much airborne sound could be generated from a beetle larva scrunching
> around inches underground (you'd have to measure it at different
> frequencies).
>
> The adults typically are out flying about now, late August, often
> cruising over lawns. As has come up before on NatureNS, there is just
> the single N. American species, and those seen are always females. Males
> are known but are much smaller and are never usually found --
> reproduction may be mostly without male intervention.
> Steve (Halifax)
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> Quoting nancy dowd <nancypdowd@gmail.com>:
>> There have been some fascinating posts lately about some of our
>> parasitic (and hyperparasitic) wasps. Here is another one that seems
>> to be everywhere right now- a Pelecinid (Pelecinus polyturator):
>>
>> http://www.flickr.com/photos/92981528@N08/9612734497/
>>
>> Another, more ominous looking, using the flash:
>> http://www.flickr.com/photos/92981528@N08/9616020630/
>>
>> They are big (40mm as shown) and hard to miss.
>>
>> Somehow (sound/vibration?) mother Pelecinid detects larvae below
>> ground, jabs her abdomen in the soil and lays a single egg on one,
>> often a June Beetle grub from what I read.
>>
>> Nancy
>
>
>
> -----
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>
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