A simple (?) question about moths!

I have been interested in Lepidoptera all my life and have been studying moths intently for about the past 20 years. A question popped into my head just this morning as I was identifying yet one more worn moth on the wall near my porchlight.

Observation: As a moth loses more scales (presumably as time passes in its short life), a common wear pattern is for the top of the thorax to be exposed, in some cases eventually being almost completely exposed. See, for example:
https://www.inaturalist.org/observations/93169376
https://www.inaturalist.org/observations/92539986
https://www.inaturalist.org/observations/92357030
This happens to medium and large moths which perhaps have longer life spans, but it is not exclusive to such species (note the 2nd example which is a relatively small Noctuid).

Question: I can understand wear and tear on wings, but what aspects of a moth’s life, actions, and movements would cause scale wear on top of the thorax?

Further musings and speculations:
– I understand that moths might come into contact with some “overhead” substrate like leaves or twigs, but how often do moths squeeze into tight places (e.g. to roost) or just bump into objects in flight?
– Do the forces of in-flight air flow itself (which must be considerable) over the head and thorax cause so much turbulence as to eventually dislodge scales?
– Do the dynamics and forces involved in wing motion have a deleterious effect on retaining scales on top of the thorax?
– In flight, would swept-back antennae whipping around in the turbulence be contacting the thorax and knocking scales off?

I’ll be interested to hear of any suggested explanations for this phenomenon. I have not done a literature search for this rather arcane question!

13 Likes

From experience I’d say first one sounds as more reasonable, both about objests when they hide and when they bump in lightbulbs. Supported by which ones we see with those scales of, mostly those hiding somewhere during the day and affected by light at night. Doubt antennae are hard enough to get scales as much as they go off sometimes.

1 Like

Ha! I wondered the same thing a week or so ago when someone made a comment about a red spot on the thorax. I explained what it was, and then thought ‘why there?’ The rest of the moth was in pretty decent shape. All I could think of was that the motion of the thorax during flight caused the scales to come off. Moth wing muscles are attached to the inner side of the thorax, so there must be some movement. I’ll dig out my insect anatomy and physiology book and see if it says anything. I had not thought about the other factors you mentioned!
EDIT Got out my Chapman, and there is substantial movement of the thoracic tergum up and down with each wing beat. There is also side to side movement. Insects use an elastic effect stored on the upswing to move the wings down. Chapman says that about 85% of the upward force is stored for the downstroke. There is also an elastic pad at the main wing hinge. The elastic properties of the pad “are almost perfect so that it absorbs less than 3% of the energy imparted to it” when stretched on the upswing. So, something to keep in mind! I don’t know if there has been any work done on scale loss patterns. I’ll check it out later.

6 Likes

Does the bald thorax mean the moth escaped an attack from a predator with a mouth full of scales?

1 Like

A couple of papers, not specifically related to scale loss:
https://journals.biologists.com/jeb/article/220/8/1382/18838/Mechanics-of-the-thorax-in-flies

https://cob.silverchair-cdn.com/cob/content_public/journal/jeb/94/1/10.1242_jeb.94.1.119/1/119.pdf?Expires=1633546767&Signature=MQsgaWMytynBYRcM0H3cLROJ-9kVI8Ng8R15uN3nt11QHu12DIGwCpQ5e1WFztRr6vJRwzV5hxDSRRfLy8k03M0~L~84jyynomI7YfJVuCha9ljv3NkPxCGUTk4H2LobyCinIooQxFxS3nB0g6~p1GDTz1~SXEyYnSTpaYewYf~KSAWdBnhrxbmJyAKk6zngZ6eA2BWYLNfdfjFWxfPIqJQb1cqes9V0RqWkRGUfENN23edKDHM-9fv-QP2l7htmOT7IyHFsgamvqmZX-NL6IAfw2DEQ7AhO0A6NQwrmDNrgXSVMlnrIoLiRDiZZBm6-VBV9qW3fyKgwnazDA0Y~-w__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA

Each night I fly to Texas where I surreptitiously sneak around gcwarbler’s house and delicately rub the scales off moth thoraxes. I see this has finally paid off!

A really good question and something I’ve noticed and wondered about for years but never bothered to ask. Thanks for bringing it up!

12 Likes

Since this topic came up, I’ve been kind of looking at wear patterns on moths. A puzzling thing - many of the moths with worn wings have the thorax intact. And the few I’m seeing with the bare thorax have the wings almost intact. My ‘sample’ size is small and in no way should this be considered scientific, but (at least for me) it’s adding to the mystery.

1 Like

Many moths feed on nectar and act as pollinators, could contact with flowers contribute to the wear patterns?

5 Likes

Perhaps it depends on the anatomy of the flowers they prefer.
Bit of a tight fit, this one.

3 Likes

Bombus also have this wear pattern. Here’s one example: https://www.inaturalist.org/observations/56857292

2 Likes

I’ve noticed that as well, but don’t know enough about them to know if they emerge like that or whether it’s wear.

1 Like

Yes, I too have noticed the wear/scale loss on top of Moths’ thorax in older Moths. You’ve proposed several possibilities…any or all of which may contribute I think. hmmmm…perhaps like Balding in older men??? Have you noticed if it is sex-associated?..males more often than females?

3 Likes

Experimental design: two groups of adult moths, all the same species. One group is fed from narrow-tubed flowers, the other from an open dish. See if one or both groups develops the wear pattern.

3 Likes

My question has elicited some great responses, speculation, and research! I appreciate all the contributions. I had not considered the thoracic movements from flight muscles, nor the flower shape for those moths that are pollinators. The latter question might be approached as follows: Is thoracic wear more apparent on species which are pollinators? What correlation is there with the shape of flowers that a given moth species visits? Many adult moths don’t feed so there might be some obvious subsamples that could be compared.
There’s also the “@tiwane Effect” to consider but unless we gain access to Tony’s travel agenda, I don’t think we’d be able to prove/disprove that hypothesis. ;-)
Great stuff!

3 Likes

This topic was automatically closed 60 days after the last reply. New replies are no longer allowed.