Lasiini

Lasiini ants are distinguished from other Formicinae tribes by a combination of morphological characters including specific arrangements of mandibular dentition, palp formula, and male genitalia structure. The tribe can be separated from Plagiolepidini by differences in antennal segment count and petiole structure. Within Lasiini, genera are distinguished by worker caste polymorphism patterns, presence or absence of ocelli in workers, and metapleural gland morphology. Lasius species typically possess a distinct acidopore at the gastral apex for chemical defense, while Nylanderia species are characterized by their extremely long antennae and legs relative to body size.

Lasiini ants occupy diverse terrestrial habitats across temperate and tropical regions. Lasius species are particularly abundant in temperate forests, grasslands, and urban environments, often nesting in soil, rotting wood, or under stones. Nylanderia species are common in tropical and subtropical forests, frequently occurring in leaf litter and arboreal microhabitats. Myrmecocystus species are specialized for arid and semi-arid environments in western North America, nesting in sandy soils. Cladomyrma species are obligate inhabitants of myrmecophytic plants in Southeast Asian forests.

The tribe has a cosmopolitan distribution with centers of diversity in the Holarctic region (Lasius), tropical Asia (Cladomyrma, Paraparatrechina), and the Americas (Nylanderia, Myrmecocystus). Lasius is primarily distributed across the Northern Hemisphere with highest species richness in Eurasia and North America. Nylanderia has undergone extensive geographic expansion and is now established on all continents except Antarctica, with numerous introduced populations. Myrmecocystus is restricted to western North America from Canada to Mexico. The extinct genus †Drymomyrmex is known from Baltic amber deposits indicating Paleogene distribution in Europe.

Most Lasiini species are generalist omnivores with a strong preference for liquid carbohydrates, primarily obtained through honeydew collection from hemipteran trophobionts. Lasius species actively tend aphids and scale insects for honeydew, and also consume nectar and extrafloral nectaries. Myrmecocystus species are notable for harvesting nectar directly from flowers and storing it as honey in the crops of specialized replete workers. Protein sources include small arthropod prey, scavenged arthropod remains, and in some species, seeds. Prenolepis species have been observed to consume fungus-growing ant excrement.

• Aphidoidea - trophobiontLasius and Nylanderia species tend aphids for honeydew
• Coccoidea - trophobiontScale insects tended for honeydew production
• Rubiaceae - myrmecophyteCladomyrma species inhabit myrmecophytic plants in Southeast Asia

Lasiini ants exhibit the typical holometabolous development of ants with complete metamorphosis through egg, larva, pupa, and adult stages. Colony founding occurs through multiple strategies: independent colony founding by single dealate queens (claustral founding without foraging), dependent colony founding through temporary social parasitism (queens usurping established colonies of related species), and in some Lasius species, colony budding. Temporary social parasitism is well-documented in Lasius, where queens of parasitic species enter host colonies, kill the host queen, and utilize host workers to rear their own offspring. Colony size varies dramatically: Lasius colonies typically contain 1,000–10,000 workers, while Myrmecocystus colonies may exceed 10,000 workers with specialized replete caste development.

Temporary social parasitism represents a key behavioral innovation in Lasius, with multiple species having evolved this strategy independently. Myrmecocystus exhibits specialized food storage behavior where workers called repletes engorge with nectar and hang motionless from chamber ceilings, serving as living food reservoirs. Nylanderia species display rapid, erratic locomotion and are known for their ability to form extremely large supercolonies in introduced ranges. Lasius species engage in conspicuous mass nuptial flights during warm, humid conditions, often synchronized across large geographic areas. Defensive behavior relies primarily on chemical defense through formic acid ejection from the acidopore rather than stinging.

Lasiini ants function as dominant ecosystem engineers in many temperate and tropical communities. Their honeydew mutualisms with hemipterans significantly influence plant phloem allocation and arthropod community structure. As major predators and scavengers of small arthropods, they regulate populations of potential pest species. Soil-nesting species contribute to soil aeration and nutrient cycling through nest construction. Myrmecocystus species serve as important food sources for specialized ant predators including horned lizards (Phrynosoma) in western North American deserts. Invasive Nylanderia species have demonstrated substantial impacts on native ant communities through competitive displacement.

Lasius niger and related species are familiar urban ants in Europe and North America, sometimes entering buildings but generally considered benign. Some Lasius species protect aphid pests in agricultural settings, indirectly causing plant damage. Nylanderia fulva (the tawny crazy ant) and Nylanderia bourbonica are significant invasive pests in the southern United States, causing ecological disruption and electrical equipment damage through their aggregation behavior. Myrmecocystus repletes were historically harvested by Indigenous peoples of the southwestern United States and Mexico as a sweet food source. The tribe serves as important model organisms for studies of social parasitism evolution, supercolony formation, and ant-plant mutualisms.

• PlagiolepidiniAlso in Formicinae with similar general body plan; distinguished by 11-segmented antennae in workers versus 12 segments in most Lasiini, and different male genitalia structure
• MyrmelachistiniSmall Formicinae ants with superficially similar habitus; separated by distinct palp formula and lack of acidopore in most genera
• FormiciniContains Formica and related genera with convergent temperate forest ecology; distinguished by presence of psammophore (basket of hairs for sand transport) in desert species and different mandibular dentition

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• Wikipedia
• iNaturalist taxon
• NCBI Taxonomy
• Catalogue of Life
• The Ants and Butterflies of Gates Canyon: Quite the Ecosystem | Bug Squad