Compiled as a final-year zoology student at the University of Edinburgh, based on the information given in lectures.
Characteristics of the phylum
Open circulatory system - main body cavity is a haemocoel.
Body segmented, with specialisation of particular segments (tagmatisation), and jointed appendages.
developed chitinous exoskeleton,
with growth by moulting (ecdysis).
bears a pair of lateral compound eyes
and one to several simple median eyes.
Nervous system with dorsal brain (cerebral ganglia) and paired ventral
nerve cords with ganglion in each segment.
cilia absent (except for sperm of a few groups).
Major groups of arthropods
Trilobitamorpha - trilobites and their relatives (extinct).
Crustacea - 5-segmented head and long body; generally have cephalic
shield or carapace; appendages primitively biramous; lifecycle usually involves
a nauplius larva. Includes (amongst other groups):
Maxillopoda - ostracods, copepods, barnacles, etc.
Malacostraca - decapods (e.g. crabs), krill, isopods (e.g. woodlice),
amphipods (e.g. Gammarus), etc
Chelicerata - body composed of anterior prosoma (cephalothorax) and
posterior opisthosoma (abdomen), prosoma bears chelicerae, pedipalps and 4
pairs of uniramous walking legs; suctorial feeders (no jaws); breathe with book
lungs, book gills or tracheal tubes. Consists of:
Merostomata - extinct sea scorpions (eurypterids), horseshoe crabs
Arachnida - spiders (Araneae), harvestmen (Opiliones), scorpions,
pseudoscorpions, ticks and mites (Acari), etc
Pycnogonida - sea spiders - weird and poorly known, related to
Myriapoda - body is a line of more-or-less similar trunk segments,
lacking clear tagmatisation. Consists of:
Progoneata - millipedes (Diplopoda etc) - herbivorous
Chilopoda - centipedes - carnivorous
Hexapoda - insects and their relatives. Consists of:
Apterygota (Ametabola) - primitively wingless. Includes: bristletails,
Exopterygota (Hemimetabola) - wings develop externally, no pupal stage
in life cycle. Includes: palaeopteran types with no wing-folding ability
(mayflies, dragonflies), orthopterid types with mandibles (stoneflies,
cockroaches, termites, stick insects, earwigs, etc), and hemipteroid types with
sucking mouthparts (bugs, lice, etc).
Endopterygota (Holometabola) - wings develop internally, full
metamorphosis. Includes: scorpion flies, lacewings, butterflies (Lepidoptera),
caddis flies, true flies (Diptera), fleas, bees/ants/wasps (Hymenoptera),
beetles (Coleoptera), etc.
Patterns in arthropod diversity
Some groups have survived, while others have
perished. Were trilobites a poor
Variation in diversity of form. Why do crustaceans and chelicerates show more
morphological diversity than insects? They are more ancient? They have more
limbs and segments to modify? (But also true of myriapods, which aren't
diverse.) They inhabit a wider range of niches?
Variation in biomass. Why is there such a high crustacean biomass? Habitat
productivity, habitat availability (oceans are vast and three-dimensional),
habitat diversity (crustaceans can survive in temporary pools etc).
Variation in species richness. Why are insects and mites more species rich than
crustaceans? Greater variety of habitats on land than in water. Adaptation to
specific hosts (Fahrenholz's Rule - host diversity drives parasite diversity) -
terrestrial plants are structurally complex, and eating them may also require
specific detoxification mechanisms. Additional reasons for insect diversity:
more localised niches (no planktonic larval dispersal), ability to locate
dispersed habitat patches (flight), ability to cope with concealed resources
(underground, inside other organisms), holometabolous life cycles (allow adults
and larvae to exploit different niches).
Variation in range of habitats occupied. Why are there hardly any marine insects? Insects
can cope with salt lakes etc on land. Insect adaptations (e.g. wings) are
inappropriate in the sea? Hard to prove. Crustaceans already occupy marine
Hormones, moulting and metamorphosis
(Nijhout 1994, Insect hormones)
Ecdysones, steroid hormones secreted by the prothoracic gland, drive
the moulting cycle. Ecdysone release is triggered by release of PTTH from corpus cardiacum of brain
(release is under neural control).
Metamorphosis appears to have evolved once - Holometabola are monophyletic (Truman & Riddiford 1999).
Juvenile Hormone (JH) is secreted by corpus allatum of brain. Absence
of JH during a sensitive period leads to metamorphosis. Artificial injection of
JH can keep insects in larval state.
There is a
delay between achievement of critical weight and pupation, due to time taken
for JH to be cleared. JH inhibits action of PTTH during
final larval instar.
Photoperiodic gates may be associated with hormone release, so it can only
occur at certain times of day.
