How Many Fruit Flies to Feed: Portion Sizes for Dart Frogs, Geckos & Spiderlings

I've fed flightless fruit flies for years as the backbone diet for the smallest animals in my collection — dart frogs, mourning geckos, juvenile mantises, and the occasional fresh spiderling that's too small for anything else. The question that trips people up isn't whether to use fruit flies. It's how many. Too few and a fast-growing froglet stalls out; too many and you've got loose flies wandering the glass, fouling the enclosure, and stressing an animal that feels swarmed at mealtime.

This guide is about getting the portion right. I'll give you concrete numbers — flies per animal, per feeding, per week — for the species people actually keep, plus how to dust them so those flies are worth eating, how to read your animal instead of a chart, and how to keep a culture producing so you're never caught empty. Where I give a number, treat it as a starting point you calibrate against the animal in front of you, not a law.

Why fruit flies are the right tool for tiny mouths

Fruit flies earn their place for one simple reason: size. A lot of the most rewarding small exotics — poison dart frogs, mourning geckos, micro chameleons, baby mantids, fresh tarantula slings — have mouths too small to take a cricket or a mealworm safely. Feed prey that's too big and you get refusals at best and impaction or a choking risk at worst. Flightless fruit flies sit right in the size window these animals evolved to hunt: small, soft-bodied, easy to swallow, easy to digest.

The second reason is movement. Most of these animals are visual hunters that key on motion. A flightless fly that crawls and hops around the enclosure triggers the strike instinct in a way that a dead or static feeder never does. That live-prey response isn't just about getting food in — it's enrichment. An animal that hunts is an animal using its body and brain the way it's built to.

The third reason is practical: you can produce them at home, cheaply, in volume, on your countertop. A couple of healthy cultures can feed a small collection indefinitely without a weekly drive to a shop. And because the feeder strains are flightless (wings reduced or a flight mutation), you're not releasing a cloud of flying insects into your house every time you open a culture.

What fruit flies are not is a magic complete food. They're a delivery vehicle. The real nutrition — the calcium and vitamins that keep small animals from developing metabolic bone disease — gets added by you, at the moment of feeding, with supplement powder. Hold that thought, because it's the single most important thing in this whole guide.

Melanogaster vs. hydei: pick the right fly first

Before you can get the quantity right, you have to be feeding the right fly. There are two feeder species in wide use, and they are not interchangeable.

Drosophila melanogaster is the small one — roughly 2mm. It breeds fast, booms quickly after you start a culture, and is the correct fly for the very smallest mouths: dart froglets, mourning gecko hatchlings, tiny spiderlings, and the first instars of small mantids. The trade-off is that it's so small that a larger animal needs a lot of them to get a meal, and cultures crash sooner.

Drosophila hydei is the larger one — roughly 3mm, noticeably bigger in the enclosure. It takes longer to get going and produces fewer flies per culture, but each fly is a more substantial mouthful. This is the fly for adult dart frogs, adult mourning geckos, bigger mantids, and small day geckos that would otherwise need to chase down 50 of the small fly to feel fed.

Here's how I think about choosing between them:

A practical move: keep both going if you keep animals of different sizes. I run melanogaster for the babies and hydei for the adults, and I always have a hydei culture as a "bigger meal" option when an adult looks like it wants more substance than the small fly delivers. Whatever you do, source clean cultures — flies free of mites and mold — from a supplier who actually maintains their strains. You can find live flightless cultures of both species, along with the other micro-feeders I mention below, at All Angles Creatures' live feeder collection.

The nutrition reality: the dust is the meal

Let me be blunt about this, because the source material I built this from was vague and the internet is full of feel-good nutrition talk. A fruit fly is mostly protein and water with some fat, B-vitamins, and a poor calcium-to-phosphorus ratio. Like nearly every feeder insect (crickets, roaches, mealworms — black soldier fly larvae are the rare exception), fruit flies are phosphorus-heavy and calcium-poor. Fed naked, day after day, they will set a young animal up for metabolic bone disease: soft bones, deformities, in frogs the classic "shaky" or spindly-leg syndrome.

