There is a family of flies, Tachinidae, which consists of different species of parasitic flies, one of which is Epalpus signifier. If you look at enough dandelions this time of year, you are likely to spot one — their white rump is a distinctive identifying feature. This fly is technically a parasitoid, and as such, spends most of its life attached to or within a host organism (Noctuid moth caterpillar) , getting nourishment from it. Unlike a parasite, it eventually kills or consumes its host
The striped caterpillar that is crawling along the surface of fresh snow is the larval stage of a noctuid or owlet moth (species unknown). Noctuids are dull-colored, medium-sized, nocturnal moths that are attracted to lights in the summer. They usually possess a well-developed proboscis (mouthpart) for sucking nectar. You may be familiar with the common garden pests, cutworms, which are also noctuid larvae. How this larva survives freezing temperatures I do not know, but I have seen several dozen at a time crawling around on top of the snow. Note: Jean Harrison, a fellow nature lover, just identified this larva as Noctua pronuba, a winter cutworm also known as the greater yellow underwing (moth), a recent immigrant from Europe.