Feeding invertebrates is straightforward once you understand what each group actually eats. The problem is that "invertebrate" covers an enormous range of animals with completely different diets. An isopod eating leaf litter has nothing in common with a jumping spider catching flies. Getting the food right matters, because the wrong diet kills slowly and invisibly.
Isopods
Isopods are detritivores. In the wild, they eat decaying plant matter, fungus, and the odd bit of dead animal material. In captivity, the diet breaks down into three categories:
- Leaf litter: oak and beech leaves are the standard. These should be present in the enclosure at all times, both as food and as habitat. Avoid conifer needles, which contain phenols that are toxic.
- Vegetables and fruit: small pieces of carrot, courgette, sweet potato, cucumber. Replace uneaten food within a day or two before it moulds.
- Protein: dried shrimp, fish flakes, or a bit of dried mealworm. Feed sparingly, maybe once a week. Overfeeding protein is the main cause of grain mite infestations. The mites aren't harmful to the isopods, but they indicate you're overdoing it.
Calcium is non-negotiable. Isopods are crustaceans that moult regularly, and they need calcium to mineralise their new exoskeleton. Cuttlebone is the easiest option: break off a piece and leave it in the enclosure permanently. Crushed eggshell or limestone also work.
Millipedes
Millipedes are also detritivores, but they eat their substrate. The mix of rotting hardwood, leaf litter, and organic soil that makes up their enclosure is their food source. They eat it constantly, gradually processing it into finer material.
On top of the substrate diet, offer fresh vegetables and fruit. Cucumber, courgette, melon, and mushrooms go down well. Some keepers offer small amounts of protein (fish flakes, dried shrimp) occasionally, though opinions vary on how much protein millipedes need.
Calcium is just as critical for millipedes as for isopods, for the same reason. Their exoskeleton is heavily calcified, and deficiency leads to failed moults, which are usually fatal. Keep cuttlebone or crushed oyster shell in the enclosure at all times.
Make sure all wood in the enclosure is hardwood (oak, beech, birch). Softwoods like pine, cedar, and spruce contain phenols and terpenes that are toxic to millipedes. This applies to food wood, substrate wood, and any decorative pieces.
Beetles
Here's where people get confused, because beetle larvae and adult beetles eat completely different things.
Larvae
Flower beetle larvae (Pachnoda, Mecynorrhina) and rhinoceros beetle larvae eat fermented hardwood flake soil. This is their only food source. They live in it, eat it, and grow in it. Regular potting soil, garden compost, or unfermented wood shavings will not sustain them. The larvae appear to be eating but don't get the nutrition they need, and they die over weeks.
Buy proper flake soil from an invertebrate supplier. Top up the substrate as the larvae consume it. You can tell it's being eaten because the substrate gradually turns into dark, fine frass (droppings).
Adults
Adult flower beetles eat sugary foods. Ripe and overripe fruit is the staple: banana, mango, peach, apple, fig. Beetle jelly (a commercially available sugar and protein gel) is convenient and clean. Flower petals are also taken, particularly roses, hibiscus, and dandelions, which reflects their ecology as flower-visiting beetles in the wild.
Replace fresh fruit every day or two. Fermenting fruit is fine and even attractive to them, but mouldy fruit should be removed.
Jumping spiders
Jumping spiders are predators. They eat live prey only. No dead insects, no fruit, no jelly. They hunt by sight and need the movement of live prey to trigger a feeding response.
- Spiderlings and juveniles: flightless fruit flies (Drosophila hydei or D. melanogaster). These are sold as cultures you maintain at home.
- Adults: houseflies, blue bottles, green bottles, small crickets, or small mealworms. The prey should be roughly the same size as the spider's abdomen or smaller.
Feed juveniles every 2-3 days and adults every 3-5 days. They'll refuse food when they're approaching a moult. That's normal. Don't panic and don't leave prey in the enclosure. An uneaten cricket left overnight can harass or injure a moulting spider.
Water comes from misting the enclosure walls. Spiders drink droplets, not standing water. Mist the walls, not the spider directly. Water droplets can trap small spiders, which is as grim as it sounds.
Mantids
Mantids are ambush predators. Like jumping spiders, they need live prey. The size of prey should be roughly the length of the mantid's raptorial foreleg (the grasping front leg) or smaller. Too-large prey can injure the mantid.
- Young nymphs: fruit flies
- Older nymphs: houseflies, small crickets
- Adults: larger flies, crickets, locusts, small roaches
Feed every 2-3 days. Mantids can overeat. An overfed mantid develops a visibly distended abdomen, and this puts it at greater risk during moulting. Restraint is better than generosity here.
Stick insects
Most commonly kept stick insects eat fresh leaves. Indian stick insects (Carausius morosus) eat bramble (blackberry) leaves, which are available year-round in the UK. Other species may eat privet, ivy, oak, or eucalyptus depending on origin.
Put fresh cut stems in a small jar of water (cover the top so the insects can't fall in and drown) and replace when the leaves dry out or get eaten. That's about it. They're the easiest invertebrates to feed.
Things to avoid
- Avocado is toxic to many invertebrates. Don't offer it to any species.
- Citrus is debated for isopods. Some keepers use it, others avoid it. If you're unsure, just don't bother. There are plenty of other vegetables.
- Pesticide residue on shop-bought produce is a real risk. Wash fruit and vegetables thoroughly, or use organic. This is especially important for small animals with high surface-area-to-body-mass ratios.
- Softwood in any form for millipedes and beetles. The phenols are toxic.
How to tell if you're feeding enough
A well-fed invertebrate is active and growing. A poorly fed one is lethargic and small. For colony animals like isopods, steady reproduction is a sign that nutrition is adequate. If breeding slows or stops, look at food availability and calcium supply.
For individual animals, regular moulting on schedule and good body condition (not shrivelled, not bloated) indicate things are on track. Don't overfeed predators. A jumping spider that's stuffed to bursting isn't healthier than one that eats a moderate amount every few days.