Mealworm Sizes Explained: Micro, Mini, Medium, and Large

Mealworms (Tenebrio molitor) are cheap, easy to store, and slow enough that jumping spiders, tarantulas, and mantids can catch them without a chase. They're a genuinely useful feeder — but they work a little differently for spiders than they do for the reptile-feeding world most mealworm care advice is written for. Here's how the sizes break down, how to store them, and what actually matters when you're feeding inverts specifically.

The Size Breakdown

Micro — the smallest size available, typically under 1/4 inch. Right for spiderlings, tiny tarantula slings, and juvenile mantids that would be overwhelmed by anything bigger.

Mini — roughly 1/4 inch. A step up for slings and juveniles that have outgrown Micro but aren't ready for a full-size worm.

Medium — around 1/2 to 3/4 inch. The general-purpose size — a solid match for sub-adult tarantulas, larger jumping spiders, and adult mantids.

Large — roughly 3/4 inch to a full inch. Best for adult tarantulas and larger mantids that can handle a bigger, more filling feeder.

As a rule of thumb: if your spider is ignoring a mealworm or seems reluctant to engage it, size mismatch is usually the first thing to check.

Storage

Mealworms store better than almost any other live feeder — with one size-specific exception worth calling out first.

Micro mealworms are the exception: do not refrigerate them. At that smallest size, cold storage kills them rather than just slowing them down. Keep Micro mealworms at room temperature and plan to use them within a shorter window than the larger sizes.

Mini, Medium, and Large store well refrigerated, roughly 42–50°F, right in their bedding (wheat bran works as both substrate and food). At that temperature they go dormant and hold in good condition for weeks — some keepers get a month or more out of a batch. Avoid freezing temperatures entirely; that kills the larvae outright rather than just slowing them.

Moisture matters, but not too much. A slice of potato, carrot, or apple on top of the bedding gives them what they need. Refresh or remove it every day or two so it doesn't mold — soggy bedding is the main way a batch goes bad in storage.

Ventilation matters too. Make sure the storage container has small airflow holes — this prevents condensation buildup and keeps the worms from suffocating, on top of helping control mold.

If you're storing longer than two weeks, take them out of the fridge periodically, let them warm and become active for a day, offer fresh moisture, then return them to the cold. A batch that gets an occasional refresh holds up better than one left static and cold indefinitely.

Troubleshooting:

• Pupating too quickly in storage — check that your fridge is actually holding 40–50°F. Pupation kicks in when it's running warmer than that.
• Mold in the bedding — remove the affected area immediately, replace the bedding, and check that ventilation is adequate and you're not over-moistening.

The Spider-Specific Safety Note (This Matters More Than Sizing)

Mealworms don't pose a chitin/digestion risk to spiders — spiders liquefy their food externally with venom and digestive enzymes, so the hard exoskeleton that's a real compaction concern for reptiles just isn't an issue here. The actual risk with spiders is different, and it's the one thing worth getting right:

Never leave a live, uneaten mealworm in an enclosure indefinitely. Left alone, mealworms will pupate and eventually mature into a beetle — and an adult mealworm beetle is hard-shelled, defensive, and strong enough to injure or kill a jumping spider or a molting/vulnerable tarantula. This is the single most important handling rule with this feeder:

• Offer, then check back. If your spider hasn't taken the mealworm within a reasonable window (a few hours to overnight, depending on species), remove it rather than leaving it to wander the enclosure indefinitely.
• For slings and small spiderlings, pre-kill the mealworm or offer just a piece (many keepers offer the head end) rather than a live whole worm — this also makes it easier for a small spider to actually process a bite that's proportionate to its size.
• Never feed adult mealworm beetles. If a worm you meant to feed off has already pupated and matured, don't offer the beetle — same defensive-chemical and hard-shell issue, worse.

Feeding: A Solid Regular Feeder, Not Just a Treat

Mealworms are actually moderate in fat — roughly 13%, well below genuinely high-fat feeders like waxworms (~20%) or butterworms (~29%). That's a common mix-up: butterworms and waxworms are the ones that should be treat-only due to fat content; mealworms don't carry the same caution. They're a reasonable regular feeder, not something you need to ration for weight-gain reasons.

The real case for rotating in other feeders alongside mealworms isn't fat — it's the usual nutritional-variety argument: no single feeder insect covers every nutrient an animal needs, so a mixed rotation (mealworms, fruit flies, roaches, etc.) beats leaning on one species exclusively.

Gut loading still helps. A few hours with fresh vegetables before feeding improves what your animal gets out of the meal, same principle as any other live feeder. Calcium/vitamin dusting, by contrast, is mainly a reptile and dart-frog practice — most spider keepers skip it, since it's not part of standard invert feeding routines.

Choosing a Size as Your Animal Grows

Because mealworms store so well in bulk, a lot of keepers stock two adjacent sizes at once — the current size their animal takes, plus the next size up — so there's no gap when it's time to size up. Worth planning for ahead of time if you're feeding something still growing.