You fix that two ways, and only one of them really works for flies.

Gut-loading — feeding the feeder something nutritious before it becomes a meal — does a lot for crickets and roaches. For fruit flies it does very little, because the fly is so small there's almost no gut to load, and they're sitting in a carbohydrate-and-yeast medium, not a calcium-rich one. Don't rely on gut-loading flies to deliver minerals. It isn't the lever.

Dusting is the lever. You coat the flies in a fine supplement powder immediately before feeding so the animal eats the powder along with the fly. This is where the calcium, the vitamin D3, and the multivitamin actually come from. For animals living on a fruit-fly staple, your dusting schedule is their nutritional plan. Get it right and flies are a perfectly good staple; skip it and no quantity of flies will keep the animal healthy.

A dusting schedule that works

There's no single universal schedule — UVB exposure, species, and life stage all shift it — but here's the rotation I use as a sensible default for small insectivores kept without strong UVB, and it lines up with how most dart frog and mourning gecko keepers run things:

• ◆Most feedings: a plain, phosphorus-free calcium powder (no D3). Light coat.
• ◆Once or twice a week: a calcium with D3 powder instead of the plain calcium.
• ◆Once a week (roughly): a reptile/amphibian multivitamin in place of that feeding's calcium.

Babies and fast growers lean heavier on calcium; established adults can ride a slightly lighter schedule. If your animal gets meaningful UVB, you cut back on the D3 because they're making their own. The cardinal sin is over-supplementing fat-soluble vitamins (A and D in particular) — more is not better, so don't dust everything with the multivitamin "to be safe." Rotate, keep coats light, and when in doubt about a specific species, check the dosing logic in a veterinary reference like the Merck Veterinary Manual's section on amphibian nutrition.

How to actually dust a fly that won't hold still

Flightless doesn't mean motionless. Here's the method that's never failed me:

1. ◆Tap a measured portion of flies out of the culture into a small deli cup, feeding cup, or zip bag. (Knock the flies down by tapping the culture on the table first; they fall off the lid.)
2. ◆Add a small pinch of supplement — you want a light dusting, not a snowball.
3. ◆Gently swirl or roll the cup/bag so the powder coats the flies. Two or three seconds. Don't shake them to death.
4. ◆Tip the dusted flies straight into the enclosure, near where your animal hunts.

Work quickly and keep a lid or hand ready — flies crawl up and out of an open cup fast, and some strains climb smooth plastic. A cup chilled briefly in the fridge (a couple of minutes, not frozen) slows them down enough to dust calmly, and they warm up and get lively again inside the enclosure, which is exactly what you want for triggering a feeding response.

How many flies per animal: the portion charts

This is the part everyone actually wants. I'll give numbers by animal type, then teach you to read the animal so you can stop counting eventually. Remember: these are dusted flies per feeding unless noted, and they're starting points.

Dart frogs (Dendrobates, Phyllobates, Ranitomeya, etc.)

Dart frogs are the classic fruit-fly animal, and they essentially live on dusted flies their whole lives.

• ◆Froglets (just out of the water, tiny): melanogaster only. Feed a small but generous pinch every single day — enough that they're hunting actively but not so many that flies pile up. Roughly 10-20 flies per froglet daily, scaled to how many froglets share the space. Daily feeding at this stage is non-negotiable for growth.
• ◆Juveniles to subadults: still melanogaster (or small hydei as they grow), daily or near-daily. Working up toward 20-30 flies.
• ◆Adults (thumbnail to standard size): roughly 20-40 flies per frog per feeding, fed daily or every other day. Larger Dendrobates (tinctorius, auratus) take the higher end and handle hydei well; tiny Ranitomeya stay on melanogaster and take fewer.

A real-world calibration that matches my own experience: one tinctorius breeder I trust dialed in at about 30-40 flies consumed per adult frog, fed in controlled daily amounts after starting out way too heavy and fouling the viv with uneaten flies. That overshoot is the universal beginner mistake.

