What Size Feeder Insect Should I Use? The Eyes-Width Rule
If there's one rule that prevents more feeding emergencies than any other, it's this: never offer a feeder insect wider than the space between your reptile's eyes. It sounds almost too simple, but it's the single most reliable guideline in reptile keeping, and it applies to every feeder you'll ever use — roaches, silkworms, hornworms, BSFL, all of them. Prey that fits the rule passes safely down the esophagus and through the gut. Prey that's too big risks choking, impaction, and stress.
Why the eyes-width rule works
A reptile's skull and throat scale together, and the gap between the eyes is a remarkably good proxy for how wide a prey item the animal can safely swallow and process. It's visible, it's easy to eyeball against a feeder in your hand, and it travels across species — a hatchling and an adult of the same species both get correctly sized food from the same rule, because the measurement grows with the animal. No charts to memorize, just a quick look.
Sizing by species
| Species | Age | Recommended size |
|---|---|---|
| Bearded dragon | Baby | Small nymphs, small silkworms |
| Bearded dragon | Adult | Large roaches, large silkworms, large hornworms |
| Leopard gecko | All ages | Small to medium — not large |
| Chameleon | Adult veiled/panther | Medium roaches, medium silkworms |
| Crested gecko | Adult | Small only — cresties have small mouths |
| Monitor / tegu | Adult | Large roaches, large hornworms, large silkworms |
Two things worth calling out. Leopard geckos are small lizards, and even as adults most should be eating medium feeders, not large — people routinely oversize them. And crested geckos have notably small mouths for their body length, so adults still take small feeders. When the species and the eyes-width rule disagree, trust the eyes.
When in doubt, go smaller
Here's the principle that makes sizing low-stress: a reptile eating several small feeders gets the exact same nutrition as one eating fewer large feeders — minus the choking and impaction risk. There is no nutritional penalty for feeding small. So if you're ever unsure between two sizes, take the smaller one. You can always size up at your next order once you've watched your animal handle the current size comfortably.
That's also why I like feeders that come in clear size grades. Buying discoid roaches in small, medium, and large lets you match the feeder to a growing gecko precisely instead of forcing one size onto an animal it doesn't fit — and discoids stay put in a smooth-walled bin while you sort them, since they can't climb glass.
The eyes-width rule costs you nothing and prevents the scariest feeding mistakes. Make it a reflex: look at the gap between the eyes, look at the bug, and if the bug's wider, don't feed it.
Apply this alongside how to choose the right feeder insect and the amounts in how many silkworms to feed your reptile.
Sources: MSD Veterinary Manual — Nutrition in Reptiles · University of Florida IFAS Extension — Reptiles as Pets