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Arthropods
form a huge assemblage of small coelomate animals with “jointed
limbs” (hence the name arthro-pods). They exhibit segmentation
of their bodies (metamerism) which is often masked in adults
because their 10-25 body segments are combined into 2-3 functional
groups (called tagmata). They exhibit varying degrees of cephalization
whereby neural elements, sensory receptors and feeding structures
are concentrated in the head region. Arthropods possess a
rigid cuticular exoskeleton consisting mainly of tanned proteins
and chitin. The exoskeleton is usually hard, insoluble, virtually
indigestible and impregnated with calcium salts or covered
with wax. The exoskeleton provides physical and physiological
protection and serves as a place for muscle attachment. Skeletal
plates are joined by flexible articular membranes and the
joints are hinges or pivots made from chondyles and sockets.
Biodiversity
The
main arthropod assemblages include crustaceans (crabs, lobsters,
crayfish, shrimp), arachnids (spiders, scorpions, ticks, mites)
and insects (beetles, bugs, earwigs, ants, bees, termites,
butterflies, moths, crickets, roaches, fleas, flies, mosquitoes,
lice). Most parasitic arthropods belong to 2 main classes:
the 6-legged insects, and the 8-legged arachnids. |
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Insects
have 3 distinct body parts, commonly called the head,
thorax and abdomen. The head has 2 antennae and the
thorax has 6 legs arranged in 3 bilateral pairs. Many
insect species also have 2 pairs of wings attached to
the thorax. Parasitic insect species include fleas,
flies and lice which actively feed on host tissues and
fluids at some stage in their life-cycles. |
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Arachnids
have
2 body parts known as the prosoma (or cephalothorax)
and opisthosoma (or abdomen). The cephalothorax has
8 legs arranged in 4 bilateral pairs and arachnids do
not have wings or antennae. Important parasitic assemblages
include the ticks and mites which bite into tissues
and feed off host fluids. |
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Collectively,
arthropods account for a substantial share of global biodiversity,
both in terms of species richness and relative abundance.
There are over 1,000,000 species of insects and over 50,000
species of arachnids. They are very successful and adaptable
organisms and are capable of forming large populations due
to their rapid and fertile reproduction rates. Many species
are also able to withstand adverse environmental conditions
by undergoing periods of developmental arrest (diapause).
The protection afforded by their exoskeletons allows them
to colonize many habitats and they overcome the problem of
growing larger in a non-expandable exoskeleton by undergoing
periodic moulting (or ecdysis) which is mediated by hormones.
Developmental stages between moults are referred to as instars.
Moulting is a complex process and its timing is mediated by
many environmental and physiological cues. It involves detachment
of the hypodermis from the procuticle, partial resorption
of the old cuticle, production of a new epicuticle, dehiscence
(splitting) of the old cuticle, emergence of the animal, stretching
and expansion of the new cuticle by air and/or water intake,
and then sclerotization of the new cuticle.
Life-cycles
Adult
arthropods are generally small in size, most are visible but
some remain microscopic. Arthropod sexes are separate and
fertilization is internal. A wide range of mating behaviours,
insemination and egg production strategies are involved. In
most species, the egg develops into a larva: i.e. a life-cycle
stage that is structurally distinct from the adult and must
undergo metamorphosis (structural reorganization) before becoming
an adult. This metamorphosis may be complete (involving major
changes during a pupation stage) or incomplete (involving
gradual changes in nymph stages). For example, the grub-like
larval stages of flies and fleas form cocoon-like pupae where
they undergo complete metamorphosis and emerge as radically-different
adult insects. In contrast, the larval instars (or nymphs)
of lice, ticks and mites undergo incomplete metamorphosis
through a series of moults gradually becoming more adult-like
in appearance. |
| complete
metamorphosis |
incomplete
metamorphosis |
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| egg----------larva----------pupa----------adult |
egg----------larva----------nymph----------adult |
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Arthropods
are involved in nearly every kind of parasitic relationship,
either as parasites themselves or as hosts/vectors for other
micro-organisms (including viruses, bacteria, protozoa and
helminths). They are generally ectoparasitic on, or in, the
skin of vertebrate hosts. Many species are haematophagous
(suck blood) while others are histophagous (tissue-feeders)
and bite or burrow in dermal tissues causing trauma, inflammation
and hypersensitivity reactions. Infestations are transmitted
from host-to-host either by direct contact or by free-living
larvae or adults actively seeking hosts. |
direct
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host-seeking
(larva or adult parasitic)
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host-seeking
(all feeding stages parasitic)
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Direct
transmission
of infective stages occurs when hosts come into close
contact with each other or share quarters, bedding or
clothing. Larvae, nymphs or adults may cross from one
host to another, while eggs or pupae may contaminate
shared environments. Insects (fleas and lice) and arachnids
(mites) rely on close contact between hosts. |
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Many
adult insects actively seek hosts
in order to feed or lay eggs. Winged insects (mosquitoes,
flies) fly to new hosts to feed while fleas jump onto
passing hosts. Some adult flies (botflies) do not feed
on their hosts but deposit eggs from which larvae emerge
and feed on host tissues and exudates. |
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Tick
larvae actively seek hosts
by climbing vegetation and questing for passing hosts.
