Dehydration kills more captive invertebrates than most keepers realise, and it can happen fast. An isopod colony can crash overnight if the substrate dries out during a heatwave. A jumping spider in a poorly ventilated enclosure near a radiator can go from fine to shrivelled in a couple of days. If you spot invertebrate dehydration signs early, you can usually fix it. If you miss them, the animal dies.
Why invertebrates are so vulnerable
Vertebrates have relatively efficient water retention. Invertebrates, for the most part, do not. Their exoskeletons limit water loss to some extent, but they are still far more susceptible to desiccation than mammals or reptiles.
Isopods are particularly at risk because they breathe through gill-like structures called pleopods on the underside of their body. These need to stay moist to function. A dry isopod cannot breathe properly. It is the equivalent of a fish out of water, just slower.
Spiders lose moisture through their respiratory system (book lungs and, in many species, tracheae) and through the cuticle itself. Millipedes lose moisture through their spiracles and cuticle. Smaller species and juveniles have a higher surface-area-to-volume ratio, meaning they lose moisture proportionally faster than larger animals. A sling (baby spider) can dehydrate in hours in conditions an adult would tolerate for days.
What dehydration looks like
The signs vary by group but share some common features. A dehydrated invertebrate is lethargic. It moves slowly or not at all, and its body looks physically smaller or more wrinkled than it should.
Isopods
A dehydrated isopod curls slightly at the edges of its body and the pleon (rear segments) looks shrunken and wrinkled. The colour may appear duller than normal. In severe cases the animal stops moving entirely and sits in a hunched position. A whole colony under heat stress will cluster tightly in the dampest corner of the enclosure. If you see every isopod in the tub piled on the wet side and nothing on the dry side, they are telling you the enclosure is too dry.
Spiders
A dehydrated jumping spider has a noticeably shrunken abdomen. In a healthy spider the abdomen is plump and rounded. When the spider is dehydrated it looks pinched and wrinkled, sometimes with visible folds in the cuticle. The spider becomes sluggish and stops hunting. It may sit at the bottom of the enclosure rather than up high where it normally perches, and it will stop building or maintaining its web retreat.
Millipedes
Millipedes show dehydration through a wrinkled, dull exoskeleton and reduced activity. They may stop coiling when handled and instead just lie limp. The segments may appear slightly separated, with gaps visible between them that you would not normally see. A millipede that is out in the open during the day and not moving is almost certainly in trouble, since healthy millipedes spend most of the day buried or hidden.
Beetles
Adult beetles are more resistant to dehydration than the other groups because their hardened elytra provide better protection against water loss. But they still need water. A dehydrated adult beetle moves slowly, struggles to grip surfaces, and may have difficulty flying. Larvae are more vulnerable. A dehydrated beetle larva looks deflated and wrinkled, and the normally taut, C-shaped body becomes limp and flaccid.
Immediate response
If you spot signs of dehydration, act now, not after you have finished reading this article. Mist the enclosure lightly with dechlorinated water. Focus on the substrate surface and the sides of the enclosure where the animals can drink from droplets. Do not flood the enclosure. You are adding moisture, not creating a swimming pool. Waterlogging is its own problem.
For an individual animal that looks badly dehydrated, you can place it on a piece of damp (not soaking) paper towel or sphagnum moss in a small, ventilated container. This gives it immediate access to moisture through contact. Spiders will often drink from droplets placed directly in front of them using a pipette or small syringe. Drip water onto the enclosure wall near the spider and watch whether it moves to drink.
Do not submerge the animal in water. Isopods can handle shallow water briefly but spiders and millipedes cannot. A spider that falls into standing water will drown.
Why it happened
Work backwards from the problem. The most common causes:
The room got too warm. A heatwave, a radiator being turned on, or the enclosure being moved to a sunnier spot. Temperature and humidity are linked. As temperature goes up, relative humidity drops. A room that was 60% humidity at 20C might be 35% at 28C, which is dangerously dry for most tropical invertebrates.
The ventilation is too aggressive. Cross-ventilation is good for preventing mould, but too many holes in the enclosure or a mesh lid with no solid sections lets moisture escape too quickly. This is a common problem with converted sweet tubs and food containers where keepers drill lots of holes. You need airflow, but you also need humidity retention. It is a balance.
You forgot to mist. It happens. Life gets busy, you skip a few days, and the substrate dries out. Setting a reminder on your phone is not a bad idea, especially during warmer months.
The substrate was not holding moisture well. Coco coir on its own dries from the top down very quickly. A mix of organic topsoil, coco coir, and sphagnum moss retains moisture much better and creates a gradient from dry surface to damp base. Leaf litter on top also helps by shading the substrate surface and slowing evaporation.
Preventing it
Monitor your humidity with a small digital hygrometer. The cheap ones are accurate enough for this purpose. Place it inside the enclosure at substrate level, not stuck to the top of the lid where the reading will be different.
Keep enclosures away from direct sunlight, radiators, and heat sources. Even a few hours of direct sun through a window can turn a terrarium into an oven. This kills animals faster than almost any other mistake.
Use a substrate mix that holds moisture. Add a layer of sphagnum moss and leaf litter on top. Mist regularly but not excessively. For most tropical species, lightly misting one side of the enclosure every two to three days is enough. For arid-adapted species, less frequently but still monitor.
And pay extra attention during summer. British summers are unpredictable, and a couple of days above 28C in a room with no air conditioning can cause problems faster than you expect. Move enclosures to the coolest room in the house if temperatures spike, and increase misting frequency until things cool down.