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Insect Relatives, Panarthropoda – Insect Relatives
Jon G. Houseman
Panarthropopda
The Panarthropoda includes today's arthropods and their close relatives the
onychophorans and tardigrades. Although they may not look like they are related to
each other they share the characteristics of the moulted alpha-chitin cuticle, loss of
external cilia, appendages with terminal claws, and the dorsal ostiate heart. Animals in
the three phyla are all that remain of the panarthropods that first appeared with the
Cambrian explosion; a period when the arthropod body plans was the most diverse. As
new fossils from this period, and those like the Burgess Shale fossils are being reexamined, the diversity of types is increasing and transitional forms that link today's
Crustacea, Chelicerata, and Atelocerata are being identified proving that the group is
monophyletic in its origins.
Onycophora
Onychophorans are found in Australasia, Southeast Asia, Africa, and Central and
South America. Their presence in these widely separated parts of the world is related
to the breakup of the ancient supercontinent Gondwana: as the earth’s tectonic plates
drifted, so did the onychophorans. Where they live today are the remnants of the
ancient continent and provides biological evidence of continental drift.
Onychophorans are commonly called velvet worms, because the many little, tubercles,
or papillae on the surface of the living animal give it a velvety look and their common
name. But this is hard to imagine when you look at a preserved specimen.
Onychophorans live in moist soil and leaf litter and are often found hiding in cracks
and crevices. Their cuticle contains the alpha-chitin typical of all Ecdysozoa, but it’s
thin and is molted in patches. The delicate structure of the cuticle is related to the
underlying sheets of circular and longitudinal muscles, and the hemocoel acts as a
hydrostatic skeleton for the animal’s wormlike movements. The thin cuticle and a lack
of epicuticle waterproofing explains why onychophorans are restricted to moist
environments, where they emerge only at night to feed on small insects that they trap
with secretions from their slime glands.
Onychophorans display unique characteristics that once gave zoologists a reason to
place them at a possible taxonomic boundary between annelids and arthropods,
particularly when annelids were viewed as the sister group to Arthropoda.
Onychophorans have fleshy appendages tipped by a tarsal claw, which may be the
forerunner of the uniramous limb. They molt like arthropods, but their cuticle is
similar to that of annelids. Their metanephridial system that depends on cilia is an
annelid trait, and the absence of cilia in the coxal glands is characteristic of terrestrial
arthropods. The branched tracheal system they use for respiration resembles that of
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primitive insects. The main body cavity is a hemocoel, and a pericardial cavity
surrounds a dorsal ostiate heart. The creation of the taxa Ecdysozoa and
Lophotrochozoa separated the Annelida from the Arthropoda. The onychophorans are
now considered the segmental ancestor, the precursor, of the arthropods, which is why
they are included in the Panarthropoda.
External anatomy
If your specimen is preserved in liquid, let it dry a bit before beginning your
observations. When it dries enough you might be able to feel the velvety texture of the
body wall. Alternatively, completely submerge the specimen under water. Whether
you use a dry or wet specimen, you need the dissecting microscope for your
observations of the external anatomy (Figure 1).
Figure 1 External anatomy of Onychophora
Head
Onychophorans show weak cephalization, and the head’s location is marked by three
pairs of appendages: antennae, mandibles, and oral papillae. The most obvious
appendages are the paired, annulated, unsegmented antennae. Look on the dorsal
surface near the base of the antennae and examine a pair of small eyes and their lenses.
On the sides of the head, the slime glands open through oral papillae that fire sticky
threads that entangle prey or, in the case of some species, harden to form a permanent
trap. The mouth opens on the ventral surface of the head and is surrounded by
peribuccal lobes. Look inside and try to locate the head’s second pair of appendages,
the chitinous mandibles. When an onychophoran feeds, it pushes its mouth against the
meal, and the mandibles tear off pieces of food. The mandibles move from front to
back, not against each other like the mandibles of arthropods. The pieces of food
become mixed with saliva that liquefies the meal before it is sucked into the digestive
tract. The fleshy lobes surrounding the mouth hold tight against the substrate ensuring
that nothing leaks out.
