Wednesday, June 12, 2013

Arthropods (Phylum Arthropoda) of Singapore

Arthropods (phylum Arthropoda) form the largest single phylum in the animal kingdom, with more than a million species that has been described, making up more than 80 percent of all known animal species.

They are bilaterally symmetrical with segmented bodies and jointed (or segmented) limbs. In fact, "arthropoda" means "jointed legs" in ancient Greek.

Their body is enclosed in an external skeleton (or exoskeleton) composed largely of a tough material called chitin. This exoskeleton functions like a protective but rigid armour, and hence as the arthropod develops through it life stages and changes in size and/or shape, it will need to moult, i.e. discard the old skeleton and grow a new one.

Most species reproduce by sexual reproduction, though some species are known to reproduce by parthenogenesis (the embryos develop without fertilisation). Some species are hermaphroditic (possessing both male and female reproductive parts), but most have separate sexes and remain the same sex throughout their lives. For those that reproduce sexually, fertilisation is usually internal. The male either inserts its reproductive structure directly into the female's reproductive parts, or deposits a sperm package to be picked up by the female. Some aquatic species may perform external fertilisation, with the female laying the eggs to be fertilised subsequently by the male.

The extant groups of arthropods that can be seen in Singapore are as follow:

A) Subphylum Myriapoda

Myriapods (subphylum Myriapoda) are typically elongate arthropods which possess numerous pairs of legs ("myriapoda" means "10,000 legs" in ancient Greek), a trunk with many segments, and a head with one pair each of mandibles and antennae at the front end. The cuticle (i.e. outer covering) of myriapods is not waterproof, and hence they are largely terrestrial. Most species are also nocturnal to avoid water loss, and are most readily found in damp habitats such as the leaf litter or underground. They can breathe by taking in air through openings in the cuticle.

Myriapods are generally harmless to human, except for some venomous species. The bites may be painful, but are rarely fatal unless the victim is allergic to the venom.

The myriapods that I have photographed in Singapore include:

1. Millipedes (Class Diplopoda)

Millipedes (Class Diplopoda)
Millipedes (class Diplopoda) are easily recognised by their long segmented body with two pairs of legs on most segments, except for the first few and last segments. The name means "a thousand legs", and the ones seen in Singapore can have more than 300 legs. Most species feed on decaying organic matter or fungi. They are not venomous, and protect themselves either by secreting toxic or distasteful chemicals, or roll up to protect their softer underparts, exposing only the tougher upperparts. To breed, the males usually twist their bodies around the females to transfer the sperm, and the females will lay the eggs inside a nest in the soil.

2. Centipedes (Class Chilopoda)

Centipedes (Class Chilopoda)
Centipedes (class Chilopoda) are myriapods with an elongate and flattened body made up of at least 16 segments. Each segment has one pair of legs. Interestingly, they always have an odd number of pairs of legs, with the last pair longer than the rest, and the first pair modified into venomous fangs (or forcipules). They use their venomous fangs to hunt smaller animals. While the venom is generally not fatal to human, been bitten by the bigger species can be very painful. To breed, the males usually drop the sperm on the ground that the females pick up. The eggs may be laid singly or brooded in batches underground.

3. Symphylans (Class Symphyla)

Symphylans (class Symphyla) are small, whitish myriapods with a soft body made up of 15 segments, including a head with a pair of long antennae and three pairs of mouthparts, and a trunk made up of 14 segments. A pair of tail-like structures (or spinnerets) extends from the last segment. They have no eyes, and are often found in the soil or in the leaf litter, feeding on plant materials. Some species are perceived as garden pest, as they feed on living plant matter, such as the roots. To breed, the males deposit sperm packages that are picked up by the females. The above photo features an unidentified symphylan found among the leaf litter.

B) Subphylum Chelicerata

Chelicerates (subphylum Chelicerata) are arthropods which possess two main body parts - the cephalothorax (head and thorax fused together) and the abdomen, though for some the division between the two parts may not be visible. They have specialised appendages called chelicerae that are used mostly for feeding, positioned before the mouth. The marine species breathe with gills, while terrestrial species may have either book lungs (composed of stacks of alternating air pockets and hemolymph-filled tissues) or tracheae (a system of tubes).

Most chelicerates are harmless to human, including many of the venomous species. The bites may be painful, but are rarely fatal unless the victim is allergic to the venom. The venoms of the species found in Singapore are not known to be lethal, and no one in Singapore is known to have died from chelicerate bites/stings.

Here are some examples of the many chelicerate groups that can be seen in Singapore.

1. Horseshoe Crabs (Class Merostomata)

Horseshoe Crabs (Order Xiphosura, Family Limulidae)
Horseshoe Crabs (order Xiphosura, family Limulidae) are the only extant members of the class Merostomata. They can be recognised by the horseshoe-shaped carapace, and a long, spine-like tail. On the underside, there are six pairs of appendages. The mouth part is located in the middle. During the breeding season, the male will be seen riding on the larger female's back. Fertilisation is external though - the female will lay the eggs, and the male will then deposit the sperm on them.

2. Arachnids (Class Arachnida)

Arachnids (class Arachnida)
Arachnids (class Arachnida) can be distinguished from other chelicerates by having six pairs of appendages on the underside of the cephalothorax - a pair of fang-like or pincer-like chelicarae, a pair of leg-like or pincer-like pedipalps, and four pairs of walking legs. Most are predators, though there are also several species that scavenge and some others that parasitise other animals. In most species, the males will pass a sperm package to the female to fertilise the eggs. Many arachnids are observed to provide parental care by guarding the eggs or even the newly hatched.

