The next slug to make your acquaintance is the Red-crossed Button Slug. This species is quite similar to one or two others as both larvae and adult, but given that most lists I have seen from Missouri list this one and not the others, I am pretty confident in this ID. This species lacks the stinging, protective hairs, going instead with a more camouflage approach of looking like a bit of leaf blight as it passes over leaves of oaks, hickories and quite a few other known woody, deciduous host plants.
The image above gives a glimpse into how the slugs get around – on a substance described as liquid silk. See the winding trails?
Here I caught one in the act. From what I’ve read, slugs leave distinctively shaped (indented) frass that is different from that of other caterpillars. I didn’t pause long enough to investigate.
Finally, the adult slug moth is pictured above. Slug moths are strongly attracted to lights and, from what I have read, are often some of the first species to show up when setting up a light and sheet/trap. I have a theory that this may be why I find many less slug caterpillars the closer I look near St. Louis. Although I found very similar habitats with the same composition and numbers of sapling oaks and hickories (the favored host plants), the closer I came to the city the number of slug caterpillars dropped significantly. Perhaps the city lights are sucking in the adults before they are able to reproduce? Probably a too simplistic idea, but it is a trend I noticed. It could just as well be due to fragmented habitat and overall less habitat available the closer one gets to the city.