Bodyforms & Lifestyles

Coping with living in soil

Soil is a dense, dark habitat and soil biota cope by developing special adaptations of body form or by changing behaviour. However, there are advantages to living in the soil. There are generally lots of organic residues overlying the soil on which to feed; shelter from above-ground predators is provided and the temperature, relative humidity and the moisture content of the environment is more stable than in the above-ground environment. Here are some ways that soil biota use to adapt to life in the soil.

Adaptations of Body Form

True soil animals live in the dark recesses of the soil and have little need for eyes. They often have small eyes or light sensitive areas of skin (e.g. earth worms) or are lacking eyes altogether (e.g. soil springtails).

Many soil animals are wingless. Wings are no use as they can’t be expanded and fluttered in close confinement in the soil. Springtails never developed any wings over evolutionary time and other animals lost theirs, all the better to live in soil and rotting logs (e.g. wingless cockroaches).

It is often the smaller species of any soil biota group which live in soil while their larger relatives inhabit the surface organic layers in plant litter and rotting logs (e.g. millipedes and centipedes).

Some soil animals have small legs and other appendages (antennae, springing organ of springtails, legs of mites and springtails) as long appendages would only hamper movement in the cramped environment of the soil. However, predatory soil animals still tend to have long legs for chasing prey (e.g. predatory soil mites).

Long, thin animals can move through the soil. Hard-bodied, rigid animals such as millipedes can push through the soil without damage while the soft-bodied earthworms can move soil particles gently aside by alternately lengthening and contracting their bodies. Earthworms lengthen and push between soil particles. They then dig tiny bristles on their bodies into the burrow walls, which helps anchor themselves before pulling the rest of their bodies forward. Earthworms and millipedes also create burrows by simply eating their way through the soil. They are called “tunnelers”.

Some insects have developed different bits of their anatomy which act as shovels. Dung beetles have flanges on their legs and heads which help move soil particles from their burrows to the soil surface – a bit like a mini-bulldozer. Mole crickets have front legs with wide flanges and spurs and the first thoracic segment behind the head is enlarged and which protects the head while digging. These animals are called “excavators” – likening their anatomy and digging activities to large earthmoving equipment.

Earthworms secrete a mucous layer to lubricate their bodies against abrasive surfaces in the soil.

Many animals that live in the surface soil and in litter, dung and logs have a flattened body shape so they can slide easily under the litter layer, stones and logs for protection (e.g. centipedes, cockroaches).

Many true soil animals are white as they don’t need pigment to protect them against ultra-violet light (e.g. springtails, millipedes).

Adaptations of Behaviour

• Many soil animals, particularly those that move up into the surface layers of litter, or onto the soil surface, are active in the hours of dark (e.g. spiders and snails). By being nocturnal, they can avoid predators (e.g. birds) and they can avoid the hotter, drier hours of the day. Many soil animals will move away from a light source, preferring darker parts of a light gradient (e.g. woodlice).

• Some social insects can create their own habitat by constructing nests where the microclimate is carefully controlled. Termites and ants excavate soil cavities and move soil particles about to create living spaces and foraging galleries. Termites nests can be sealed or opened to the air depending on prevailing climatic conditions.

• Predators in soil can have a lie-in-wait strategy (antlions, trapdoor spiders, scorpions) while other predators are active hunters. The “lie-in-wait “ animals, use strategies that conceal them in the soil (e.g. antlion pits, trapdoors into underground tunnels of trapdoor spiders) and then spring out and capture their prey as it comes too near their lair. The “active hunters” leave their daytime homes in the soil for the night shift (e.g. wolf spiders and centipedes). Protected by the night from things that eat them, they roam about on the soil surface hunting for prey (wolf spiders, centipedes). These animals generally have long, “all the better to catch you with”, legs.

• Large soil animals have various ways of moving soil about so that they can progress through it (e.g. shoveling it aside, eating through it). Although smaller invertebrate animals, such as springtails and mites, are not strong enough to move soil particles aside, they are tiny enough to use the existing pore spaces and channels between soil particles, to move from place to place.

Springtail Life Forms

We can see some of these adaptations to underground life in springtails as they are a diverse group of biota that have species that live not only in soil and litter, but also in the vegetation.