Acanthomyrmex concavus

Acanthomyrmex concavus

Acanthomyrmex concavus is a tiny leaf litter ant from the rainforests of Borneo, instantly recognizable by the deeply concave rear margin of the minor workers' heads ^[1]. These ants measure only about 2-3mm in length and inhabit the damp leaf litter of lowland rainforests in Sarawak and Sabah ^[2][1]. Unusually for the genus, only minor workers are known for this species, major workers have never been found despite extensive collecting ^[1]. Recent research reveals they reproduce via ergatoid queens, which are wingless reproductive females that resemble oversized workers rather than the typical winged queens seen in most ants ^[1].

Quick Summary

• Difficulty: Expert
• Origin & Habitat: Borneo (Malaysian states of Sarawak and Sabah) and Indonesia ^[2][1]. Found in leaf litter of lowland rainforests, mature wet forests, and alluvial forests ^[1][3].
• Colony Type: Single-queen colonies with ergatoid replacement reproductives documented. Ergatoid queens are wingless and intermediate in size between workers ^[1].
  • Colony: Monogyne
• Size & Growth:
  • Queen: Unknown for this species, ergatoid queens likely 3-4mm based on related Acanthomyrmex species ^[4].
  • Worker: Approximately 2-3mm total length (minor workers: head width 1.1mm, head length 0.9mm) ^[1]. Major workers unknown.
  • Colony: Likely small, probably under 100 workers based on related Acanthomyrmex species ^[4].
  • Growth: Slow to moderate.
  • Development: 6-10 weeks at 25-27°C (estimated based on similar small tropical ants). (Direct development data unavailable. Timeline inferred from small-bodied tropical Myrmicinae.)
• Antkeeping:
  • Temperature: Warm and stable, approximately 24-28°C. Tropical species requiring consistent warmth without seasonal cooling.
  • Humidity: High humidity essential. Keep nest substrate damp but not waterlogged, mimicking rainforest leaf litter conditions. Ventilation must balance humidity retention with fresh air.
  • Diapause: No. This is a tropical species that remains active year-round ^[1].
  • Nesting: Small cavities in dead twigs, rotten wood, under stones, or in leaf litter in nature ^[4]. In captivity, use small test tubes or mini naturalistic setups with tight spaces appropriate for tiny ants.
• Behavior: Slow-moving leaf litter specialists that prefer dark, humid conditions. Non-aggressive but shy and easily stressed by light or vibration. Because of their very small size, escape prevention must be excellent, they can squeeze through the tiniest gaps in standard setups ^[1].
• Common Issues: workers are tiny and will escape through gaps that larger ants cannot fit through, requiring fine mesh barriers., colonies are small and easily overfed, excess food can mold quickly in humid setups., require consistently high humidity, dry conditions kill them quickly., may refuse artificial or large prey items, need appropriately small live food., wild-caught colonies may carry mites or parasites from leaf litter habitats.

Identification and Appearance

Acanthomyrmex concavus minor workers are instantly recognizable by their deeply concave head margin when viewed from above, which gives the species its name ^[1]. The head width measures about 1.1mm with a head length of 0.9mm, making these ants extremely small ^[1]. The head capsule has rounded pits with thick walls, and the antennae are relatively short ^[1].

Uniquely among Acanthomyrmex, major workers have never been collected for this species despite extensive sampling in their native range ^[1]. This suggests either that majors are extremely rare, seasonal, or potentially absent in this species. Keepers should expect to see only one worker size in their colonies.

Natural Habitat and Distribution

These ants are found exclusively on the island of Borneo, specifically in the Malaysian states of Sarawak and Sabah ^[2][1]. The type specimen comes from Gunung Mulu National Park in Sarawak, collected from leaf litter in lowland rainforest ^[1].

They appear to be true leaf litter specialists, living in the damp decomposition layer of rainforests and mature wet forests ^[1][3]. Stable isotope analysis of a single specimen suggests they occupy a higher trophic level, indicating predatory or scavenging habits rather than reliance on plant-based foods ^[3]. In the wild, they likely nest in small cavities within this microhabitat.

Colony Structure and Reproduction

Unlike most ants that have winged queens, Acanthomyrmex concavus reproduces via ergatoid queens ^[1]. Ergatoid queens are wingless reproductive females that look like large workers rather than having the typical enlarged thorax and wings of standard queens. Based on related Acanthomyrmex careoscrobis, these ergatoid queens are intermediate in size between major and minor workers ^[4][1].

This reproductive strategy means colonies likely function with a single primary egg-layer (monogyne), but can replace her with one of these ergatoid reproductives if she dies ^[1]. Colonies remain small, probably containing fewer than 100 workers based on patterns seen in related species ^[4].

Housing and Nest Setup

Because these ants are extremely small and require high humidity, standard test tube setups work well for founding and small colonies, but the water reservoir must be carefully managed to prevent flooding of the small chamber ^[4]. For larger colonies, small naturalistic setups with rotting wood, dead twigs, or tight-fitting stone cavities mimic their natural nesting sites ^[4].

Avoid large open spaces. These ants prefer narrow chambers and tight passages scaled to their tiny size. The nest must maintain damp conditions, the substrate should feel moist to the touch but not dripping wet. Use fine mesh (stainless steel or similar) for ventilation to prevent escapes while allowing air exchange.

Temperature and Environmental Needs

As a tropical rainforest species, Acanthomyrmex concavus requires warm, stable temperatures between 24-28°C year-round ^[1]. They do not require and should not receive any cooling or diapause period. Temperature fluctuations should be minimized.

Humidity is critical. The nest environment should replicate the damp leaf litter of a rainforest floor. This means consistently moist substrate, not occasional misting. However, stagnant air promotes mold, so provide gentle ventilation while maintaining humidity. A heating cable on one side of the nest can create a beneficial thermal gradient, but ensure it does not dry out the substrate.

Diet and Feeding

Stable isotope data suggests these ants are predators or scavengers rather than relying heavily on sugar sources ^[3]. In nature, nests of related species often contain tiny seeds, suggesting they may collect and store small seeds ^[4].

In captivity, offer tiny live prey appropriate for 2-3mm ants, springtails, very small fruit flies, or pinhead crickets chopped into manageable pieces. They may also accept small seeds like chia or poppy seeds. Sugar sources such as honey water or sugar water can be offered but should not be relied upon as the primary food source. Feed small amounts frequently rather than large amounts rarely to prevent mold in their humid environment.

Colony Founding and Growth

Founding behavior for this species is unconfirmed. Given the presence of ergatoid queens, new colonies may form through budding (where a queen and some workers leave the parent nest) rather than traditional lone queen founding. If you acquire a founding queen, provide her with a small, humid test tube setup and offer food regularly, as semi-claustral founding is possible given the small size and predatory habits, though this remains speculative.

Growth is likely slow given the small colony size. Do not expect rapid expansion typical of larger species. Patience is required, and overfeeding is a common mistake with small colonies of this type.

Frequently Asked Questions

References