Acronicta dactylina
  
The genus Acronicta comprises the moths known as "daggers" 
  on account of a dagger-shaped mark that is more or less prominent on the forewings 
  of the many species. (76 were listed for North America in 1983, and there are 
  yet more in Europe and Asia.) The adult moths are quite similar to each other, 
  and are not easy to identify, and one may even doubt that many of them are full 
  species. All doubt is removed, however, on seeing the caterpillars, for there 
  is a wonderful variety of striking caterpillars; the caterpillars of two almost 
  indistinguishable adults may be quite different in appearance. That of dactylina 
  feeds in August on alder, and overwinters as a pupa in a parchment-like cocoon, 
  which is too tough to be torn apart by hand. The moths appear the following 
  June. The genus in the past has been called Apatele or Apatela.