Mourning geckos and small day geckos

Mourning geckos (Lepidodactylus lugubris) are tiny, parthenogenetic, and thrive on a mix of dusted flies and a powdered fruit/nectar diet (like a crested gecko diet).

• ◆Hatchlings: melanogaster, small daily offerings of 5-10 flies, alongside the powdered diet.
• ◆Adults: roughly 10-20 flies per gecko, 3-4 times a week, plus the prepared diet 2-3 times a week. Hydei works for adults. Because they also eat the fruit mix, flies aren't their only food, so you don't need to push fly volume the way you do with frogs.
• ◆Small day geckos (e.g., dwarf Phelsuma): similar logic — flies plus prepared diet; scale fly count up with body size and offer hydei to adults.

Mantises (juvenile instars)

Young mantids take fruit flies until they're big enough for houseflies and small crickets.

• ◆L1-L2 (newly hatched nymphs): melanogaster, and offer 2-4 flies per nymph at a time, keeping a few available so the nymph can hunt at its own pace. Mantids hunt one prey item at a time, so you don't dump a swarm — you keep a modest standing supply in the enclosure.
• ◆L3-L4: can move to hydei; a few flies available daily.
• ◆Feed nymphs roughly daily; back off if the abdomen is plump and they're refusing.

Spiderlings (tarantula and true-spider slings)

Fresh slings too small for a pinhead cricket take a fruit fly beautifully.

• ◆Offer 1-2 flies per sling at a time. Slings eat one item, then need to digest. Feed every 3-5 days for most slings — they're far slower-metabolizing than frogs.
• ◆Use melanogaster for the smallest slings; pre-kill or crush a fly if the sling is reluctant to chase. Always remove uneaten flies from a sling's enclosure — loose feeders stress a molting spider, and a fly can even nibble a freshly molted, soft sling.

Other animals people ask about

• ◆Newts/efts and tiny amphibian juveniles: small portions similar to froglets; many prefer prey on land or at the waterline. Don't foul the water with excess.
• ◆Insectivorous nano-fish (bettas, killifish, guppies): a few floating flies as an occasional treat, not a staple — feed only what's eaten off the surface in a couple of minutes and skim the rest, because drowned flies wreck water quality fast.

Here's the consolidated view:

Reading the animal instead of the chart

Counting flies is training wheels. The goal is to read your animal and adjust, because appetite shifts with temperature, season, breeding condition, and growth stage. Here's what I actually watch.

Signs you're feeding too much

• ◆Flies wandering the glass an hour later. The clearest sign. If there's a standing population of loose flies in the viv well after feeding, you offered more than the animal wanted. Cut the next portion.
• ◆Weight gain / a fat animal. A dart frog that's gone from athletic to balloon-shaped, a gecko with fat deposits, an obese mantis with a distended abdomen between molts.
• ◆Lethargy. An overfed animal often just sits. Strike response goes dull.
• ◆Fouling and pests. Excess flies die in the substrate, feed mold, and can seed fungus gnats or grain mites. A viv that suddenly has a hygiene problem is often an overfeeding problem.

Signs you're underfeeding

• ◆Constant hunting / pacing. An animal patrolling the glass near feeding times, striking at anything, is telling you it's hungry.
• ◆Weight loss. Visible thinning, sunken look, hip bones or ribs showing on animals that shouldn't show them.
• ◆Stalled growth. A froglet or nymph that isn't gaining size on schedule is under-fed (or under-supplemented, or too cold).
• ◆Dull condition. Poor color, low energy, weak strikes.

The "fed within X minutes" rule

A good practical test for many of these animals, borrowed from experienced keepers: offer enough that the animal feeds actively, then check whether the flies are essentially cleaned up within 10-15 minutes of hunting. If they vanish fast and the animal still wants more, bump the next portion slightly. If lots are still crawling around well after, you overshot. For frogs and geckos that graze, "cleaned up by the next feeding" is the looser version of the same idea. For mantids and slings, the rule flips — you want a small standing supply they pick off slowly, and you remove leftovers before they accumulate.

