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>>> I'm confident they are nothing more than highly evolved "hairs", --_55d8c3f1-6232-4be0-9f4c-d7d2ed8f7065_ Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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