Although we certainly could use more rain around here, we’re far wetter than last year. This increased rain has been reflected in the plants and the animals that eat them, including caterpillars.
Caterpillars are the young, or larval, forms of Lepidoptera (this is the order of insects that contains butterflies, skippers and moths). This group of insects has a “complete” or holometabolous life cycle. Here, the adult lays an egg, which hatches into a larval insect. The larval insect develops through several molts and eventually goes into a pupal stage. In a butterfly or moth, we call it a chrysalis or cocoon.
Inside the pupa, the insect undergoes metamorphosis into the adult form. That is how we get from the “worm-like” caterpillar to the flight of an adult butterfly! This also means that if you find a small butterfly of a species that is usually larger, it’s not a baby. It’s just a small adult. Perhaps the caterpillar didn’t eat as much food as some of its cousins.
The other form of insect metamorphosis is called “incomplete” or hemimetabolous. There, the insect just keeps molting until it reaches the adult, winged form (not all adults will have wings, but anything with fully developed wings is a mature adult). Insects with that type of metamorphosis include grasshoppers (you can see tiny grasshoppers with little stubs of wing pads), true bugs (this includes critters like stink bugs, squash bugs and cicadas) and dragonflies and damselflies (the young forms live underwater and crawl out into the air for the final molt into the flighted adults).
So we have all these caterpillars. What do they eat? It depends. Some are very picky and will only eat plants of one species (and most are going to eat plants). Hackberry trees seem a very popular host plant this year. They are covered with caterpillars ranging from the specialized Hackberry Emperor to a species like the Question Mark that eats several tree species (hackberries and elms mainly).
Other species will eat a wide range of foods, such as the Salt Marsh Caterpillar (Estigmene acrea). The Salt Marsh Caterpillar, a member of the tiger moth family (Arctiidae), is common here despite our distinct lack of salt marshes.
There are a few caterpillars with very unique diets: some moths in the Arctiidae (tiger moths) eat lichens. We’re still looking for those, although I’ve seen the black-and-yellow and black-and-red adults of several species of lichen moths. The young of the Harvester butterfly (found east of here) eat larval aphids. They are the only exclusively predatory caterpillar in North America (some other species will cannibalize their relatives).
Many caterpillars are “hairy” like the Salt Marsh Caterpillar. In the same group are the famous wooly bear caterpillars. On these, the hairs, which are called setae (plural) or seta (singular) in insects, are harmless. Other hairy caterpillars such as the “asp” (which develops into a moth in the family Megalopygidae, the flannel moths) have terrible stings hidden under their hairs. So, identify your fuzzy friend before touching it!
Other caterpillars have more amusing defenses that are incapable of harming humans. Some jump off their host plant as you approach. At first it seems like you are just disturbing the bushes or trees, but then you realize that many have a good grip and manage to cling even on our windy Texas days.
Swallowtail butterfly caterpillars (Papilionidae) have a unique organ near their heads called an osmeterium. They evert the osmeterium, releasing a foul-smelling chemical to frighten off potential predators. The accompanying photo shows this organ on a Black Swallowtail. It is the orange fork at the caterpillar’s head.
Several of the swallowtails also have large eyespots near the head in hopes of fooling predators into thinking they are small snakes. Other caterpillars will shake their heads violently from side to side when disturbed.
It’s a jungle out there for these cats, so they need all the defenses they can get. Hungry birds, from cuckoos to chickadees, have been having a field day this year feeding their young big caterpillars. I watched a titmouse earlier this spring eating a big hornworm (similar to the tomato hornworm but likely a different species; other members of the family Sphingidae feed on plants ranging from trees to vines to forbs).
The bird must have really needed energy after caring for the brood of five fledglings following her and the male around, begging constantly, as she spent a good five minutes thrashing the hornworm on a branch and pulling out bits of it to eat. Once it was down to the manageable size of about a half-an-inch long, she fed the remainder to the nearest fledgling.
There are now several great resources available for identifying caterpillars. A good starting place with a few species is the “Peterson First Guide to Caterpillars of North America” by Amy Bartlett Wright and Roger Tory Peterson. It has nice illustrations of both the caterpillars and of the adult Lepidoptera that it will become. It covers many common species.
A newer book with a lot more variety is David L. Wagner’s “Caterpillars of Eastern North America” (it is also the source of much of my information on caterpillar species and their host plants). He also has a new book out more detail on Owlet Moth Caterpillars of Eastern North America. The owlet moths (Noctuidae) are one of the more diverse families of moths, so it covers an additional range of caterpillars for identification.
Keep your eyes peeled this summer as you explore the fields and woods. You never know what amazing caterpillars you may find!
The next monthly field trips on the Lyndon B. Johnson National Grasslands will be July 11 (moved from the first Wednesday due to the Fourth of July holiday) and Aug. 1. We will depart at 9 a.m. from the Forest Service District Office in Decatur. For information, contact Mary Curry (see below) or the Forest Service District Office, (940) 627-5475.
Claire and Mary Curry are nature enthusiasts based in Greenwood. If you would like to contact them, email them at email@example.com.