Allacma fusca

Allacma fusca in Estonia

Allacma fusca is a large species of springtail with a dark brown globular abdomen; adults reach a length of around 3.5 mm (0.14 in).[3] There is often a "snowflake"-like marking on the abdomen, which is most noticeable in paler coloured individuals.[4] The mouthparts are internal, and the head bears a pair of antennae, two groups of eyespots known as "composed eyes" and a pair of small post-antennal organs. The setae (bristles) on the third antennal segment are unequal in length, some being much longer than others, and the fourth antennal segment is divided into sixteen sub-segments. There are three thoracial segments, each bearing a leg, and five abdominal segments. The fourth abdominal segment bears an organ known as a "dens", and at the tip of this is a structure known as a "mucro". This species has mucros with smooth outer edges and saw-edged inner edges. In females, the appendage on the fifth abdominal segment is unforked.[3][5]

This springtail is native to Western Europe. It is common in the British Isles and inhabits moist terrestrial environments such as plant litter.[3]

Invertebrates living among the soil particles in underground habitats often experience raised levels of carbon dioxide, which tends to increase with depth. Researchers have found that the surface-dwelling Allacma fusca can tolerate a 10% level of carbon dioxide for a few hours while in contrast, the springtail Folsomia candida, which lives deeper in the soil, can survive under the same conditions for more than six weeks.[6]