Body divided into regions;
either head and trunk, or head, thorax and abdomen
One pair of antennae and
mandibles, and one or two pairs of maxillae
Mouth, straight gut and
Branched tracheal tubes with
one pair of spiracles on each segment
Sexes separate with internal
Mainly terrestrial, but some
freshwater, and a very few marine species
The Uniramians are thought
to have evolved on land, after the Silurian,
where they became the dominant invertebrates. About one million species have
been described so far; this is thought to be only a small fraction of the
actual number of living species. The Phylum is divided into two subphyla the insects and everything else.
During the Carboniferous many of the insects and related arthropods reached a huge size. There were dragonflies as big as seagulls are today, and 225 million years ago some millipedes were 2.5 metres long and had a diameter of a few centimetres. There are actual fossilized animals and also evidence in fossilized footprints. It may be that this was a time when there were high oxygen levels in the air. This would have allowed the animals to grow bigger while still enabling them to breathe through spiracles (the paired openings down the abdomen and thorax through which they breathe). The oxygen production may have been higher because of the greatly increased photosynthesis by the high and rapid expansion of plants which could have temporarily outstripped the capacity for oxidation from the air. It is these plants we now mine as coal. High oxygen content would have increased the number of forest fires which would have left behind easily fossilized, partly-burned trees.
This Subphylum contains four
classes, Chilopoda (centipedes), Symphyla, Diplopoda (millipedes) and Pauropoda. All follow
the body plan of head with a single pair of antennae, mandibles and 1 or 2 pairs of maxillae and trunk with more than 4 similar leg-bearing segments.
Symphyla are small, from 2 mm - 10 mm. They are usually white and have no eyes. They resemble centipedes, and there are around 160 species world wide. They are found in moist habitats such as leaf mould. Most can run fast. They feed on vegetation.
Scutigerella immaculata (above) can be a pest in greenhouses, as it feeds on soft plant material such as roots. It is from 2.5 - 8 mm long, pale coloured, and with 12 pairs of legs - one pair per segment. It can be found under stones, in tree stumps, dead wood, leaf litter, and soil as far as 2 m deep, and can live for as long as 4 years.
Scutigerella immaculata unusual mating habits
The male deposits his spermatophore at the end of a stalk. When the female finds it she eats it, but she does not swallow the sperm. She stores it in special pouches in her mouth. When she is ready to lay an egg she removes it as it emerges using her mouth, and attaches it to the soil or leaf mould. In doing this each egg gets smeared with sperm. There are usually around 35 eggs and they are attached to the leaf mould, moss or lichen. Parthenogenesis is also common. Once the female has laid her eggs they hatch in 1 or 2 weeks. The young have only 6 pairs of legs. They will gain an additional pair with each moult. The number increases with each moult. They moult even in adulthood.
All in this class are less than 2 mm long, and have soft bodies with 9 - 11 segments. There are around 500 species world wide. They have branched antennae (see left and below) and no eyes. Usually they have nine pairs of legs, however on hatching from the egg the young have only 6 pairs of legs. It is believed that they are related to the millipedes, but more primitive. They inhabit moist environments such as leaf litter, under bark and in decaying vegetation and debris. They are usually a pale, whitish colour.
On the left is Euypauropus sp. it is a browser on minute pieces of plant debris where it moves around in a clumsy and slow manner.