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message.<BR>Checked by AVG - <A 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", that can >> be moved in a defensive manner. >> Best, Rick. >> >> >> On Sat, Aug 17, 2013 at 6:41 PM, nancy dowd <nancypdowd@gmail.com> wrote: >> >>> A week ago I took a picture of this fast-moving and impressive, albeit >>> common, White-Marked Tussock Moth Caterpillar (Orgyia leucostigma). >>> Pls correct the ID if wrong: >>> http://www.flickr.com/photos/92981528@N08/9501075283/ >>> >>> Then it reared its head up at me and waved its front tufts in the air. >>> Closeup of front end here: >>> http://www.flickr.com/photos/92981528@N08/9503879386/ >>> Details are in photo captions