Caterpillar spiracle - 2nd image added

[NikonUser]

Rik's photo of a cutworm's spiracle has bothered me since it was posted.
HERE

Morphologically and physiologically it makes no sense.
Spiracles need to be open spaces for the passage of air in and CO2-laden air out. The spiracle in Rik's image appears not to allow any gas movement.

I have finally found a decent-sized caterpillar so as to look at it's spiracle. As suspected it is an obvious opening guarded by branching hairs that I suspect can be moved so as to partially cover the opening to reduce air flow (most likely to reduce water loss under very dry/hot conditions) and perhaps even as a filter to keep out dust particles.
(The hair crossing the spiracle is a body hair).

So, what is Rik's spiracle showing? My guess is a disease organism such as a fungus that has invaded the caterpillar.

American Tent Caterpillar, Olympus 40x SPlan Apo +1.25x intermediate lens +2.5x relay lens. 1µ frames, ZS PMax

NU11097

[rjlittlefield]

Spiracles need to be open spaces for the passage of air in and CO2-laden air out. The spiracle in Rik's image appears not to allow any gas movement.

Well, they certainly need to be open for respiration. However, as noted in "The Insects / Structure and Function", pages 449-450,

The spiracles of most terrestrial insects have a closing mechanism which is important in the control of water loss.
...
The spiracles are normally open for the shortest time necessary for efficient respiration presumably to keep water loss from the tracheal system to a minimum

I discount the possibility of fungus since the caterpillar I photographed appeared perfectly healthy from the time I noticed its appearance until several weeks later when I inadvertently froze it while trying to chill it into complete quiescence. Also there was nothing special about that one particular spiracle -- all spiracles on the caterpillar looked the same.

My own guess is that the spiracle I photographed just happened to be closed at the time I photographed it, and we're looking at exterior texture of the closing flaps. Alternatively, the spiracle may be open, but filled with a dense mat of filter hairs such as illustrated in the SEM image HERE. Google image search on spiracle shows many cases that look similar to mine, though not at such high magnification.

It would be nice to have pictures of the same individual with spiracles open versus closed, but I couldn't find any of those. The idea of open versus closed is well illustrated HERE, last image, but that's with two very different critters.

I do agree that NikonUser's image is an excellent illustration of an open spiracle. The filtering hairs are very well captured. I haven't seen those clearly rendered in optical before, only in SEM.

--Rik

Edit: 06 Sept 2022, fix broken link for spiracle filled with dense mat of filter hairs

[NikonUser]

Thanks Rik, the references you cite certainly point to a closed spiracle.

Reminds me of a quote from a renowned entomologist (don't remember who) when asked if he was an entomologist.
He answered along the lines that there was no such person, we are all just students of entomology.
I must change my signature

[rjlittlefield]

The world is just a very complicated place, and that's a good thing!

In the case of the spiracle I photographed, I still don't know what I'm dealing with. I kept the specimen in alcohol, did some probing just now, and learned almost nothing. I had hoped that I could pull or prod so as to get the two lips to separate, but no such luck.

Chapman notes that in most insects "Spiracle closure results from the sustained contraction of the closer muscle, while opening commonly results from the elasticity of the surrounding cuticle when the closer muscle is relaxed". But of course I'm dealing with a critter that died by freezing and then got stuffed in alcohol, neither one necessarily causing a muscle to relax.

Adding to the complexity, Chapman notes that "The closing mechanism may consist of one or two movable valves in the spiracular opening or it may be internal, closing off the atrium from the trachea by means of a constriction." And also that "In some Diptera, Coleoptera and Lepidoptera the spiracle is covered by a sieve plate containing large numbers of small pores. In the fifth stage larva of Bombyx (Lepidoptera) the pores are oval (6x3 µm)." The resolution of my optical setup could easily resolve pores of that size, but I guess they could be indefinitely smaller also. Diatoms, anyone?

So I dunno! I guess that either I'm looking at a spiracle with external closure that's stuck in the closed position, or I'm looking at a spiracle that's open but filled with a fixed cover of filter stuff, or I'm looking at something else. I take comfort in the theory that the last option covers all the possibilities, but I'm not completely sure it does...

--Rik

[Linden.g]

There seems to be a wide range of feather/hair like structures, I imaged a hawk moth spiracle a while ago and was also surprised by its complexity. http://www.flickr.com/photos/13084997@N ... 1935366797. In this case it looks like the plates may be closed. The plates also look like they consist of criss-crossed hairs acting like filters.

With respect to rules on posting images on someone elses post, are links ok or is this considered not acceptable?
Linden

[rjlittlefield]

Thanks for the further information.

Posting links in someone else's gallery thread is recommended practice. Problems are only caused by images inlined with [img].

--Rik

[NikonUser]

The 1st image was looking at the spiracle from the outside.
This shot is looking from the inside.
On the left can be seen the air tubes (trachea) which leave the spiracle in several branches.
The trachea have been separated from the spiracle by compression of the coverslip. The thickened bar adjacent to the spiracle is
a flattened ring that is attached to the inside of the spiracle.

Similar setup as above but with 20x SPlan Apo and with less bellows extension above the relay eyepiece.

NU11098

[rjlittlefield]

Excellent!

--Rik