The Common Porcupine (Erethizon dorsatum) was nowhere in sight, but clearly it had been feeding on the bark of a nearby tree. The freshly-gnawed bark wasn't an unusual sight, but the exposed, sappy area was covered with raisin-like specks that deserved a closer look. The specks turned out to be flies known as Sapromyza brachysoma. Although the "sapro" part of the fly's name sounds like it refers to the sap the flies were apparently enjoying, "sapro" actually refers to decay or rot, which gives you an idea of other means of sustenance for these insects. "Myza" refers to feeding by suction.
Thank you to Tom Murray, a local insect expert, for ID help. Both Charley Eisenmen (https://bugtracks.wordpress.com/) and Tom note that these flies can be seen at relatively cool times of the year. Below is Tom's far better photograph of S. brachysoma, which he posted on bugguide http://bugguide.net/node/view/475548/bgimage
12 FEBRUARY through 15 FEBRUARY 2016
Eastern Bluebirds, Northern Cardinals, American Tree Sparrows, Dark-eyed Junco
Westfield Farm, Hubbardston, 2/14/16
This colorful caterpillar was found on Ware River watershed property in Barre on October 6th. The Symmerista family of caterpillars present identification challenges, so we pulled out our copy of David Wagner's Caterpillars of Eastern North America. The head color, something between orange and lemon-yellow, threw us off at first. But Red-humped Oakworm (Symmerista canocosta) and White-headed Prominent ( S. alibifrons)--difficult to separate in the field-- dine on beech, chestnut, and oak. This critter was chowing down on the last leaf remaining on a small birch sapling. Also, the dorsal stripe pattern is consistent with Orange-humped Mapleworm (Symmerista leucitys).
A web search brought us to the blog of DenPro, a field biologist and educator in southeastern Ohio (thanks, DenPro, whoever you are), who points out that S. leucitys can also be hard to separate from the other two Symmeristas in the field, presumably because of changes that occur at different instars (stages of growth).
Obviously we didn't investigate to this level. So what about that birch food plant? On the site The Lepidoptera of Wayne County, Ohio, we found this information that states that the larval host plant(s) of S. leucitys is/are unknown. But Wagner lists maple, especially sugar maple, as the host. Maybe S. leucitys often eats birch, unnoticed. Maybe it's a new behavior. The larvae of many butterflies and caterpillars are sometimes found on new and unexpected host plants; this is a topic too large for this page. But our little mystery demonstrates the benefits and enjoyment of the close observation of the nature around us.