Moult cycle is tied to growth - insects can be artificially kept in larval
stages by restricting food intake.
hemipteran bug Oncopeltus, moulting
occurs when a critical weight is reached (Nijhout 1971).
Restricted feeding experiments on the moth Manduca - if size is below critical threshold at end of 'final'
larval instar, extra instars result (Nijhout 1975). Contrast with Drosophila, where there are always three
Hemiptera, abdominal stretch provides a measure of growth - artificially
inflating bugs triggers moulting. In Lepidoptera and Diptera, nutrient levels
appear to be the cue.
undergo diapause in response to short day length, although exact mechanism
varies between species.
In Manduca, brains
removed from larvae, exposed to long photoperiods in vitro, and implanted into short-day larvae 'reprogram' them into
long-day larvae (Bowen et al 1984).
blowfly, Calliphora, diapause in larvae depends upon
photoperiod experienced by the mother. Critical day length is higher in
populations from more northerly latitudes (McWatters
& Saunders 1996).
The ability to develop
in different ways depending upon the conditions experienced during development...
Seasonal diphenism in butterflies - in Araschnia levana, short-day larvae diapause and emerge as red
spring morphs, long-day larvae don't diapause and emerge as black & white
summer morphs. Experiments involving parabiosis and ecdysone injection show
that timing of ecdysone release determines which morph develops. In other
butterflies (e.g. Polygonia),
seasonal diphenism is controlled by a brain-borne 'summer morph producing
hormone' rather than by timing of ecdysone release (Koch & Buchmann 1987). The butterfly Precis coenia has beige long-day morph (linea) in spring/summer and
reddish-brown short-day morph (rosa) in autumn;
temperature as well as photoperiod determines which morph develops (Rountree & Nijhout 1995).
Horn polyphenism in dung beetle Onthophagus taurus - larger males have disproportionately large
horns (for fighting), small males have tiny, useless horns. Metamorphosis
occurs when food runs out; body size at metamorphosis determines whether large-
or small-horned form develops. Horn development triggered by JH at a critical
period - demonstrated by artificially applying methoprene, a JH analogue (Emlen & Nijhout 1999).
Soldier determination in ant Pheidole bicarinata – development as a soldier depends on JH and on
a critical body size. Addition of methoprene to larger larvae causes them to
develop into soldiers; addition of methoprene to smaller larvae does not
(Wheeler & Nijhout 1983). Nursing of larvae by soldiers, rather than by workers,
inhibits them from becoming soldiers themselves, apparently by a pheromonal mechanism
(Wheeler & Nijhout 1984).
Functions of exoskeleton: acts as a template for body form, facilitates
levers for locomotion, allows development of hard structures such as jaws, and
reduces water loss.
Annelids are segmented like arthropods (though not closely related),
and can make sclerotised appendages (e.g. jaws), but their bodies never became
entirely hardened due to limitations of hydrostatic skeleton.
Dyar's law - amount of growth at each moult is predictable: instar
number is proportional to log(body dimension).
Przibram's law - at each instar weight doubles and dimensions increase by
1.26 times (assuming isometric growth).
Exoskeleton is like a strong tube. Increasing diameter increases strength, but also
weight, limiting arthropod size.
Layers of cuticle: basement membrane (with epidermal cells), procuticle, and
Procuticle is made of criss-crossed strands of chitin (a
polysaccharide), with high flexibility and tensile strength, and protein
(arthropodin), tanned to form sclerotin. Tanning occurs from outside in,
resulting in hard exocuticle and plastic endocuticle. At hinges in exoskeleton,
exocuticle is absent and resilin (elastic protein) gives flexibility.
Epicuticle comprises inner and outer layers of cuticulin
(protein/lipid mixture) covered by waterproof wax layer (secreted by epidermal cells
through pore canals) and cement layer (secreted by extensions of dermal glands).
Stages of moulting: apolysis (splitting of epidermis from base of old cuticle),
ecdysis (shedding of cuticle).
Inactive moulting fluid fills gap between old
and new cuticle while epidermis lays down cuticulin layer of new cuticle; fluid
is then activated and digests old endocuticle. Insect expands by swallowing air
or water (haemocoel may have evolved to facilitate this); old exocuticle and
epicuticle rupture along ecdysial lines
(where exocuticle is thin) and are shed. New cuticle is sealed, waterproofed,
and finally sclerotised.
