Catch a Glimpse of Caterpillars During National Moth Week

By Yahel Ben-Zvi

Lepidoptera (butterflies and moths) are among the most popular and widely recognizable insect groups in the world, with showy butterflies incorporated into art and society for hundreds of years. Unfortunately, this love of butterflies often overshadows the beauty and allure of many moths.

That’s one reason why National Moth Week was founded 10 years ago. The nonprofit, grassroots organization showcases and celebrates the elegance of moths and their life cycles and habitats through mothing events and is now a global citizen science project that partners with iNaturalist to document the moths of the world.

National Moth Week 2022 takes place July 23-31, and “moth-ers” of any age are encouraged to learn about and document moths and participate in mothing events. These events usually involve setting up black lights (or other types of traps) to attract moths and to document and appreciate them. Anyone can sign up. Currently, over 630 events are scheduled in over 50 countries across the world for National Moth Week 2022!

This year, National Moth Week will shine a spotlight on caterpillars. So, what do caterpillars look like, and where do they live?

Caterpillar Shapes, Sizes, and Colors

Perhaps the easiest way to differentiate butterfly and moth caterpillars is the lack or presence of silk; moths often utilize silk throughout their larval stages and eventually use it to make a cocoon, whereas butterflies usually make a hard chrysalis out of a protein. Meanwhile, butterfly caterpillars tend to be smooth, while moth caterpillars can be either smooth or fuzzy.

Other prominent features in caterpillars can be the hairs, horns, and prolegs. The hairs on a caterpillar can come in the form of setae or spines (also called scoli when branched) and on occasion can cause irritation when they contact skin or sensitive areas. Prolegs, or little outpockets of flesh along the abdomen that resemble legs, are another point of identification. Prolegs are unjointed, with little musculature, and thus are normally moved by the blood pressure in the insect’s body rather than with muscles.

Many non-lepidopteran insect larvae out there may look similar to caterpillars, particularly sawfly larvae. A good method of differentiation between sawfly larvae and moth or butterfly caterpillars is that sawfly larvae have six or more pairs of prolegs while caterpillars have five pairs or less. That also means that different types of caterpillars can have different numbers of prolegs.

Last, caterpillars come in all sorts of colors. Some caterpillars are one solid color, some have just a different colored head capsule, some have different colored scoli, and some simply come in a plethora of colors. Even within one species, caterpillars can have different colors based on the larval instar or life stage.

Here are some easy identifications of moth caterpillars by family based on physical features:

Saturniidae. Saturniid moths are amongst the largest in the world, including imperial moths, emperor moths, giant silk moths, and luna moths, and, as can be expected, they have some large caterpillars, too. These caterpillars need to eat a lot, because as adults they lack a digestive tract or mouthparts. While not always the case, many saturniid caterpillars are known for being hairy and having lots of colorful scoli present that can be urticating (i.e., stinging) and cause irritation. Not all hairy caterpillars are Saturniidae, nor do they all have stinging hairs, but it is wise to be careful in handling any hairy caterpillar in case it does sting.

Sphingidae. Sphinx moths and hawk moths are in the family Sphingidae, but their caterpillars are often referred to as hornworms. Unsurprisingly, hornworms were given that name due to a very large spine protrusion coming from their rear that looks like a horn. These horns are not venomous but rather are used to scare predators. Many of these hornworms grow up to be important and specialist pollinators for several nocturnal flowers around the world.

Geometridae. Geometridae (also known as inchworms, spanworms, and loopers) are a family in which the caterpillars are known for their prolegs, or rather lack thereof. Compared to many caterpillars, which have five pairs of prolegs, geometrids only have two or three pairs. Geometridae is a very large family that has species all over the world, a few of which are notorious pests while others are very appreciated for their usefulness in making measurements as inchworms!

Caterpillar Habitats

Other than shapes, sizes, and colors, caterpillars can also live in a variety of habitats. Caterpillars have mandibles that they use to chew their food, and the vast majority are herbivores. As such, finding and identifying caterpillars is frequently done according to the host plant on which it is found. Depending on the species, caterpillars can either be generalists or specialists, and they can create optimal conditions for themselves to minimize competition or to augment defenses. These conditions can include exploiting the plant or surrounding materials and also gregarious living—i.e., gathering in large numbers.

Caterpillars may live socially for a variety of reasons. For instance, by living together, caterpillars can overwhelm their host plant defenses to feed and forage better. They can also leave pheromones to lead each other to more food sources. Some caterpillars make a shelter together that can help regulate their temperatures, which otherwise would not be as achievable on an individual basis. Moreover, gregariousness can be a passive defense against predators and parasitoids, using the safety-in-numbers approach.

Caterpillars can also differ in seasonality. At different times of the year, different caterpillars may be found, and often that is related to the phenology and timeline of host plants. On top of that, different species can have different voltinity, or generations per year. In some cases, members of the same species can have different voltinities depending on the location in which they are found. This is important to consider when going out to look for caterpillars.

All in all, a caterpillar’s habitat and lifestyle can be a useful indicating feature in identification. Below are some more easy identifications of caterpillar families, according to their habitats:

Tortricidae. To look for caterpillars of Tortricidae, colloquially called leaf rollers, requires some physical work. This is because, as the common name suggests, the larvae roll up leaves to form shelters. From far away it may seem like a couple leaves are at an awkward angle, but up close the leaf roller’s shelter is more evident, showing leaves that are stuck to each other or folded in on themselves to provide a home and food for the larvae. Unfortunately, a lot of torticids are very serious pests to agricultural enterprises because these caterpillars can also bore into fruits, stems, and roots. The proverbial worm in the apple is a tortricid!

Psychidae. Psychidae is a relatively small family of moths, also known as bagworms, found across the world. Bagworms are usually inconspicuous and lead an interesting life. From hatching, the bagworms start sticking nearby materials (leaves and sticks and such, but sometimes other objects too) onto themselves using silk. As they grow, they continue to build up their so-called “bags.” These caterpillars are often identifiable by the materials stuck to their backs. Psychids that emerge from cases are normally only males who go on to mate and die.

Lasiocampidae. Most tent caterpillars belong to a couple genera in the family Lasiocampidae. These caterpillars are social and, not surprisingly, build silken nests that look like tents. These tents can be easy to spot but can be high up in trees. Tent caterpillars start building their tents soon after they hatch and are in tune with their host plant’s phenology, syncing their stages of life with the tree. Tent caterpillars are known to use their social tents to help regulate their body temperatures.

Happy Moth-ing!

I hope this short guide sparks your interest in moths, and my fellow moth-ers look forward to seeing you at a National Moth Week event near you.

Yahel Ben-Zvi is a student in the Department of Entomology at Rutgers University and U.S. student coordinator for National Moth Week. Email: y.bz@rutgers.edu.