Relative to other caterpillars, slug caterpillars seem more fantasy than reality.
~David Wagner
It is getting to be that time of year – caterpillar time! As summer draws to a creeping close, one of the things that lifts my spirits above the stifling heat waves is the increasing abundance of larval Lepidoptera. And one of our favorite groups, the slug caterpillars, is starting to show up in greater numbers in our woods and yard. Earlier this week, Melissa was out in the garden and harvested some of our collards, since it was obvious they were becoming riddled by insect chewing. When she pulled one leaf she saw two tiny Saddleback caterpillars, Acharia stimulea. The female moth tends to lay clusters of eggs and the young feed gregariously at first. They are extremely variable in their choice of host plants. We have found them on tomatoes, various tree leaves, iris leaves, and now, collards. This may be why so may people recognize this as one of our most common so-called, stinging caterpillars as you can find them almost anywhere. You may accidentally brush up against one while weeding your garden and you won’t soon forget that encounter as they pack a powerful punch resembling the pain associated with a wasp sting. You can read more about them in an earlier blog post here.
Two tiny Saddleback Caterpillars feeding on a collard leaf (click photos to enlarge)
They are already sporting the pattern that gives them their name – the distinctive brown saddle outlined in white in the middle of their back
Ballpoint pen tip for scale
Though these guys are extremely small (the tiniest Saddlebacks either of us has ever seen), I think they have probably molted at least once already. Online descriptions say that the earliest instars lack the prominent tubercles on either end.
After one day, the caterpillars’ colors had already darkened and taken on more of the pattern of later instars
One scientific study I found said it was extremely difficult to accurately determine how many times this species molts during its larval development since the head capsule is hidden beneath the body and they almost always eat their shed skin. It is certainly more than the usual five molts of many butterfly and moth species, and may be as many as eleven or more and may require several months before pupation. Once again, I’m afraid we have taken on more than we bargained for in raising some caterpillars (we still have a few Cecropia larvae that hatched on June 10!). But, Saddlebacks will eat a variety of leaves are are not nearly as voracious in their feeding habits as most other species. I’ll try to keep you posted as they mature.
What they will look like in a month or two
Roads End Naturalist 
I just love all your posts! Thanks so much for the learning opportunities each time. So enjoyable to see your fabulous photos and learn new things!
Thanks, Cathy. I love to learn as well, which is why I keep doing this…learn something new each time I research for a post.
Very cool. I’ve only seen these once at the NCBG when someone got stung. I would love to find some again. Thanks for posting the early instars.
Interesting critter.
Hello, great post! I’m having a bit of an issue finding information about saddlebacks online and i’m wondering if you can help. All summer i’ve been dying to find a saddleback. I’ve only seen them once and they are my favorite caterpillar species. Yesterday after a strenuous hike through lots of bushes, I was in my car taking an important phone call and I happened to look over at the passenger seat.. and there was a saddleback! I must’ve picked him up on my camera’s flash diffuser(I was using it to gently push leaves out of my face.) How’s that for luck? Now the issue is, he hasn’t eaten. Or really moved at all. I put him in my butterfly house which thankfully was empty, and gave him plenty of food options. He doesn’t seem to be on the last instar(he’s smaller than the other one i’ve seen) and i’m hopeful he’s not dead. I may have seen him wiggle but I can’t be sure. He’s sticking to the plant I put him on and not curling, oozing etc. I guess this whole paragraph is just to ask: Have yours gone without eating or moving for over a day? I realized while writing this that he could be molting, but honestly I am very inexperienced with slug cats. I’ve raised plenty of monarchs/swallowtails and they are voracious eaters, so this behavior is very concerning to me. I’m really hoping he is okay but it’s been almost impossible for me to find info on them.
Hi Alicia…my guess is it may be getting ready to molt. Some caterpillars take a day or two to do that and, as you are describing, just sit there the entire time. I had a Hickory-horned Devil once sit for over 48 hours before finally molting. Good luck!