triggered by eclosion hormone (EH). Sclerotisation of cuticle is also driven by
waxes lose much of their waterproofing ability when a certain temperature is
of gut and tracheal tubes are also lined by cuticle and must be shed during
Nutrition, growth and body size
(Nijhout 2003, Stern
Metamorphosis in Manduca is triggered
by drop in JH levels when critical body size is reached. Lab populations of Manduca kept since 1970s have increased
in size, due to greater growth in final larval instar (higher critical weight,
longer time taken to clear JH, and higher growth rate). Why? Inadvertent selection
by lab technicians for body size? Release from constraints imposed by natural
(e.g. Drosophila) won't pupate at all
if they fail to reach a critical body size; others (e.g. Manduca) will eventually metamorphose into undersized adults if
Artificial selection can increase body size in Drosophila,
with increased critical weight and increased delay between achievement of
critical weight and metamorphosis. Increase is in cell number rather than cell
Control of imaginal disc growth in Drosophila
is largely intrinsic to the disc - it grows to a predefined size. There is also
extrinsic control by insulin-like
proteins, whose production depends on nutrient levels (Ikeya
et al 2002).
feedback from the growing imaginal disc to the rest of the body - delaying disc
growth (by removing parts and forcing regeneration) will delay pupation of the whole
lepidopteran imaginal discs in vitro requires the insulin-like
(Nijhout & Grunert 2002).
Drosophila, like many ectothermic animals, grows more slowly but
ultimately larger at low temperatures. Natural selection under cold temperatures
produces cool lines that develop
faster and larger at normal temperatures. Experiments suggest these differences
are adaptive (cold types are fitter under cold conditions and vice versa).
Latitudinal clines in size of Drosophila
are found in Australia and South America. In Australia (where spread may have
been from north to south so selection was for increased size) the difference is
mostly in cell number; in South America (where selection may have been in both
directions) it is in both cell size and number.
Size evolution may reflect trade-offs - restricted feeding experiments show that
growth is more efficient at lower temperatures, so low-temperature populations
incur fewer costs in growing larger (Robinson & Partridge 2001).
Butterfly wing patterns
Seasonal polyphenism - in Pontia occidentalis,
spring morph darker than summer morph; experiments (correlation between
melanism and survival, rearing butterflies with wrong photoperiods, painting
butterflies) suggest this is related to thermoregulation (Kingsolver 1995). In Bicyclus anynana, wet
season morph with prominent eye spots develops in warm conditions; camouflaged
dry season morph develops in cooler conditions (Brakefield
Eye spot colour develops in response to signal emitted from centre (focus)
- grafting experiments demonstrate this (McMillan et al 2002).
Artificial selection can produce variation in eye
spots. There is a correlated
response between different eye spots, although this correlation can be broken
by deliberate selection.
Muuml;llerian mimicry in Heliconius - H. erato
and H. melpomene
have different morphs in different regions of South America. Local morphs of the two species mimic one another.
Capture-release experiments show that this mimicry is linked to survival.
Colour patterns are due to 20 tightly-linked genetic loci. Where H. melpomene
coexists with closely-related H. cydno, its mating preferences are stronger - selection
against hybridisation (Jiggins et al 2001).
in Heliconius either arose
historically when habitat was split into refugia, or results from one species
adapting to local environments and the other arriving later and mimicking it.
Phylogenies of H. erato and H.
melpomene are different, and colour morphs do not
correspond to genetically distinct races, suggesting the latter hypothesis.
Divergence in H. melpomene is more recent, so maybe it was the newcomer.
eyes are rhabdomeric, consisting of many ommatidia.
cells have many microvilli, carrying
visual pigment molecules.
Visual pigment consists of a chromophore (a Vitamin A aldehyde derivative)
bonded to a protein (opsin). Photon capture alters the isomeric status of the
chromophore molecule from an 11-cis (bent)
conformation to an all-trans
(straightened) form, starting a cascade of reactions leading to perception of
compound eye: Apposition eyes, with
photoreceptors directly under the lenses, allow the formation of an (inverted)
image. Superposition eyes, with
photoreceptors at the bottom of the ommatidia (separated from the lenses), are
good in low-light conditions and are used by night-flying insects. Neural superposition eyes, containing
optically-isolated ommatidia in which photoreceptors all point in the same
direction, allow the formation of complex images, and are found in dipterans.
arthropods also have simple eyes (ocelli).
have hexagonal lenses, because this
shape packs together most efficiently. Crustacean eyes are composed of square
units, each side of which is a shiny mirror (mirrors work better than lenses
ommatidium of a dipteran eye has a set of 8 photoreceptors, which are sensitive
to different wavelengths, allowing perception of different colours (including
Visual pigment is bistable, with visually-active xanthopsin (similar to
rhodopsin in vertebrate eyes) present under dark-adapted conditions, and
inactive metaxanthopsin predominating under light-adapted conditions.
from active to inactive pigment is triggered by blue light; reactivation of
pigment is triggered by red light (Wright & Cosens
1977). This process is much faster than the reactivation of pigment in
vertebrate eyes (which is enzyme-catalysed), allowing insects to cope with a
rapidly-changing visual-environment (e.g. when flying through a forest), and
permitting a high flicker-fusion rate
(which requires a high level of active visual pigment). Red pigment in eyes encourages reactivation of visual pigment
absorption of light occurs when plane of polarisation is parallel to long axis
of pigment molecule.
eyes pigment molecules are parallel to long axis of microvilli, permitting sensitivity to polarisation.
ommatidia, eight retinular cells are twisted 180° along their length (so have
no overall polarity-detection ability), but a ninth is not (Wehner
et al 1977). However, this turns out not to be the polarisation detector.
