Short term scale wear on a moth
As all of us do sometimes, Jack (@jcochran706) photographed the same moth at different times on the same evening and inadvertently (?) uploaded the images as separate observations. His original uploads of two crisp images of the Texas endemic Capps’ Petrophila can be seen here and here. Here are medium-sized versions of each image:
The two images were taken about 25 minutes apart and the moth had evidently moved from one perch to another. (Jack indicated he had two moth setups that night and couldn’t tell me on which each image was captured…but that’s beside the point.)
What first drew my attention to the possible duplicate offerings was the wear on the face of this individual moth—the creamy white scales on the face of the moth are absent in both images. I enlarged each of Jack’s sharply focused images to examine them carefully to verify my suspicion. From the wear on the head and thorax it was easy to discern that both photos showed the same, slightly worn moth.
The two images both show the moth in a relatively similar posture, which allowed me to examine each further in excruciating detail*. Based on this exceptional opportunity, I quickly went down a rabbit hole of examining minute details of the wing patterns. Many Petrophila species have a forewing pattern of a pale ground color liberally sprinkled with tiny black or dark brown dots—often single black scales. In the present instance, I began looking carefully at the set of black dots in the basal third of the forewings. And—no surprise—the two images showed some scale loss from the earlier photograph to the later one. But keep in mind, this scale wear happened in only 25 minutes!
Scale Loss in 25 Minutes
Above is a side by side comparison of a closely cropped portion of the base of the left forewing of this moth. The earlier image (10:53 PM CDT) is on your left; the later image (11:18 PM CDT) is on the right. I have highlighted a few of the changes that I can recognize. The four numbered red arrows point to black dots that are present in the earlier photo and absent later. I interpret these as scale loss. I’m sure a careful examination of the entirety of the wings would show more. It’s just easy to spot when black dots are missing on a white background. The wing itself, under the scales, is a pale translucent membrane. It would be hard to spot missing white scales for that reason.
Jack pointed out something else which is apparently further scale loss: Notice the dark area in the oval in the upper right of each image. These are NOT areas of dark scales; this is an area of the dark integument of the moth’s thorax showing through between gaps in the white scales of the thorax and adjacent scale tracts. In this case, the loss of some white scales exposed a little more of the dark thorax underneath.
Can Moth Scales Wiggle or Move?
I normally think of the scales on a moth or butterfly wing as being fixed in position, like overlapping shingles on a roof. But I was puzzled by some additional details that I recognized in the two images above. Notice the dark dots I’ve highlighted by a “?” and black arrows, and compare these to their apparent positions in the later image. Each of these (and perhaps a few more) seem to have shifted laterally just a little bit to the right between the two time marks. IF each represents a dot that is about a single scale wide, the shift looks to be about 1 to 2 scale-widths to the right. I would hope that this isn’t happening with my roof shingles. So what does this represent? It seems that the scales are not fixed in place like the roof analogy. My best guess is that the black dots actually show only the tip of a wing scale; the basal part of the scale attached to the wing is hidden some distance underneath the overlapping scales above it. This might make them behave more like a bird’s feathers which can rearrange (or be rearranged, in the case of the bird) somewhat. Is there anyone out there who is more familiar with the micro-structure of Lepidopteran scales who might shed some light on this apparent scale movement?
Conclusions
Moths lose scales over time. The wing wear on “older” moths is apparent to anyone who looks at lots of these. But we typically lack any time scale on this scale loss. In the uncommon instances when a moth is photographed from one night to the next, or from one evening to the next morning, wear may or may not be evident. In the present instance, a small amount of scale loss is evident in just a 25-minute span of time.
Another point may be apparent: It takes exceptionally detailed and close-up photography to approach questions like those above. My thanks and congrats to Jack for—time and time again—offering up such beautiful natural history imagery which makes such an investigation possible!
(* As a side note, for several years now I have been examining hundreds of images of Two-banded and Capps’ Petrophila in Texas attempting to find ways to distinguish the two species when views of the diagnostic hindwing patterns were not evident. So far I have failed in that task. Much to his credit, Jack is not only the most prolific documenter of the endemic Capps’ Petrophila, but his photography is of such high quality that it allows me chase such arcane questions.)