Weigh growers periodically if you're serious. A cheap digital scale and a logged weight every week or two turns "I think it looks okay" into actual data, and it catches both obesity and decline before they're a crisis.

Feeding frequency by life stage

Quantity and frequency work together. The single most reliable rule across all these animals: the younger and faster-growing the animal, the more often it eats.

• ◆Babies (froglets, hatchling geckos, early mantis instars, fresh slings): daily for frogs/geckos/mantids; slings every few days because of their slow metabolism. Growth happens now and you can't get it back later — do not skip days on a growing froglet.
• ◆Juveniles/subadults: daily to every other day. Still growing, still hungry, but you can start watching condition and easing off if they're getting chunky.
• ◆Adults: maintenance, not growth. Many do well on 4-5 feedings a week rather than daily. Breeding females need more; cooled or brumating animals need far less. An overfed adult is a more common problem than an underfed one.

Consistency beats volume. An animal fed a steady, appropriate amount on a predictable rhythm does better than one that's binged then starved. I feed at roughly the same time of day so the animals anticipate it, and so I notice immediately when one doesn't show up to hunt — that's often the first sign something's wrong.

Delivery: getting the flies where they'll be eaten

How you put the flies in matters as much as how many.

Frogs and small geckos (planted vivariums)

Dust, then tip the flies into the enclosure near a feeding station — many keepers use a low dish, a piece of cork bark, or a leaf where flies congregate and the animal learns to patrol. A small smear of fruit or a fruit-fly feeding station keeps flies in one area instead of scattering into the substrate. In a humid, heavily planted viv, the flies will climb to the canopy; that's fine — frogs and geckos hunt them there. Just avoid drowning them in standing water features.

Mantids and spiders

Free-range delivery: release the dusted flies (or place a couple of pre-killed ones) into the enclosure and let the animal hunt. Keep the standing number low and appropriate to one or two meals, and remove uneaten flies before they stress the animal — especially around a molt, when a mantis or spider is defenseless and a loose fly is a liability.

Aquatic and surface feeders

For anything taking flies off a water surface, float only what gets eaten in a couple of minutes and skim the rest immediately. Drowned flies decompose fast and tank a small water volume's quality. This is a treat method, not a staple-delivery method.

Maintaining cultures so you never run dry

You can have the perfect portion plan and still fail if your flies crash. Culturing is its own craft, but here's what keeps me supplied.

The basics of a culture

• ◆Medium: a commercial fruit-fly media is easiest and most consistent. The DIY version is a cooked mix of something starchy (instant mashed potato works), water, a sugar, a pinch of active yeast, and a mold inhibitor. Yeast drives the boom.
• ◆Container: a clear 32oz-ish cup or jar with a breathable lid (fine mesh or a fabric/foam plug) — ventilation without escapes.
• ◆Climbing surface: crumpled coffee filter, excelsior (wood wool), or plastic mesh wedged in vertically. Flies pupate on it and adults rest there; it dramatically increases your usable population and gives you a clean surface to tap from.

Conditions

• ◆Temperature: 70-78°F is the sweet spot. Warm (low 80s) speeds production but burns the culture out faster and invites mites and runaway mold; cool (mid-60s) slows everything to a crawl. Keep cultures out of direct sun and away from drafts and temperature swings.
• ◆Humidity: the medium should stay moist, not soupy and not dried to a crust. Too wet drowns larvae and grows bad mold; too dry stalls the boom.

The rhythm that prevents gaps

Cultures boom and bust. A culture peaks, pumps out flies for a couple of weeks, then production falls off as the medium is exhausted and the adults age out. If you wait until a culture dies to start the next one, you'll have a gap with no flies — and a hungry collection.

Stagger your cultures. Start a fresh one every 1-2 weeks so you always have a young culture coming online as an old one fades. For a small collection, two or three cultures at staggered ages is plenty; scale up from there. I start new cultures on a calendar, not "when I remember," because the one time you forget is the week everything is hungriest.

