Eupithecia moth

2007 August 25

Back in April, this moth was on our front window, and didn’t show any inclination to move. It had a wingspan of a bit under an inch, and as near as I can tell, it is in the genus Eupithecia, based on the way that the wings are held splayed out when at rest, rather than folded over the back. These overwinter as pupae, and so are one of the moth species that appear as adults very early in the season.


Basically, this is one of the moths that inchworms grow up into. The pictures of its back are OK, but it is hard to see the actual structure of the moth, because (a) I was a little bit out of focus, due to hand-holding the camera instead of being able to use the microscope stand, and (b) the mottled camoflage pattern breaks up the outline quite a bit [1]. It is possible to see that there seems to be some feathering of the trailing edge of the wings, but the antennae and even the boundaries between the forewings and hindwings are hard to make out.

However, since we were on a sheet of glass, it was easy just to go inside and look out.[2] It was the middle of the day, so the moth was well backlit, making an image in transmitted light that was almost like an X-ray [3]:


Now we can definitely see the feathering on the trailing edges of the wings, the overlap between forewings and hindwings, and the antennae that are tucked underneath the wings. I’m really wondering about the function of that feathering. The other place I have seen something like this is on owl feathers, which have kind of a “fuzzy” appearance. I understand that this makes owls very quiet flyers by muffling the sound of air rushing over the wings, and I suspect that the moth wings are like this for a similar reason. As for why it would be important for them to fly quietly, I don’t know, unless it is to keep bats from hearing them. Since bats mostly eat moths (big, fat, juicy moths)[3], this would be a big survival issue.

[1] I expect that, if this moth were on the trunk of a tree, I would have thought it was just a patch of lichen. This explains why it stayed so still for photography: it was behaving as if it were on a tree trunk (and therefore practically invisible) instead of on a clear sheet of glass (and therefore painfully obvious).

[2] This was very easy, due to a fortuitous feature of our front window: the glass is a double pane, and the spacing between the panes is right in the middle of the correct focal distance for my improvised macro lens. This meant that I could hold the lens right up against the inside glass to hold it steady while photographing the moth on the outside glass.

[3] Near the entrance to the building I work in, there are some concrete structures that bats like to hang out in during the day. A while back, right below the bat hangout I found a complete set of cecropia moth wings, and a set of luna moth wings. These are both really large, fat moths, maybe a quarter of the weight of the average bat. I bet the bats that caught those were pretty pleased with themselves.

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