Lasius fuliginosus, commonly known as the Jet Ant, is easily recognized by its striking shiny, jet-black exoskeleton. Workers possess a distinctive heart-shaped head with rounded lobes, complemented by a prominent notch at the rear—a key feature that differentiates them from other members of the ラシウス genus. Their bodies exhibit sparse pubescence alongside scattered erect hairs on the dorsal surface. In contrast, their legs and antennae display a brownish-yellow hue, and their movements are precise and rapid.

Workers measure approximately 4 to 6 mm in length (0.16-0.24 inches), with queens slightly larger at 6 to 6.5 mm  (0.24–0.26 inches), while males are somewhat smaller, ranging from 4.5 to 5 mm  (0.18–0.20 inches). Notably, Lasius fuliginosus is a monomorphic species, meaning worker sizes are uniform without the presence of larger soldier castes. It ranks among the largest species within the genus ラシウス.

An intriguing identification tip: the gaster (abdomen) of a full-fed ant may appear swollen, shifting the segmented plates and revealing connective tissues—sometimes leading to misidentification as a different species.

The Jet Ant exhibits sophisticated chemical defenses. When threatened, workers release dendrolasin—a terpene secreted from their mandibular glands. While the scent is mild to humans, it acts as a potent olfactory alarm to other ants, signaling danger and even repelling rival species due to its toxic properties.

Lasius fuliginosus demonstrates aggressive competitive behaviors. They actively interfere with neighboring ant colonies by raiding nests, stealing food resources, and abducting brood. These strategies enhance their ecological dominance and have been shown to suppress species richness in their vicinity.

For instance, while diverse ant species such as ラシウス・ニガー, ミルミカ・ルブラ（Myrmica rubra, カンポノータスそして Tetramorium often coexist near フォルミカ・ルファ colonies, the presence of a Lasius fuliginosus nest correlates with a marked reduction in local ant diversity—highlighting their role as ecosystem engineers, albeit at the expense of some biodiversity.

Interestingly, the beetle Amphotis marginata, nicknamed the “highwayman” of the ant world, has evolved to exploit Lasius fuliginosus. It mimics trophallaxis behavior to solicit food from returning ants, evading aggressive responses by flattening against the ground when attacked.

Lasius fuliginosus forages both diurnally and nocturnally, favoring warm conditions. Their primary food source is honeydew produced by aphid colonies they tend on trees and shrubs. This mutualistic relationship sees the ants shepherding aphids, removing diseased individuals to safeguard the colony’s resource. They repeatedly return to the same aphid colonies, fiercely defending them from competitors.

Though their mandibles are relatively weak, Jet Ants scavenge dead or immobilized insects such as cocoons, mosquitoes, and beetles, supplementing their diet with protein-rich prey.

Jet Ants are renowned for their remarkable nest construction inside hollow trees. They build “cardboard” nests with papery walls, akin to termite architecture. Contrary to earlier beliefs, mandibular gland secretions are not used in nest building; instead, the ants employ a glue-like mixture of honeydew, sap, and nectar to bind materials.

The nest’s walls consist of wood particles and soil combined with this sugary mortar, providing an ideal substrate for symbiotic fungi. The fungus strengthens the nest structure, which can house up to 15,000 workers.

Nest construction is a highly organized task: three specialized worker groups—builders, material carriers, and honeydew collectors—collaborate to maintain the colony’s home. Typically concealed within trees, nests reveal themselves through entrances either above ground in tree cavities or near underground chambers. Surrounding refuse, resembling sawdust piles sometimes caught in spider webs, offers clues to their presence.

Also, colonies are often polycalic, comprising multiple interconnected nests with several queens. During winter, ants retreat deeper underground, creating insulated chambers to overwinter safely alongside their brood.

Lasius fuliginosus queens exhibit a unique reproductive strategy. Rather than founding nests independently, they invade existing nests of other ラシウス species—primarily Lasius umbratus, itself a social parasite of ラシウス・ニガー. The invading queen eliminates the resident queen and manipulates workers into raising her offspring. Over time, the original colony is entirely replaced by Jet Ants, a form of hyper-parasitism that reflects complex evolutionary interactions.

The Jet Ant (Lasius fuliginosus) exemplifies the remarkable diversity and adaptability of ants. As builders, invaders, aphid farmers, and chemical warriors, they reshape their environment and outcompete rivals with impressive versatility. Their ability to manipulate ecosystems, craft intricate nests, and engage in sophisticated social parasitism underscores their ecological significance.

This species not only challenges traditional notions of ant behavior but also invites further study into the complexities of social insect interactions and ecosystem dynamics.