[NatureNS] caterpillar question- tuft control? (long, sorry)

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Date: Sun, 18 Aug 2013 18:16:28 -0300
From: nancy dowd <nancypdowd@gmail.com>
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Thank you Steve!!!!!!

That was a great explanation for the basis of the caterpillar's tuft
movements but also for a whole lot of things- such as the perfectly
preserved hairs etc on insect exuviae (that I see when I hold my
Cicada skin up to the light).

Neat how Crickets and others triangulate with their tympanae to hone
in on their calling mates.


On Sun, Aug 18, 2013 at 3:22 PM, Stephen R. Shaw <srshaw@dal.ca> wrote:
> Don't know who wrote the second paragraph quoted by Rick but it is only
> partly correct.  All of all parts of insect hairs are modified extensions of
> the insect's exoskeleton (the cuticle) and all are therefore
> 'cuticularized'.  Cuticle is basically a matrix made of a complex
> polysaccharide chitin plus other chemicals, and comes in a variety of forms,
> from some that are hard and very stiff (the hard bits) and some very soft
> and flexible (for instance, the inter-segmental membranes of the abdomens in
> insects that allow a huge extension of the abdomen to oviposit in soil or
> sand, as locusts do).  Where the hairs bend, at their bases, the cuticle is
> much thinner, allowing more flexibility.
> Hairs in insects are generally classified as microtrichia and macrotrichia.
> The former are small simple extensions of the surface cuticle, are not
> hollow, have no associated nerve cell and don't do much that's known.  The
> latter are larger, longer, often hollow. The type under discussion on
> caterpillars usually act as single mechanoreceptors connected to the central
> nervous system (CNS), because they are associated with a single bipolar (=
> 'has 2 processes') nerve cell.  The dendrite (outer process) of this cell
> has specialized membrane channels that are sensitive to mechanical
> deformation when the hair and the dendrite inserted in it are bent in a
> particular direction, but often not in other directions, so the CNS can tell
> from which direction a deflection has come.  This triggers nerve impulses in
> the second process, the centrally directed nerve axon that ends up reaching
> one of the 'ganglia' (nerve centres, like little brains) of the ventral
> nerve cord (= part of the CNS).  Insects in early evolution developed upside
> down from vertebrates, so the nerve cord is on the ventral side of the body,
> versus dorsal for vertebrates (= the spinal cord). The tormogen
>  and trichogen cells around each hair mechanoreceptor are specialized
> support cells that modify the local environment for the bipolar neuron.
> None of these hairs have direct muscle insertions upon them, so when the
> hairs move it is because the local body muscles nearby are deforming the
> head end, the body surface, etc. The tufts of hairs move because they are
> mechanically connected to local body movement, by virtue of being carried on
> the body's exoskeleton or an appendage.
> Some of the directionally selective hairs are used to detect the close
> approach of predators/parasites.  When a disturbance of the air is generated
> from a vibrating source like the wing beats of a wasp (colloquially called
> 'sound' if the frequency is high enough for us to hear, but not too high),
> two types of usable information result.  The first is effective only very
> close in, a so- called 'near-field effect' by which the local air molecules
> incur large displacements.  Hairs of the properly 'tuned' length for the
> particular frequency can couple effectively to the displacement and get
> stimulated, and so tell the caterpillar that a predator is hovering nearby,
> which may lead to defensive reaction.  This near field effect dissipates
> very rapidly with distance away from the source, and is useless only a few
> wavelengths away.
> The second effect also dissipates with distance but less severely (inverse
> square law to be specific), so can be detected usefully at some distance. It
> consists of propagated waves of compression and rarefaction of the air
> molecules which we hear as sound, but to which the hairs do not respond at
> all.  To hear that, you need a pressure-gradient detector like your
> tympanum, but some insects also have them, like mantids, crickets and also
> katydids, which have the most complex acoustic inputs known.  In Nancy's
> recent katydid, the main tympanum can be seen as the dark elliptical area on
> the inside edge in the yellowish zone just below the 'knee', on the tibia of
> the front legs, in her nice second photo:
> http://www.flickr.com/photos/92981528@N08/9526735582/
> We have started to hear crickets sing in the last week or two.  In some
> species, female crickets compare the sound intensity of the male song at the
> two tympana on their front legs as a pressure difference, and use this
> difference to locate the male by zig-zag walking towards him.
> Agreed that one should not 'underestimate the power of natural selection'
> but I'd disagree that 'it just might happen', if this is meant to imply that
> anything could happen: not so.  Evolutionary modification only works on what
> went before it in that species line, therefore the range of possibilities
> for change, while large, is limited and rather narrowly channelled.  For
> instance, tympanic ears above have evolved independently at least 14 times
> at last count in insects, on different parts of the body, but all versions
> are based on the chordotonal receptor, a pre-existing mechanoreceptor cell
> complex that is found throughout the body and that monitors its internal
> state.
> Steve (Halifax)
>  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Quoting Rick Whitman <dendroica.caerulescens@gmail.com>:
>> From wikipedia, from which all good things come:
>> "The larvae <http://en.wikipedia.org/wiki/Larva> are brightly coloured,
>> with tufts of hair-like setae <http://en.wikipedia.org/wiki/Setae>."
>> "Setae in entomology <http://en.wikipedia.org/wiki/Entomology> are often
>> called hairs or chaetae <http://en.wikipedia.org/wiki/Chaeta>. They are
>> unicellular and formed by the outgrowth of a single epidermal cell
>> (trichogen). They are generally hollow and project through a secondary or
>> accessory (tormogen) cell as it develops. The setal membrane is not
>> cuticularized and movement is possible. This serves to protect the body."
>> On Sat, Aug 17, 2013 at 7:30 PM, Rick Whitman <
>> dendroica.caerulescens@gmail.com> wrote:
>>> I feel that you are under-estimating the power of natural selection i.e.
>>> if it benefits the organism, in terms of survival, it just might happen.
>>> I'm confident they are nothing more than highly evolved "hairs",