Some species complete their life-cycle on the same host
(one-host ticks) while others detach after feeding and
drop to the ground to moult before seeking new hosts
as nymphs or adults (two-host or three-host ticks). |
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Taxonomic
overview
Insects
exhibit extraordinary biodiversity, both in terms of
species richness (numbers of species) and relative abundance
(population sizes). Most parasitic species belong to
three main groups: the jumping fleas (Siphonaptera);
the winged flies (Diptera); and the wingless lice (Phthiraptera). |
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fleas
are
bilaterally-flattened wingless insects with enlarged
hindlimbs specially adapted for jumping (up to 100 times
their body length). Jumping feats are accomplished using
elastic resilin pads which expand explosively when uncocked
from the compressed state. Fleas undergo complete metamorphosis
whereby grub-like larvae form pupae from which adult
fleas emerge. The larvae are not parasitic but feed
on debris associated mainly with bedding, den or nest
material, whereas the adult stages are parasitic and
feed on host blood. There are some 2,500 flea species,
most parasitic on mammals (especially rodents) and some
on birds. They vary in the time spent on their hosts
ranging from transient feeders (rodent fleas) to permanent
attachment (sticktight fleas and burrowing chigoes). |
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flies
and mosquitoes
are
winged insects with two pairs of wings attached to the
thorax and a well-developed head with sensory and feeding
organs. They undergo complete metamorphosis involving
a pupation stage. Different species vary in their feeding
habits, both as adults (parasitic or free-living) and
larvae (parasitic or free-living). There are over 120,000
species belonging to 140 families. Two main suborders
are recognized on the basis of structural differences,
Nematocera (adult stages parasitic, larval stages often
free-swimming) and Brachycera (adult stages parasitic
or free-living, larvae stages often predaceous). |
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lice
are
small wingless insects, dorsoventrally flattened, with
reduced or no eyes and enlarged tarsal claws for clinging.
All lice undergo gradual metamorphosis and there are
no free-living stages. Eggs are cemented to hair/feathers
whereas nymphs and adults cling to hair/feathers. Two
orders of lice are recognized on the basis of their
mouthparts: the Mallophaga (chewing/biting lice) with
some 3,000 species infesting birds and mammals; and
the Anoplura (sucking lice) with 500 species found on
mammals. |
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Many
non-spider arachnids (subclass Acari) are found as parasites
on animal or plant hosts. They belong to two main groups:
the macroscopic ticks and the microscopic mites. Many
species are important in human and animal medicine as
causes of disease or as transmission vectors for other
pathogens. |
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ticks
are
epidermal parasites of terrestrial vertebrates that
may cause anaemia, dermatosis, paralysis, otoacariasis
and other infections (transmit viral, bacterial, rickettsial,
spirochaete, protozoal and helminth pathogens). They
feed mainly on blood and their mouthparts are armed
with small backward-facing teeth to aid in attachment.
All ticks undergo gradual/incomplete metamorphosis whereby
larval and nymphal instars resemble adults. The integument
is relatively thick and respiration occurs via spiracles
(usually only one pair) and trachea. Two major families
of ticks are recognized on the basis of many morphological
features: the Ixodidae (hard ticks with a tough cuticle
and a large anterodorsal scutum) with some 650 species
that infest mammals, birds and reptiles; and the Argasidae
(soft ticks with a leathery integument and no scutum)
with 160 species that infest mainly birds and some mammals. |
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mites
are
microscopic arachnids which undergo gradual or incomplete
metamorphosis. Adults and nymphs have 4 pairs of legs
whereas larvae have 3 pairs. Over 30,000 species of
mites have been described, many are free-living species,
some are plant parasites while others are parasitic
on terrestrial and aquatic hosts. Most parasitic species
feed on skin debris or suck lymph, some burrow into
the skin, some live in hair follicles, and some in the
ear canals. Their mouthparts are variable in form but
the hypostome is never armed with teeth. The integument
is usually thin and three orders are recognized on the
basis of their respiratory systems: the Mesostigmata
with respiratory spiracles (stigmata) near the third
coxae; the Prostigmata (Trombidiformes) with spiracles
between the chelicerae or on the dorsal hysterosoma;
and the Astigmata (Sarcoptiformes) without tracheal
systems as they respire through the tegument. |
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