In bees and Cataglyphis ants, polarisation is
detected in the dorsal rim area of the eye, where there is a two-channel system
- some (UV-sensitive) cells respond most strongly to light with a particular
plane of polarisation, and others to light with a different plane (Labhart 1980).
Endothermy and temperature regulation
(Heinrich 1993, The hot blooded insects)
(constant body temperature) and poikilothermy (body temperature varying with
environment) are states. Endothermy (generating heat internally) and ectothermy
(relying on external heat sources) are ways of achieving those states.
birds maintain a constant temperature (36-40°C) by catabolism in the liver and
in brown adipose tissue. Smaller mammals and birds lose heat faster; they
compensate by having higher proportion of brown adipose tissue, elevated
metabolic rate, and periods of torpor in which they thermoregulate at a lower
vertebrates have a minimum size of around 2g. Large insects are capable of
thermoregulation despite being much smaller than this.
in insects allows high-performance activities (flight, singing in katydids and
cicadas, dung-rolling in competitive dung beetles) to be conducted over a range
of environmental temperatures. When not performing these activities, insects do
not thermoregulate, yet are not totally inactive, unlike torpid mammals or
insects maintain a high temperature only within the thorax. The thorax is
relatively spherical and well-insulated (with scales, hairs, or air sacs at
front of abdomen) to minimise heat loss.
up by vibrating their powerful flight muscles while the muscles are
disconnected from the wings. They
may also generate heat by performing futile enzyme cycles, but this is
uncertain. Time and cost of warm-up depend on the ambient temperature; below a
minimum temperature, warm-up will not be attempted.
In bees, a
counter-current heat exchanger at the petiole (waist) keeps warmth in the
thorax when necessary.
down by pulsing haemolymph between thorax and abdomen, transferring heat to the
abdomen where it is lost through an uninsulated 'thermal window'. Bees can also
cool down by 'tongue-lashing' (regurgitating nectar onto tongue where it
of body heat by bees: incubation of larvae, maintenance of colony temperature,
killing hornets - bees surround an invading hornet and heat it to a temperature
that kills the hornet but not the bees (Ono et al 1995).
ability varies between bee species, and within species (e.g. Amegilla sapiens, found at different
altitudes in New Guinea), depending on climate.
which must fly regularly to stock cells with food for young, are usually more
endothermic than males (exception: the carder bee Anthidium, in which males guard flowers territorially and are
bees that deposit larvae in other bee species' nests are less endothermic than
The origin of arthropods
were traditionally thought to be related to annelids, which are also segmented
and have paired segmental appendages. However, there are differences: annelids
do not moult; annelids have ciliated trochophore larvae while crustaceans have
nauplius larvae, most annelids have a coelom while arthropods have a haemocoel.
data has now led to arthropods being grouped with nematodes and related phyla
(which also possess a haemocoel and chitinous cuticle, and moult) into the
Ecdysozoa (Aguinaldo et al 1997). They are not
closely related to annelids, which evolved segmentation independently.
(onychophorans and tardigrades) are the arthropods' closest living relatives.
Arthropods probably evolved from a lobopod-like ancestor.
trilobites, chelicerates, crustaceans and onychophorans are all ancient, dating
back to Cambrian times.
(1977) wrongly grouped insects, myriapods and onychophorans - which share
uniramous limbs - together as the Uniramia. Manton believed that the Uniramia
have jaws derived from whole-limbs while other arthropods have gnathobasic jaws
(derived from a basal limb part). More recent evidence, from expression patterns
of distal-less gene, suggests that
all arthropods have gnathobasic jaws (Popadić et
data shows that the arthropods are monophyletic, and that insects are most
closely related to crustaceans (they are grouped together as the Pancrustacea).
Very recent evidence (Nardi et al 2003) suggests that
the Hexapoda are polyphyletic.
Some group Pancrustacea
with myriapods as 'mandibulates' (Giribet et al
2001); others disagree (Hwang et al 2001).
centipedes, arachnids, crustaceans and the ancestors of insects all invaded the
and malpighian tubules of insects and myriapods are convergent adaptations to
life on land.
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