Uniramia (insects, centipedes, millipedes, symphyta, pauropoda,)

Latin: unus = one; ramo = branch

UNIRAMIA CHARACTERISTICS AND FAST FACTS

Bilaterally symmetrical
Body shape varied from spherical to long and thin
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 anus
Branched tracheal tubes with one pair of spiracles on each segment
Sexes separate with internal fertilisation
Mainly terrestrial, but some freshwater, and a very few marine species
Exoskeleton of chitin
Jointed legs

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.

Giant insects

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.

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SUBPHYLUM MYRIAPODA

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.

Class Diplopoda

tropical millipede

These are the millipedes (see the tropical millipede on the left), there are over 10 000 described species (62 species in Britain, 250 in France, 160 in Germany, 45 in Holland, 39 in Denmark), and perhaps as many as 70 000 undescribed. In the U. K. there can be 100 - 400 individuals in a m2 of grass and forest soil, and they play a significant role in the decomposition of the litter layer in soils, especially of forest soils. All are terrestrial. They can reach 28 cm in length, and have fairly cylindrical bodies of between 25 and 100 segments. The antennae are short, the mandibles are strong, and they have 1 pair of maxillae fused together to form a lower lip. The body of millipedes is often specialized to enable the animal to roll into a ball (see the pill millipede below) or coil (see the tropical millipede on the left. They usually coil up when disturbed.

Millipede legs

Unlike the Chilopods (centipedes), each body segment has two pairs of legs and two pairs of spiracles. No millipede has a thousand legs, the largest, from West Africa, barely reach 400. However a tiny millipede, Illacme plenipes, found in one single ravine in San Benito, California, and thought to have gone extinct until recently has up to 750 legs in the female. She is only 3.2 cm long and 0.05 cm wide. The male has between 300 - 400 legs.

Millipede development

larval millipede newly hatched

Millipede larvae have only one pair of legs to each segment, and fewer segments than the adult form, see left. Usually they hatch with 4 fully developed segments and 3 pairs of legs between them. Behind this there are 1 - 3 other segments, then the final segment. At every moult the animal gains more legs until it has the full compliment for that species.

Millipede eyes

Not all millipedes have eyes, those that do usually have two groups of simple eyes (ocelli), again the number of ocelli that go to make up each eye increase with each moult until the full number for that species is reached. The ocelli in a mature millipede form a triangle.

Glomeris marginata, pill millipede

Glomeris marginata

Above is Glomeris marginata, the pill millipede. It is often mistaken for a woodlouse, but it has more legs (17 or 19 pairs when fully grown) and a shinier body. When disturbed it can roll itself into a ball - hence its common name.

It tends to be found in calcareous, grassy soils, and in the leaf litter of deciduous forests - especially beech. It feeds on withered leaves. When fully grown it can be up to 2 cm long.

The female does not reach maturity until she is 3 years old, but a male is mature at 2. Females in the Glomeris genus surround their eggs with a sticky secretion exuded from the rectum, and mixed with soil particles.

Millipede reproduction

The sexual organs of both sexes are on the second segment behind the head, just behind the second pair of legs. The female stores sperm in a spermatheca and fertilizes the eggs with sperm as they leave her body. She lays eggs in a nest.

Millipede feeding habits and behaviour

Millipedes are mainly herbivorous or detrivorous. They can escape predation by rolling up or secreting toxic fluids from repugnatorial glands. They tend to prefer dark, moist habitats. Like earthworms they break down leaf litter and other organic debris, and are important contributors to improving soil structure. In colder climates millipedes don't hibernate in the winter or aestivate in the summer, nor are their eggs capable of undergoing a period of dormancy.

Millipede moulting

Most millipedes moult underground in a recess or chamber they have constructed. They usually lie in a spiral position during moulting, and they eat the moulted exoskeleton before they leave the chamber.

Millipede lifecycle

In the U. K. most millipedes lay their eggs in the spring or early summer. Some, but not all, species of millipede die after reproducing - this is termed semelparity. Some species of pill millipedes (see the drawing above) can live for as long as 11 years, some others live for as long as 7 years. In many species the males are smaller than the females and reach maturity in a shorter time.

Millipede predators

Most millipede predators are vertebrates. in the U. K. starlings, blackbirds, toads, lizards and hedgehogs are the main predators.

Millipede habitat preferences

Most live near the soil surface in the litter layer on which many of them feed. They will go deeper underground to moult, to lay eggs, and to avoid cold or drought.

Keeping millipedes as pets

The large tropical millipedes make popular and easy to care for pets, and surely few things on this planet are as elegant as the red-legged millipede out for a stroll.

Pauropus silvaticus

Above is Pauropus silvaticus.

Scutigerella immaculata, Class Symphyla

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.

Class Symphyla

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

Scutigerella immaculata (left) 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.

Euypauropus, a Pauropoda

Class Pauropoda

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 right) 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.

Pauropoda
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