10Jul2022 I was incorrect in my identification of this moth caterpillar and the additional two images. Its name is actually Triprocris lustrans. It was identified by David Wagner. I do not know how to change the present name to the correct name. Please advise.

This aposmatic caterpillar is feeding upside down on a leaf of Hairy Four O'clock (Mirabilis hirsuta = Oxybaphus hirsutus) in the Four O'Clock Family (Nyctaginaceae) in the foothills of Colorado at about 6400' elevation. On June 20, 2007 we saw a colony of these cats feeding on the same kind of Four O'Clock in exactly the same place. At that time I brought one larva into the house to raise. However, it refused to eat. I felt such a colonial larve might prefer the company of siblings and returned it to the plant. Since then we have looked for it every year. This year we were successful, as we found two dispersed individuals, indicating more mature larvae.

Checking in Powell and Opler , Moths of Western North America, I checked out all the moths they had listed as feeding on the Four O'Clock Family. There are not many. A Gelechiidae moth whose caterpillars mine leaves. A few Heliodinidae whose larvae are colorless grubs. A Geometridae which lacks prolegs on segments 3-5. The sphinx moth Hyles lineata. A noctuid whose larvae are velvety black with some orange spots and orange striped head. And a Zygaenidae, Triprocris smithsoniana. Of all these species, only the latter in a possibility. It occurs in eastern and central Colorado, including foothills of the Front Range which is where we found it.

They describe larvae in Family Zygaenidae as "stout and broad with the head retractile, body roughened and covered with dense secondary setae arranged in rows of dense clumps (verrucae)." On page 162 the caterpillar of Triprocris smithsoniana is described: "the mature larva has a black dorsal longitudinal strip, white intersegmental spots, and black, white and salmon colored lines and bands." There seem is describe our caterpillar. I have checked other caterpillars in this family Zygaenidae and they seem to have similar structures along the side. Any help with this caterpillar and its identification is much appreciated. I will post two more images.