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

Date: Mon, 19 Aug 2013 16:10:46 -0300
From: "Stephen R. Shaw" <srshaw@dal.ca>
To: naturens@chebucto.ns.ca
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&gt; &gt; We have started to hear crickets sing in the last week or two. In so
Hi Angus,
On part (1) below, you gave up too easily and obviously need to think  
more like a caterpillar.  I used to rear a few species (mostly moths)  
as a kid in the UK and encountered the following defensive reactions,  
at least when I went near caterpillars or tried to touch them (might  
vary if the approach was from a wasp, and obviously wasps aren't the  
only things that eat caterpillars, either).

-  drop like a stone off a leaf or stem on to the undergrowth or soil  
below, making itself difficult to find; climb back up later (some  
noctuids would do this).
-  curl up into a millipede-like tight circle with defensive bristles  
sticking out, that may be noxious to some predators, or make  
parasitoids harder to gain a close approach to oviposit.  Some tiger  
moths (arctiids) do this. Somebody is going to ask why the nasty  
irritating hairs break off so easily and I don't know, but presumably  
there's a weak spot near the base that's responsible.  'Deliberately'  
weakened areas underlie defensive reactions called 'autotomy'  
elsewhere, as when a crab defensively sheds a leg at the leg's  
specialized autotomy zone.
-  regurgitate noxious fluid from the mouthparts, that may smell bad  
and deter some attackers, suggesting a potentially unpleasant meal if  
pursued.  This would be more common in insects like grasshoppers.
-  defensive writhing of the back end with the prolegs still attached  
to a stem, presumably in an attempt to prevent the predator or  
parasitoid gaining a foothold.  I've seen a sphinx caterpillar here do  
this.
-  defensive front-end rearing to show fearsomely threatening eyespots  
- large elephant hawk moth caterpillars in UK do this (Deiliphilia  
elpenor, spelling from memory).
-  defensive rearing or writhing to remind predator of warning  
coloration that advertizes toxicity: orange-black banded cinnabar moth  
caterpillars (an arctiid? - I can't remember) that live in small  
colonies on ragwort do this.  I was once involved in breeding these en  
masse for pocket money, in an ill- conceived scheme to export pupae to  
Australia to control invasive ragwort, bad for livestock.  Apparently  
someone forgot that the seasons are 6 months out of sync so the adult  
moths emerged in winter.

No doubt the lep specialists here or others can add a few more tricks?

Re. damned if you do or don't, biologists view these sorts of  
adaptations as evolutionary 'arms races'.  The prey species evolves a  
new or modified defense, then the predator evolves a countermeasure;  
well known in plants too.  Usually, neither strategy is perfect so an  
equilibrium prevails for a while: sometimes a defense succeeds and  
sometimes it fails.

On (2), as you may know, one of the largest groups of brachyceran  
flies, tachinids, reproduce almost exclusively by parasitizing other  
insects either in the adult or larval state.  They lay one or more  
eggs on the body of the insect host and the fly larvae slowly eat it  
up from the inside and then pupate, a pretty gruesome fate.
A question is, how do they locate the host?  There are a couple of  
genera of tachinids, one of which is a yellowish nondescript fly  
Ormia, that home in on singing male crickets, as you say.  According  
to work done in Ron Hoy's lab in Cornell Univ, the fly shows quite  
exceptional directional selectivity of around 1°, so can  
accurately/quickly locate the sound source at night.  Interest in this  
is that flies generally do not possess eardrums (tympana).  This genus  
has evolved a pair of these on its 'chest' (ventral thorax) from  
pre-existing chordotonal organs there, that in other species respond  
just to internal stretch (this reprises an earlier comment about  
evolutionary adaptations usually being based on pre-existing  
structures).

The main scientific interest is that the system is seemingly far too  
small to work at all acoustically, because the two tympana appear to  
be too small and too close together to allow either a significant  
sound intensity difference between them, or a significant sound time  
delay (in humans, directional hearing works because of sound-shadowing  
by the head at the frequencies we resolve, which generates a sound  
intensity difference between the eardrums, and there's also a time  
delay from most positions of the sound source because of the large  
distance between the ears). These crickets sing dominantly at ~6 kHz  
(from memory) where the sound wavelength is ~60 mm, while the  
separation of tympana is less than 1 mm (from memory).  Daniel Robert  
originally in Hoy's lab came up with an explanation for how it works,  
involving coupling between the two tympana* via some fancy chitinous  
rods that amplify the sound delay, such that the fly can respond to  
time differences (that indicate direction), in the nanosecond range.   
It's quite a challenging system to understand, technically.
Steve (Halifax)
*P.S.
tympanum, eardrum (tympana = plural; Latin noun, neutral gender) = OK
tympani, orchestral drums (plural only used, italian noun) = OK, but not here
tympanae = hypothetical but non-existent plural Latin noun, female gender
Steve (Hfx)

Quoting Angus MacLean <cold_mac@hotmail.com>:

> (1) Steve speaks of the caterpillar's defensive reaction to a  
> predator being nearby. So I'm thinking, 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 detection device, there  
> must be more immediate steps they can take. (perhaps lay on their  
> back?).
>
>
>
> (2) Speaking of such things like tympanae, 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, you're dammed if you do (vocalize) & dammed if you  
> don't.
>
> Angus
>
>
>
>  Date: Sun, 18 Aug 2013 18:16:28 -0300
>> Subject: Re: [NatureNS] caterpillar question- tuft control? (long, sorry)
>> From: nancypdowd@gmail.com
>> To: naturens@chebucto.ns.ca
>>
>> Thank you Steve!!!!!!
>>
>> That was a great explanation for the basis of