Polyrhachis mjobergi

Polyrhachis mjobergi

Polyrhachis mjobergi is a medium-sized ant species belonging to the subgenus Hedomyrma. Workers are typically 6-8mm with the characteristic spines on the thorax that give Polyrhachis their common name of 'spiny ants'. They have a dark reddish-brown to black coloration. This species is lignicolous, meaning it nests in rotting wood in its natural habitat. Like all Polyrhachis, they spin silk to help construct their cocoons during pupal development. The genus Polyrhachis contains over 600 species and is found throughout the tropical and subtropical regions of the Old World, particularly in Asia and Australia.

Quick Summary

• Difficulty: Medium
• Origin & Habitat: This species is found in forested habitats of Southeast Asia. As a lignicolous species, it nests in decaying wood rather than soil or under stones. The lignicolous nesting habit suggests preference for humid, shaded microhabitats with access to rotting wood material.
• Colony Type: Colony structure is unconfirmed. Polyrhachis species can be either single-queen (monogyne) or multi-queen (polygyne) depending on the specific species, and this has not been documented for P. mjobergi specifically.
• Size & Growth:
  • Queen: Estimated 10-12mm based on related Hedomyrma species
  • Worker: Estimated 6-8mm based on genus patterns
  • Colony: Estimated several hundred workers based on typical Polyrhachis colony sizes
  • Growth: Moderate
  • Development: Estimated 6-8 weeks at optimal temperature based on typical Polyrhachis development (Development time is inferred from genus-level data, specific timing for this species has not been documented)
• Antkeeping:
  • Temperature: Keep at 24-28°C. As a tropical species, they prefer warm conditions. A heating cable on one side of the nest can create a gentle gradient if room temperature is below this range.
  • Humidity: High humidity is essential. Keep the nest substrate consistently moist but not waterlogged. Lignicolous species need damp wood material to prevent desiccation. Aim for 70-80% humidity in the nest area.
  • Diapause: Likely minimal or no true diapause. As a tropical species, they probably do not require hibernation, though activity may slow during cooler months.
  • Nesting: Provide a naturalistic setup with rotting wood pieces or a Y-tong/plaster nest with wood material included. They prefer enclosed spaces with high humidity. Avoid dry, airy setups.
• Behavior: Polyrhachis mjobergi is generally calm and not particularly aggressive. Workers are active foragers that will search for sugar sources and protein. They have spines which they may use for defense, but stinging behavior is not typical for this genus. Escape risk is moderate, they are not tiny but can squeeze through small gaps. Ensure your setup has secure barriers.
• Common Issues: high humidity requirements can lead to mold if ventilation is poor, balance is key, lignicolous nesting means they need wood material in the nest, plain test tubes may not be ideal, tropical species may decline if temperatures drop below 20°C, colony growth may be slow, which can frustrate beginners, wild-caught colonies may carry parasites that affect captive survival

Nest Preferences and Housing

Polyrhachis mjobergi is a lignicolous species, meaning it naturally nests in rotting wood. In captivity, you should provide a nest setup that includes wood material or a moist substrate they can tunnel through. A Y-tong (acrylic) nest with a chamber filled with rotting wood pieces works well, or you can use a plaster nest kept consistently moist. Avoid completely dry setups like bare test tubes, these will cause the colony to desiccate. The nest should have high humidity while still allowing some air flow to prevent mold buildup. A naturalistic setup with cork or wood pieces embedded in moist soil can also work, but monitor for mold carefully.

Feeding and Diet

Like most Polyrhachis species, P. mjobergi likely accepts a varied diet including sugar sources and protein. Offer sugar water, honey, or diluted syrup regularly as a constant energy source. For protein, provide small insects like fruit flies, small crickets, or mealworms. Polyrhachis are generally not specialized predators, so standard ant feeding protocols apply. Feed protein 2-3 times per week, and keep sugar available at all times. Remove uneaten prey after 24-48 hours to prevent mold.

Temperature and Humidity

As a tropical species from Southeast Asia, Polyrhachis mjobergi requires warm and humid conditions. Keep the nest area at 24-28°C. You can use a heating cable placed on top of the nest (not underneath, as this can dry out the substrate) to maintain warmth if your room temperature is lower. Humidity is critical, aim for 70-80% in the nest area. The substrate or nest material should feel consistently moist to the touch. Mist the area occasionally, but rely on moist substrate rather than misting schedules. Too much moisture without ventilation causes mold, so balance humidity with adequate air flow.

Behavior and Colony Care

Polyrhachis mjobergi workers are active foragers that will readily explore their outworld searching for food. They are generally docile and unlikely to bite aggressively, though their spines serve as a passive defense. Colonies grow at a moderate pace, don't expect rapid expansion. First workers (nanitics) will be smaller than mature workers. The colony will likely reach several hundred workers over time. Watch for signs of stress: workers clustering on moist areas suggests they need humidity, while staying near heat sources means they need warmth. Escape prevention is important though not as critical as with tiny species, secure lids and check for gaps, but standard barriers are usually sufficient.

Frequently Asked Questions