Hornworm Instars and Size Stages, Explained

What Are Instars?

Hornworms are caterpillars, and like all insects with complete metamorphosis (caterpillar → pupa → adult), they grow through distinct developmental stages called instars. Each instar represents the period between molts. A hornworm hatches, grows, molts (sheds its skin), and then enters the next instar at a slightly larger size. It repeats this cycle several times before reaching full maturity and entering the pre-pupal stage.

Unlike some other feeders (discoid roaches, for example, which have many instars over months or years), hornworms progress through their instars relatively quickly. A hornworm from egg to full size takes roughly 4-6 weeks under optimal conditions, with about 5-6 instars total. This fast development is both an advantage (size progression happens quickly) and a limitation (your storage window is short—1-2 weeks for a cup of feeder-sized hornworms).

Instar One Through Three: Early Development

First, second, and third instars (the earliest stages) are tiny—barely visible to the naked eye or perhaps only a few millimeters long. These early-stage hornworms are so small that they're impractical for most reptile feeding. A juvenile bearded dragon or similar mid-sized reptile would struggle to even reliably locate such tiny prey, and the nutritional value is minimal.

Early instars are rarely available in the hobby because they're difficult to raise in quantity and not useful for most keepers. Breeders and suppliers typically skip past these stages and begin selling hornworms once they reach late third or early fourth instar, when they become visible and substantial enough to be worth handling and shipping.

Instar Four and Five: Feeder Stage

Fourth and fifth instars are the sweet spot for most feeder situations. A fourth-instar hornworm is roughly the size of a small caterpillar, substantial enough to feed a juvenile to small-adult reptile comfortably. Fifth instar is larger, appropriate for medium to large adult reptiles. These are the stages you'll typically receive when you order a cup of hornworms from a supplier.

The hornworms you get in a standard feeder cup are usually fourth to early fifth instar, or a mix of both. They're in the optimal size range for feeding most reptile species. This is the "feeder size" that AAC ships and that most keepers use as their primary offering. A single fifth-instar hornworm is substantial—it's a real meal for a medium-sized reptile, with genuine nutritional weight.

During these instars, hornworms are actively eating the formulated chow they're raised on, growing visibly day by day. If you keep a cup of fourth/fifth instar hornworms in warm conditions, you may notice them grow noticeably larger over 1-2 weeks. This growth is normal and, depending on your reptile, might actually be useful: a small reptile might eat a fourth-instar hornworm, while a larger reptile would wait until it grows to fifth and eat something more substantial.

Sixth Instar: Late-Stage and Pre-Pupal

The sixth instar is the final caterpillar stage before pupation. A sixth-instar hornworm is the largest it will be as a caterpillar—roughly 3-4 inches long and noticeably thicker than earlier instars. Some hornworms from a feeder cup will reach this stage if stored long enough, especially in warm conditions.

The problem: sixth instars are approaching the pre-pupal phase. As a hornworm prepares to pupate, it stops feeding. Its behavior changes noticeably—restlessness, loss of color intensity, general agitation. A non-feeding caterpillar is worthless as a feeder. It has no chow in its digestive system, and its nutritional value drops significantly. Additionally, once pre-pupal behavior starts, the animal is days away from burrowing and pupating.

If your hornworms reach sixth instar while you're storing them, you'll notice the behavioral change. At that point, they're no longer ideal feeders. Some keepers do offer very large sixth-instar hornworms to very large reptiles (large monitors, for example), but most keepers will find sixth instars are past their usefulness and should either be discarded or allowed to complete their lifecycle separately if you're interested in watching them pupate and emerge as moths.

Size Progression and Storage Temperature

Temperature dramatically affects how quickly hornworms progress through instars. In warm conditions (75-80°F or warmer), hornworms grow and molt rapidly. A cup of fourth-instar hornworms might noticeably progress toward fifth within 5-7 days. In cool conditions (50-55°F), growth slows dramatically. The same cup might take 2-3 weeks to show visible size progression.

This is why storage temperature matters. If you want your hornworms to stay at a consistent size and maintain their feeder window longer, keep them cool. If you want them to grow progressively (which can be useful if you have multiple sizes of reptiles to feed), keep them at room temperature and feed from the cup at different intervals as they size up.

Most keepers keep their cup at room temperature, feed the smaller hornworms first to smaller animals, then feed the larger ones a week or two later to bigger animals. This gives natural size progression without requiring separate containers or active management.

Matching Instar Size to Reptile Size

Fourth-instar hornworms work for juvenile reptiles and some small adult species (like many gecko species). A bearded dragon hatchling or a young anole will handle a fourth-instar comfortably.

Fifth-instar hornworms are ideal for medium to large adult reptiles: adult bearded dragons, monitors, larger iguanas. This is probably the most common size that most keepers use.

A very large sixth-instar is only useful for very large reptiles: large Nile monitors, large iguanas, and similar top-end sizes. Most keepers never deliberately use sixth-instar because their animals are smaller and fifth-instar is already the maximum.

Very small reptiles (hatchling geckos, tiny species, snakes with small mouths) may find even fourth-instar hornworms too large. For those animals, hornworms might not be appropriate at all—smaller feeders are needed.

The Practical Timeline

When you receive a cup of hornworms, they're usually a mix of fourth and early fifth instars. In warm storage, they'll progress over 1-2 weeks: fourth-instars grow into late-fourth or early fifth, and fifth-instars grow noticeably larger toward sixth. As this progression happens, their feeding window stays open for about 1-2 weeks, after which pre-pupal behavior begins and they're no longer ideal feeders.

Plan to use your cup within that window. As you feed, you're naturally selecting smaller animals first (who eat the smaller hornworms) and larger animals later (who eat the larger ones as the cup ages). This natural progression is one of the nice features of hornworm feeding and happens without requiring any special management on your part.