3. Sea Spiders (Class Pycnogonida)

Sea Spiders (Class Pycnogonida)
Sea spiders (class Pycnogonida) are not true spiders, but a group of marine arthropods with mostly four pairs of long walking legs (some species may have five or six pairs), hence resembling the latter superficially. They have a head with two pairs of eyes, a proboscis used for feeding, and most species have two clawed appendages (or chelifers) and a pair of pedipalps. Fused to the head is a cylindrical thorax (usually with three segments) where the legs are attached to, and at the end is a short abdomen without any segments. Sea spiders lack respiratory organs and gases are absorbed/released by diffusion. They usually feed on sessile marine invertebrates. There is much dispute over whether the sea spiders should be a chelicerate or a separate lineage of their own.

C) Subphylum Hexapoda

Hexapods (subphylum Hexapoda) are arthropods with six walking legs. Their body is divided into three main parts - the head, the thorax and the abdomen. The legs are attached to the thorax. Most species have a pair of antennae on their head. They usually reproduce by sexual reproduction, though some species are known to reproduce by parthenogenesis.

Here are the hexapod groups that can be seen in Singapore.

1. Insects (Class Insecta)

Insects (Class Insecta)
Insects (class Insecta) form the largest group of animals on Earth, with more than a million species being described. Like other hexapods, they have three pairs of legs, and the body is made up of a head, thorax and abdomen. They are distinguished from other hexapods by their external mouthparts. Many insects also have wings, and they are the only group of arthropods capable of powered flight. They mostly reproduce by sexual reproduction, with the male either inserting its reproductive structure directly into the female's reproductive parts, or depositing a sperm package to be picked up by the female.

2. Non-insect Hexapods (Class Entognatha)

Springtails (order Collembola)
Non-insect hexapods (class Entognatha) include the springtails (order Collembola), Diplurans (order Diplura) and Proturans (order Protura). They are distinguished from the insects by having their mouthparts enclosed within a pouch on the underside of their head. They also lack wings, and some species may even lack eyes and antennae. The above photo features a springtails about 3mm long. Most springtails have a tail-like structure folded beneath the body which snaps against the ground to fling the animal into the air when threatened. They mostly feed on decaying organic materials, though some are carnivorous. They usually reproduce sexually, with the males depositing sperm packages to be picked up by the females. The eggs are laid in the soil or leaf litter, and upon hatching, develop quickly into adults, and continue to moult after reaching adulthood.

C) Subphylum Crustacea

Crustaceans (subphylum Crustacea) are arthropods with two pairs of antennae, compound eyes on stalks, and an exoskeleton that is usually reinforced with calcium carbonate. For most species, the head and the thorax are covered with a shield-like structure called a carapace, and the abdomen ends off with a tail-like structure called a telson. They often have a variety of appendages for feeding, walking, swimming, mating and sensory purposes.

Most crustaceans reproduce sexually, though some are known to reproduce by parthenogenesis (the embryos develop without fertilisation). A number of species are hermaphroditic (possessing both male and female reproductive parts), and some may change sex during the course of their lives. Some aquatic species broadcast their fertilised eggs into the water column, while others may carry the eggs until they are ready to hatch, or attach the eggs to rocks, plants or other objects. For aquatic species, the eggs will hatch into larvae that usually appear very different from the adults. For terrestrial species like the woodlouse, the young may appear similar to the adult but has fewer legs.

A wide variety of crustaceans can be found in Singapore, but only a few groups are readily seen and easy enough to distinguish from others. Hence, I will only include the few crustacean groups that I have photographed in Singapore here.

1. Malacostracans (Class Malacostraca)

Malacostracans (class Malacostraca)
Malacostracans (class Malacostraca) have bodies made up of three main parts - a head with five segments, a thorax with eight segments, and an abdomen with 6 segments. The head may be fused with the thorax to form a cephalothorax. They often have a tough exoskeleton strengthened with calcium carbonate, and the carapace (if present) covers the gills but not the abdomen. Most species have stalked eyes. They have prominent antennae, and up to three pairs of appendages may be modified into mouthparts for manipulating food. The tail usually has a flattened tail fan. Most malacostracans have separate sexes, though a number of species are hermaphroditic (possessing both male and female reproductive parts).

2. Barnacles & Allies (Class Maxillopoda)

Barnacles (infraclass Cirripedia)
Barnacles (infraclass Cirripedia) and several tiny crustaceans such copepods and fishlice made up the class Maxillopoda, of which the barnacles are the most readily seen on our seashores. Most barnacles appear like miniature volcanoes or limpets attached to rocks and other hard structures. During their larva stage, however, they resemble small shrimps with a segmented body and jointed legs. The larva eventually finds a suitable substrate and attaches itself permanently to it with a cement-like substance it produces, and secrete a conical shell around it. There is a hole at the top of the shell, which allows the animal to extend its hairy appendages to filter plankton from the water during high tide. During low tide, the animal will hide in the shell and block the opening with a trapdoor-like operculum. Not all barnacles have this volcano-like appearance, and some species are stalked - appearing somewhat like a germinating seed. Some barnacles are parasitic, and have no legs and shells.

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