Quick Answer: Melopoeus lividus, the Cobalt Blue, is a 4-5 inch Old World fossorial from Myanmar and Thailand with intense iridescent cobalt blue legs. It needs a deep-substrate terrestrial enclosure (8"+ for adults), 78-86°F, and 75-85% RH. Old World venom — medically significant. Advanced keepers only. Download the free care sheet PDF.
Introduction
Melopoeus lividus, the Cobalt Blue tarantula, is one of the most sought-after Old World fossorials in the hobby. The common name is not an exaggeration. Adult females reach 4 to 5 inches with femurs and legs covered in iridescent cobalt blue that shifts in intensity with the light angle. On a freshly molted specimen it looks almost electric. No photo fully captures it.
Native to the tropical rainforests of Myanmar, Thailand, and Cambodia, the Cobalt Blue lives almost entirely underground in self-excavated burrow systems that can extend 12 inches or more into the substrate. In the enclosure you will see it briefly at the burrow entrance during feeding, and almost never otherwise. This is not a display animal in the traditional sense. It is a species keepers maintain for the moments when it does appear, and for the knowledge of what is living in that setup.
Old World temperament in full: no urticating hairs, no warning behavior, extremely fast, bites as a primary defense. The venom is medically significant. This guide covers what it takes to keep Melopoeus lividus correctly.
Quick Stats
| Parameter | Detail |
|---|---|
| Scientific name | Melopoeus lividus |
| Common name | Cobalt Blue |
| Origin | Myanmar, Thailand, Cambodia |
| Habitat | Fossorial; tropical rainforest floor |
| Adult female legspan | 4-5 inches; intense iridescent cobalt blue on femurs and legs |
| Adult male legspan | 3-4 inches; less vivid coloration |
| Female lifespan | 15-20+ years |
| Male lifespan | 3-4 years post-maturity |
| Experience level | Advanced |
| Urticating hairs | None (Old World species) |
| Venom | Old World — medically significant; bite requires immediate medical attention |
| Webbing | Heavy; extensive silk-lined burrow system, 12"+ deep in adults |
Enclosure Setup
Substrate depth is the primary design constraint for this species. Adults that cannot dig to their preferred depth will sit in the open, stressed and exposed. Eight inches of packable substrate is the minimum for an adult; ten to twelve is better. The enclosure footprint matters less than depth. A 10x10x12 inch setup with 10 inches of substrate is more appropriate than a large shallow tub.
| Life Stage | Enclosure Size | Notes |
|---|---|---|
| Sling (under 1") | 6-16 oz deli cup | Pack substrate tight; moisten lower half |
| Juvenile (1-2.5") | 6"W x 6"D x 8"H | 5"+ substrate; side ventilation; starter burrow |
| Sub-adult / Adult | 10"W x 10"D x 12"H | 8-10"+ substrate; cross-ventilation; permanent water dish |
Substrate: coconut fibre mixed with topsoil or peat (60/40). The mix needs to hold tunnel walls without collapsing. Avoid pure coconut fibre for adults — it does not pack firmly enough to maintain burrow structure. Create a starter burrow at a 45-degree angle against one wall by pressing a pencil or dowel rod into the substrate. The spider will extend and modify it. Cross-ventilation is critical at this humidity level; side vents work better than top-only vents for maintaining moisture without stagnation.
Temperature and Humidity
Tropical rainforest with consistent warmth and high humidity year-round. This is not a species that tolerates dry conditions. A dropping water dish and a drying substrate will stress the spider out of its burrow and into a defensive position on the surface — not where you want it.
| Parameter | Target Range | Avoid |
|---|---|---|
| Daytime temperature | 78-86°F (25-30°C) | Below 72°F or above 90°F |
| Night temperature | 72-80°F (22-27°C) | Sustained below 68°F |
| Ambient humidity | 75-85% RH | Below 65% or sustained above 90% |
| Substrate moisture | Moist throughout; slightly drier surface | Completely dry or waterlogged |
A heat mat on the side of the enclosure at the low setting handles the temperature requirement in most rooms. Never place heat under the substrate. Overflow the water dish weekly; at this humidity level the substrate should feel consistently damp several inches down. The surface can be slightly drier to allow gas exchange and prevent surface mold.
Feeding
A reliable feeder when comfortable in its burrow. Most feeding happens at the burrow entrance after dark. Drop prey near the entrance; do not place it directly on or inside the burrow. For slings and juveniles, pre-kill prey before dropping it at the entrance. Adults that have not been seen in weeks are almost certainly still alive in the burrow — continue offering prey on a regular schedule and remove anything uneaten by the next morning.
| Life Stage | Prey Item | Frequency |
|---|---|---|
| Sling | Fruit flies, pinhead crickets (pre-killed) | Every 4-5 days |
| Juvenile | Small crickets, small dubia roaches | Every 5-7 days |
| Sub-adult / Adult | Large crickets, large dubia roaches, superworms | Every 10-14 days |
| Pre/Post-molt | None — remove all prey | Resume 14-21 days post-molt for adults |
Remove uneaten prey within 24 hours. Pre-molt fasting in adults can last 2-3 months. Do not force-feed a fasting spider. If prey consistently goes uneaten for more than 6 weeks outside of pre-molt, check temperature and humidity before assuming something is wrong — environmental issues are the most common cause of extended feeding refusal in this species.
Watering
A permanent water dish at substrate level is required. Overflow it once a week to maintain substrate moisture throughout the column. Change the water every 3-4 days. For slings and juveniles, moisten one side of the substrate by overflowing a bottle cap weekly — no water dish at small sizes. Never mist directly onto the spider or into the burrow entrance. If the substrate surface is pulling away from the enclosure walls and cracking, it is too dry.
Molting
Pre-molt signs: prey refusal for weeks to months, abdomen darkening noticeably, heavy webbing or complete sealing of the burrow entrance with thick silk. Adults may disappear underground for 2-3 months before a molt and remain below for weeks afterward. The sealed burrow is the clearest sign a molt is in progress or recently completed.
Do not probe or excavate a sealed burrow under any circumstances. Remove all live prey immediately when pre-molt behavior starts. Post-molt wait: 14-21 days minimum for adults before offering food. Fangs must be fully dark before feeding resumes. If you have not seen the spider in 3+ months and the burrow is still sealed, leave it alone. The spider is almost certainly fine.
Temperament and Handling
Do not handle this species. The Cobalt Blue is one of the more dangerous tarantulas in the hobby relative to its size. It is fast, it gives no warning signal before striking, and Old World venom at 4-5 inches body size is a genuine medical concern. A bite from a mature female warrants emergency evaluation, not a wait-and-see approach.
All enclosure maintenance is done with a large clear acrylic catch cup and long metal tongs. Confirm the spider's location before opening the enclosure. If the spider is at the burrow entrance when you need to work inside, close the enclosure and come back later. There is no scenario where reaching bare-handed into this enclosure is acceptable.
Is This Species Right for You?
The Cobalt Blue is not for keepers who are new to Old World species. It is one of the most beautiful fossorial tarantulas available, with coloration that earns the common name completely, but it is almost never visible and it is extremely dangerous to work with. If you keep it for the rare glimpses of cobalt blue at the burrow entrance, and you have the discipline to leave the enclosure alone during the months when the spider disappears underground, it is a genuinely rewarding long-term animal. Females can live 15-20 years. An established adult in a properly built setup is one of the more satisfying Old World specimens to maintain.
If you have not kept a fast, bitey Old World species before, start with something that at least postures before it strikes. Come back to the Cobalt Blue once you understand what zero warning behavior actually looks like in practice.