Keeping cultures clean

• ◆Mites are the great enemy — they crash cultures and ride into your enclosures. Keep cultures on mite paper or in a quarantine setup if you've had problems, never set a new culture on a surface where an infested one sat, and toss anything that looks dusty/grainy with movement that isn't flies.
• ◆Mold beyond the expected light surface growth means too wet, too warm, or contaminated — restart it.
• ◆Quarantine new cultures before parking them next to your established stock, and wash your hands between handling different cultures. Source from a supplier who keeps clean strains so you're not importing problems on day one.

A clean, well-fed culture also makes better feeders — flies raised on good media are more nutritious vehicles for your dust than starved, stressed ones from a dying culture.

Rounding out the diet beyond flies

Even for animals that staple on fruit flies, variety helps close nutritional gaps and keeps interest up.

• ◆Springtails are the perfect co-feeder for dart frogs and froglets — tiny, self-sustaining in the viv, and a constant micro-snack between fly feedings. They also clean up, doubling as part of your bioactive cleanup crew.
• ◆Isopods (dwarf species) similarly serve double duty as cleanup crew and occasional prey for the animals big enough to take them.
• ◆Pinhead crickets and other small feeders come into play as animals outgrow flies — a juvenile gecko or larger frog appreciates the variety, and bigger prey delivers more per bite.
• ◆Powdered fruit/nectar diets are a real staple (not a supplement) for mourning and day geckos — flies are the protein-and-movement half of their diet.

The principle is the same as portioning: rotate, observe, and adjust. No single feeder is complete, but a fruit-fly staple plus smart dusting plus a little variety is a proven, healthy foundation for the smallest animals in the hobby. If you keep roaches as part of a broader feeder rotation for your larger animals, the same dust-and-portion discipline applies — see my guide on keeping discoid roaches alive for that side of the feeder room.

The most common mistakes, and how to skip them

After years of this, the failures cluster into a short list. Avoid these and you're most of the way there.

1. ◆Feeding naked flies. The number one killer of small insectivores isn't quantity — it's skipping the calcium and D3. The dust is the meal. Build the dusting schedule before you worry about counting.
2. ◆Overfeeding and fouling the viv. Beginners dump flies. Loose flies stress the animal, foul the substrate, and breed pests. Start conservative, watch what's eaten, and scale up only if the animal asks.
3. ◆Wrong fly for the mouth. Hydei to a fresh froglet is a refusal or a struggle; a swarm of melanogaster to a hungry adult is a lot of effort for a small meal. Match fly size to animal size.
4. ◆Letting cultures crash. No staggering, no backup, then a week with no food. Start fresh cultures on a schedule.
5. ◆Inconsistent feeding. Bingeing then starving stresses high-metabolism animals. Pick a rhythm and hold it.
6. ◆Over-supplementing vitamins. More multivitamin is not safer — fat-soluble vitamin excess is its own disease. Light coats, sensible rotation.
7. ◆Leaving feeders in with molting animals. A loose fly around a soft, freshly molted mantis or sling is a genuine hazard. Remove leftovers.

Putting it all together

Getting fruit-fly feeding right comes down to four moving parts, and they reinforce each other:

• ◆Right fly: melanogaster for the tiniest mouths, hydei for the bigger ones.
• ◆Right amount: start from the per-animal numbers above, then read the animal — clean-up speed, body condition, hunting behavior — and adjust.
• ◆Right schedule: daily for growers, easing toward maintenance for adults, on a consistent rhythm.
• ◆Right supplementation: light calcium most feedings, D3 and multivitamin on rotation, because with flies the dust is the nutrition.

Do those four things, keep your cultures clean and staggered, and feeding the smallest animals in your collection stops being fiddly and becomes routine. The animals tell you when you've got it dialed in: steady growth in the young ones, solid condition in the adults, active hunting at feeding time, and a clean enclosure with no loose flies wandering an hour later. That's the target — and it's very reachable once you stop guessing and start reading the animal in front of you.

For more on building out your feeder room and the animals these flies support, see my guide to keeping discoid roaches alive or browse the full exotic animals hub.