Blue Bottle Fly Spikes: Storage, Hatching, and Feeding Guide

Blue bottle flies (Calliphora sp.) are a step up from house flies in both size and activity level — a noticeably bigger, more active fly that's good for larger tarantulas and mantids, and for snapping picky eaters out of a feeding slump. They're sold as "spikes" (the larval/maggot stage, not pupae), which gives you a bit more flexibility than a pure pupae product: you can hatch them out into flies, or in some cases feed the spikes themselves. Here's how to handle them either way.

What "Spikes" Means

Spikes are the larval stage — small, cream-colored, roughly 3/8 to 1/2 inch — before they pupate into the hard-shelled chrysalis stage and eventually emerge as adult flies. Because of this, spikes can arrive at slightly different points in their development depending on shipping temperature: sometimes still active larvae, sometimes already pupated. Either is normal and both are viable.

Only use spikes that are dry and cream-colored. If any have turned black or look wet/slimy, discard those — that's a sign of spoilage, not a normal part of the life cycle.

How to Store Them

Keep spikes refrigerated, ideally 34–38°F, in the bedding they arrived in.

• Shelf life: Properly refrigerated spikes hold for roughly 2–4 weeks, though hatch quality declines the longer they sit. If you want the best hatch rate, use them within the first couple of weeks rather than waiting until the end of that window.
• Humidity matters too. Maintain moderate moisture — a damp paper towel in the container helps keep spikes from drying out. Don't oversaturate; too much moisture invites mold.
• Ventilation. Make sure the container has small airflow holes. This prevents condensation buildup, which otherwise leads to mold or suffocation.
• Watch the temperature ceiling. Run warmer than this range and spikes will start pupating whether you want them to or not. If you're trying to hold them in the larval stage longer, keep them on the colder end.
• Stagger your fridge pulls. If you're working through a batch over time, pull out a small group every 2–3 days rather than leaving the whole batch at room temp at once. This keeps a rolling supply of freshly-hatching flies instead of one big batch all emerging together.

Troubleshooting:

• Spikes pupating too quickly — check that your fridge is actually holding 34–38°F. Temperature fluctuations are the usual cause of unwanted early pupation.
• Mold in the bedding — reduce moisture, replace the bedding with fresh, dry sawdust or bran, and check ventilation.
• Spikes look dried out — lightly mist the bedding. Don't over-water; the goal is slightly damp, not wet.

How to Hatch Them

Move however many spikes you want to hatch into a separate container at room temperature (or slightly warm — around 80°F will speed things up).

• Timeline: The full pupal stage runs longer than most people expect — roughly 11 days at 80°F, stretching to 13–18 days at cooler room temperatures. If a batch you're hatching still has a while to go, that's normal, not a failed batch.
• Why some sellers advertise "48 hours": Spikes are often shipped already partway through pupation, so the time from when you receive them to hatch can be much shorter than the full pupal stage — sometimes just a couple of days if they arrived nearly mature. Don't confuse that shipping-to-hatch window with the actual biological pupal duration; if you're timing a fresh batch that just pupated, plan for the longer end.
• Give them room. A single spike produces a surprisingly large fly, so don't crowd a big batch into a tiny container — give them space so newly emerged flies aren't climbing over each other.
• Cold flies can't fly. Once hatched, a quick trip to the fridge (or a very brief stint in the freezer) will slow adult flies down enough to handle and dose out without a swarm loose in the room.

Feeding: Spikes vs. Hatched Flies

This is the one meaningful judgment call with this feeder. Some keepers feed spikes directly — gutloaded overnight — while others strongly prefer to only feed the hatched adult fly. The concern with feeding spikes as-is: they're raised on a protein substrate that can leave them carrying more ammonia and bacterial load than a properly gutloaded feeder should have. If you do feed spikes, gutload them first (a few hours to overnight, with a mild gutload mix) rather than offering them straight from the fridge. (No need to dust either spikes or hatched flies — spiders don't need calcium supplementation the way reptiles do; gutloading is the only step that matters here.)

Feeding hatched flies is the lower-risk, more common approach:

• Let spikes pupate and hatch fully.
• Gutload the newly emerged flies for a few hours before feeding, the same way you would with house flies.
• Feed live, and remove anything uneaten within a reasonable window.

Blue Bottles vs. House Flies

If you're deciding between the two: blue bottles are larger (about 10–11mm vs. house flies' 8–9mm) and noticeably more active once hatched, which makes them a strong choice for larger tarantulas, bigger jumping spiders, and mantids that respond well to visual movement triggering a feeding response. House flies are the easier, lower-maintenance default for most setups, and better sized for smaller spiders and slings. Many keepers rotate between the two for variety rather than picking one exclusively.