Double-lined Prominent Caterpillar Moth (Lochmaeus bilineata) - Or is it?

I’ve talked about identifying whether a particular insect is a butterfly or a moth, but what if it’s still a caterpillar? Often caterpillars have distinctive colors and patterns as well and can be identified that way. Some, however, like the Variable Oakleaf Caterpillar and the Double-lined Prominent Caterpillar, fall into the variable category, meaning that different individuals of the species may have substantially different coloration.

It’s not a minor variation either. It can be the difference between a caterpillar that is all green with narrow yellow stripes and a caterpillar that is green with a broad irregular reddish brown patch down the length of its back.  If I saw the two extremes of the Variable Oakleaf Caterpillar side by side, I would never have guessed that they were the same species.

That can be tricky if you are trying to identify a caterpillar by searching through photos. If you happen upon a photo of a color variant much different from your specimen, you may rule it out too early. That happened with me on the caterpillar pictured below.

Double-lined Prominent Caterpillar (Moth) - Lochmaeus bilineata (probably)
Photo by Brad Sylvester, copyright 2011, all rights reserved

At first, I thought that it could not be a Lochmaeus manteo (Variable Oakleaf Caterpillar), based on a few images I saw with a pronounced reddish saddle. Neither, however, could I find much else that was similar except the closely related Lochmaeus bilineata (Double-lined Prominent). By the way, Bugguide.net says the two may be indistinguishable in larval form, so technically, my specimen could be either species.  With some further searching and the use of a handy identification tool at Discover Life, I came back again to Lochmaeus bilineata and discovered some images of variants without the red marking on the back. These were nearly identical to my specimen, with the exception of two pairs of little red wart-like structures which should be present, but are not visible in my photos. Despite this, I am as certain as I can be that it is one of the two Lochmaeus species, and most probably Lochmaeus bilineatus or the Double-lined Prominent.

Clues to caterpillar identification

besides the color, pattern, presence of bristles, horns on its head or tail, and other distinguishing physical features, another clue to use to identify caterpillars is the type of food, usually leaves of some sort, which they eat. Some species of caterpillar are very picky about what they eat. In this case, both of the Lochmaeus caterpillars eat oak and other tree leaves. The Double-lined Prominent caterpillar that I found was eating a beech tree leaf, when I encountered it, although there is certainly plenty of oak on the property as well.

Another way to identify a caterpillar’s species is to see what it turns into after it undergoes metamorphosis. This, obviously, takes a little more time, and involves some risk to the caterpillar if you can’t keep it in a proper environment. If you aren’t sure what it eats, you shouldn’t even try to keep it captive until it pupates and forms a chrysalis or a cocoon. Even the chrysalis itself most likely requires the humidity and temperature to stay within a certain range to keep it viable. Ambient outdoor conditions for your region can be a good clue though.

If I had tried to keep the Double-lined Prominent caterpillar until it became a butterfly or moth, in this case, a moth. It would have formed a silken-thread cocoon and emerged as a pretty non-descript gray-patterned moth of small to medium size.

Quick facts about the Double-lined Prominent Caterpillar Moth (Lochmaeus bilineata)

Diet: Tree leaves: beech, oak, birch, elm, basswood, linden

Range: From New Mexico in the Southwest and Saskatchewan in the northwest eastward to the Atlantic Coast.

IUCN Redlist Status: Prevalent, not-listed

When they are here: Adult Double-lined Prominent moths can be found from June through August. I didn’t have a resource with good information on the larval life-cycle, but my BEST GUESS is that they overwinter in caterpillar form and form a cocoon when it gets warm in the spring, perhaps late April or early June based on the presence of the adult moth. The larva is certainly present in early-mid July, when I found this specimen.

Habitat: Deciduous woods

How to tell a moth from a butterfly

Some people just don't like bugs. Even those people, however, often have a favorable view of butterflies. The colorful patterns of their wings make them like little abstract paintings on the wing, a mobile art gallery in the garden, as it were. In my yard, fortunately, there are a number of different butterflies and colorful moths.

The first step to identifying one of these insects, is to determine whether it is a butterfly or a moth. A good close-up view of the antenna can usually put this question to rest. Although there are some exceptions to the rule, the antennae of butterflies are structurally different than those of most moths.

The antenna of a moth, upon close inspection, is feathery in structure with a main "stem" and smaller little "branches" all along its length. The two photos below show the difference. Note that the butterfly antenna as shown in the top photo almost always has a thickening at the end, almost like a club.

Antenna of the Eyed Brown Butterfly with club-like end
Photo by Brad Sylvester, copyright 2011, all rights reserved

Antenna of the Polyphemus moth showing feathery branches
Photo by Brad Sylvester, copyright 2011, all rights reserved

Another way to tell the difference between a moth and a butterfly is the cocoon or chrysalis. Most all moths spin a thread cocoon. This cocoon will usually be on the ground or attached firmly to a solid object such as the underside of the eaves of your house. Butterflies, on the other hand, tend to form a chrysalis which has smooth walls and is usually hanging in the air from a branch. Of course, by the time you see it flying around, it's too late for this test.

Generally. moths are active at night, but are occasionally seen moving about during daylight hours (at least they are in my yard). Butterflies are active during the daytime. This helps, but is really not reliable enough of a clue to make the determination between moth and butterfly on its own.

Another clue is the way the insect holds its wings when at rest. Butterflies wings stay extended and may be upright and together, sticking straight out to the sides or slowly moving back and forth between the two positions. Some of the larger moths do this too, but many moths at rest have swept-back wings that fold back along their body.

Moths and butterflies have two sets of wings, the forewings and the hindwings. To fly effectively, the forewing and the hindwing need to be held together. In many moths, there's a kind of tiny row of hook-like structures on the hindwing that catch and hold bristles from the forewing. In butterflies, however, usually just have a lobe of the hindwing that overlaps the forewing. However, it can be very hard to see this unless you are handling the butterfly or moth which risks damaging them.

Another clue, but not a sure thing, is the color of the wings. Moths tend to be nocturnal and hide during the daytime, so most often, their wings are a drab brown, gray, white, or other color that allows them to more easily blend into their surroundings. Butterflies, on the other hand, tend to be bright and visually distinctive, using their coloration and intricate patterns to self-identify their species to prospective mates. Their wing patterns may also serve to deter potential predators, sometimes exhibiting large false eye spots that make their wings look a bit like a large predatory face.

Butterflies often, but not always, have long slender bodies, while many moths have short, more bulbous bodies. Again, though, this is not always a reliable indicator.

Although each individual clue may be less than 100% reliable, when you start stacking them together and all of them point to either moth or butterfly, then their value increases. The antenna difference is the easiest and most reliable differentiator between a moth and a butterfly, although, as I mentioned there are some exceptions to the rule.

Once you have it narrowed down to moth or butterfly, you at least know where to start looking for a specific species identification