Buying healthy invertebrates is one of those things that sounds obvious until you are standing in front of a tub at an expo and realise you have no idea what you are looking at. Is that isopod sluggish because it is unwell, or because it just moulted? Is the spider sitting still because it is stressed, or because it is a spider and that is what spiders do? Here is what I actually look for.
General signs across all groups
There are a few healthy invertebrate signs that apply regardless of whether you are buying isopods, beetles, spiders, or millipedes. The animal should be responsive to gentle stimulation. Tap the side of the container or carefully nudge the substrate nearby. A healthy animal will react, even if that reaction is "curl into a ball" or "run away from you." No reaction at all is a bad sign.
Look at the body condition. The exoskeleton or cuticle should be intact, not cracked, flaky, or discoloured in ways that do not match the species' normal appearance. Limbs should all be present and functional. Missing legs happen and are not always a dealbreaker (many species can regenerate limbs over successive moults), but an animal missing several legs has been through something rough.
Isopods
Healthy isopods are active when disturbed. They should scatter or curl up depending on the species. Armadillidium species will conglobate (roll into a tight ball), Porcellio species will run. If they do neither, something is wrong.
Check for a plump, smooth body. A dehydrated isopod looks wrinkled and shrunken, particularly around the pleon (the rear end). The colour should be even and match what you expect for the morph. Pale, washed-out colour can indicate a recent moult, which is normal, or chronic poor conditions, which is not. Ask the seller when the last moult was if you are unsure.
Avoid buying from a culture that has visible grain mites. Those tiny white specks clustered on food or substrate surfaces are Acarus siro, and while they are not directly harmful to isopods, they indicate overfeeding and excess moisture. You will bring the mites home with you and then have to deal with them in your own setup.
Jumping spiders
A healthy jumping spider is alert and tracks movement. Those big anterior median eyes should follow your finger if you move it slowly near the enclosure. If the spider does not react to visual stimuli at all, that is concerning. They are visual hunters with excellent eyesight, and healthy ones are attentive.
The abdomen should be rounded but not hugely distended. A very shrunken abdomen means the spider has not eaten in a while or is dehydrated. Check that all eight legs are present and that the spider can move normally. Watch it walk if possible. Stumbling or dragging legs is not good.
Look at the web retreat if one is visible. A well-constructed silk sac is a sign of a spider that has been settled and healthy enough to build. No retreat in a container is fine if the spider was recently rehoused, but a sparse, ragged-looking web in a container the spider has been in for weeks might suggest ongoing stress.
Beetles
For adult beetles, check that the elytra (the hard wing covers) are intact and sit flat. Gaps between the elytra, or elytra that do not close properly, can indicate a problem during pupation. Some cosmetic imperfection is normal, but severely deformed elytra affect the beetle's ability to protect its hindwings and body.
Pick the beetle up gently if the seller allows it. A healthy adult beetle will grip firmly with its tarsi (feet) and try to walk or fly. Limp legs and no grip strength suggest the beetle is dying or badly stressed. Check the underside too. Mites sometimes cluster around the leg joints and mouthparts.
If you are buying larvae, this gets trickier. A healthy L3 larva should be plump, creamy-white or yellowish (depending on species), and curl into a C-shape when disturbed. Dark discolouration on the body, particularly around the rear, can indicate infection. The substrate the larva is sold in matters too. It should smell earthy, not sour or rotten. Larvae that come in poor-quality substrate have probably not been eating properly.
Millipedes
A healthy millipede should curl when you pick it up. The degree of curling varies by species, but some defensive response is normal. A completely limp millipede that does not react to being handled is either very unwell or dead.
Run your eyes along the body and count segments if you can. You are looking for a smooth, unbroken exoskeleton. Dents, holes, or soft patches are bad news. The legs should move in coordinated waves. Millipedes with legs that do not synchronise properly may have neurological issues from mite infestations or toxin exposure.
Smell the animal. Seriously. A millipede that has been under chronic stress may have depleted its defensive secretions, and the substrate might smell off. Benzoquinone secretions (common in Spirostreptida species) have a distinctive chemical smell. That is normal. What is not normal is a sweet, rotting smell, which can indicate bacterial infection.
Mantids
Mantids should track movement immediately. Their triangular head should swivel to follow your finger or any movement nearby. If a mantid is not tracking, it may be approaching a moult (they become sluggish beforehand) or it may be unwell. Ask the seller when the last moult was.
Check the raptorial forelegs. They should fold neatly and snap open when the mantid strikes. Forelegs stuck in an unusual position can mean a previous moult went partially wrong. Also check that the abdomen is not enormously distended, which suggests overfeeding and puts the animal at risk during its next moult.
Red flags at any seller
Dead animals in the same enclosure as the ones for sale. It happens occasionally and is not automatically a disaster, but multiple dead animals in the same tub should make you walk away. Overcrowded conditions with species that should be housed individually (jumping spiders, mantids) are a hard no. Mould-covered substrate or standing water in enclosures meant for terrestrial species tells you the seller does not understand the basics.
The best test is to ask the seller a specific care question. Not "is this easy to keep?" but something like "what humidity range does this species need?" or "what substrate are the larvae in?" A seller who knows their stock will answer without hesitation. One who does not know their animals will give you a vague answer or change the subject. Buy from the first type.