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

From: Angus MacLean <cold_mac@hotmail.com>
To: naturens <naturens@chebucto.ns.ca>
Date: Mon, 19 Aug 2013 13:17:24 -0200
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&gt;&gt;&gt; I'm confident they are nothing more than highly evolved "hairs",
--_55d8c3f1-6232-4be0-9f4c-d7d2ed8f7065_
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Steve speaks of the caterpillar's defensive reaction to a predator being ne=
arby. So I'm thinking=2C what defensive reaction would a caterpillar enable=
 to avoid a wasp? ...hmm there's a leaf I could hide under..shouldn't take =
me more than five mins to get there!! However since they've evolved the det=
ection device=2C there must be more immediate steps they can take. (perhaps=
 lay on their back?).

=20

Speaking of such things like tympanae=2C I read recently that certain flies=
 that prey on orthoptera use similar adaptations to home in on the specific=
 species they will parasitize. So like many things in nature=2C you're damm=
ed if you do (vocalize) & dammed if you don't.

Angus

=20

 Date: Sun=2C 18 Aug 2013 18:16:28 -0300
> Subject: Re: [NatureNS] caterpillar question- tuft control? (long=2C sorr=
y)
> From: nancypdowd@gmail.com
> To: naturens@chebucto.ns.ca
>=20
> Thank you Steve!!!!!!
>=20
> 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).
>=20
> Neat how Crickets and others triangulate with their tympanae to hone
> in on their calling mates.
>=20
> Nancy
>=20
> On Sun=2C Aug 18=2C 2013 at 3:22 PM=2C Stephen R. Shaw <srshaw@dal.ca> wr=
ote:
> > 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 extension=
s 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=2C and comes in a variety of=
 forms=2C
> > from some that are hard and very stiff (the hard bits) and some very so=
ft
> > and flexible (for instance=2C the inter-segmental membranes of the abdo=
mens in
> > insects that allow a huge extension of the abdomen to oviposit in soil =
or
> > sand=2C as locusts do). Where the hairs bend=2C at their bases=2C the c=
uticle is
> > much thinner=2C allowing more flexibility.
> >
> > Hairs in insects are generally classified as microtrichia and macrotric=
hia.
> > The former are small simple extensions of the surface cuticle=2C are no=
t
> > hollow=2C have no associated nerve cell and don't do much that's known.=
 The
> > latter are larger=2C longer=2C often hollow. The type under discussion =
on
> > caterpillars usually act as single mechanoreceptors connected to the ce=
ntral
> > nervous system (CNS)=2C because they are associated with a single bipol=
ar (=3D
> > 'has 2 processes') nerve cell. The dendrite (outer process) of this cel=
l
> > 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=2C but often not in other directions=2C so the CNS=
 can tell
> > from which direction a deflection has come. This triggers nerve impulse=
s in
> > the second process=2C the centrally directed nerve axon that ends up re=
aching
> > one of the 'ganglia' (nerve centres=2C like little brains) of the ventr=
al
> > nerve cord (=3D part of the CNS). Insects in early evolution developed =
upside
> > down from vertebrates=2C so the nerve cord is on the ventral side of th=
e body=2C
> > versus dorsal for vertebrates (=3D 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=2C so when =
the
> > hairs move it is because the local body muscles nearby are deforming th=
e
> > head end=2C the body surface=2C etc. The tufts of hairs move because th=
ey are
> > mechanically connected to local body movement=2C by virtue of being car=
ried 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 gener=
ated
> > from a vibrating source like the wing beats of a wasp (colloquially cal=
led
> > 'sound' if the frequency is high enough for us to hear=2C but not too h=
igh)=2C
> > two types of usable information result. The first is effective only ver=
y
> > close in=2C a so- called 'near-field effect' by which the local air mol=
ecules
> > incur large displacements. Hairs of the properly 'tuned' length for the
> > particular frequency can couple effectively to the displacement and get
> > stimulated=2C and so tell the caterpillar that a predator is hovering n=
earby=2C
> > which may lead to defensive reaction. This near field effect dissipates
> > very rapidly with distance away from the source=2C and is useless only =
a few
> > wavelengths away.
> >
> > The second effect also dissipates with distance but less severely (inve=
rse
> > square law to be specific)=2C so can be detected usefully at some dista=
nce. It
> > consists of propagated waves of compression and rarefaction of the air
> > molecules which we hear as sound=2C but to which the hairs do not respo=
nd at
> > all. To hear that=2C you need a pressure-gradient detector like your
> > tympanum=2C but some insects also have them=2C like mantids=2C crickets=
 and also
> > katydids=2C which have the most complex acoustic inputs known. In Nancy=
's
> > recent katydid=2C the main tympanum can be seen as the dark elliptical =
area on
> > the inside edge in the yellowish zone just below the 'knee'=2C on the t=
ibia of
> > the front legs=2C 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 so