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Trunk
The only visible part of the trunk’s underlying segmental body plan is the fleshy,
conical lobopods extending from the sides of the animal. Although the number of legs
varies between species, the legs move by an internal hydrostatic skeleton, and the tip
of each leg has a pair of chitinous claws similar to those of other panarthropods. At the
base of the claws, locate the spiny pads that grip the substrate and the papillae of the
crural gland below the spiny pad. Note: Depending on the species available for this
lab, the crural gland may not be visible. Look closely under the ventral surface of the
legs where they connect with the trunk. On all but the fourth and fifth pairs of legs you
can see a groove running from the leg’s junction with the body toward its tip. This
segmental groove surrounds the metanephric opening which won’t be visible and
neither will be the small spiracle in the grooves of the annulations that encircle the
body. Look closely at the body surface and examine the tubercles and the sensory
spine that extends from the tip of the tubercle. Do all the tubercles have these
spines? At the posterior tip of the animal is the anus. Depending on the species
available for this lab, the gonopore may be visible where it opens between the last two
pairs of legs.
Tardigrada
Tardigrades are microscopic in size and only a few get any larger than a millimeter or
two in length. When they were first observed crawling up small pieces of vegetation,
how they used their four-paired legs to paw the surface was reminiscent of bears
pawing at their food. Tardigrades are all aquatic and found in both freshwater and
marine environments. The chitinous cuticle covering a tardigrade's body has an outer
epicuticle, formed of cross-linked proteins; middle intracuticle containing lipid; and
inner procuticle, a mixture of chitin and proteins. The cuticle is not waterproofed.
Tardigrades are capable of withstanding adverse conditions by cryptobiosis. The
dormant forms, called tuns, drop to only 3 percent water and load up with trehalose
and glycerol to protect them from ice crystal formation. The tuns can survive
temperatures from -272 degrees C to 151 degrees C, live for over 100 years, and
survive radiation levels 1,000 times what humans can handle. If you're using prepared
slides be sure to look at a number of different individuals. Some will be squished
making it impossible to see any structures, while others may be on their sides, backs or
fronts - take advantage of these different orientations in your observations.
Head
Cephalisation in these animals is weak (Figure 2), and there are no appendages like
those found in other panarthropods. They may have been lost as these unique little
animals adapted to their miniature world. The mouth is located at the anterior tip of
the animals. In prepared slides the stylets of the feeding apparatus should be visible
through the transparent cuticle. Tardigrades feed by piercing their food and sucking
the fluids out using the muscular pharynx that is also visible.
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Figure 2 External anatomy of the dorsal surface of a tardigrade.
Trunk
The trunk consists of four segments covered with sculpted cuticular plates. Four pairs
of fleshy lobe-like legs include three that extend from the sides of the tardigrade and
the posterior most pair extending behind. The lobe-like legs are extended by
hydrostatic pressure and retracted by bands of muscle that extend to the tip of each
leg. The legs terminate in a claw, or in some species an adhesive pad.
Horseshoe Crab - Chelicerata
The arthropod subphylum Chelicerata includes spiders, scorpions, the ancient
horseshoe crabs, and the unusual sea spiders. Almost all chelicerates are predators and
some, the ticks and the mites, have become specialists at miniaturization and exploit
their predatory existence as ectoparasites. Chelicerates are primarily fluid feeders and
either liquefy their prey before ingestion or squeeze the juices from their food using
pedipalps and chelicerae, the feeding appendage that gives the subphylum its name.
Don’t let the common name of the horseshoe crab fool you; this is a very ancient
chelicerate dating back to the Silurian, not a crustacean crab. One species, Limulus
polyphemus, comes ashore on the east coast of North America, and three others inhabit
Asian oceans. These living fossils are all that remain of this ancient chelicerate
lineage. Each spring in North America, when the full moon and new moon create the
spring tides, thousands of these ancient animals come ashore to mate. In the past, the
adults were harvested and used as fertilizer and animal feed. Today, horseshoe crabs
are important in medicine because their blood clots very easily. This property is
applied in the medical and pharmaceutical industry to make products that are free of
bacterial and endotoxin contaminants. Each female lays up to 20,000 eggs, more than
enough to ensure the survival of the species and to fill the appetites of predators that
feed on the eggs. Among their predators is the red knot (Calidris canutus), a migrating
bird that synchronizes its arrival in spring from the southernmost tip of South America
to the horseshoe crab breeding grounds. Feeding on the horseshoe crabs’
proteinaceous eggs helps the red knot produce its own eggs at the end of its migration
to the Canadian arctic.
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External anatomy
The massive, tough exoskeleton of the horseshoe crab is hardened, and its leathery
texture is due to large amounts of a sclerotized protein. exoskeleton. The cuticle
protects the legs and appendages underneath the body when the horseshoe crab crawls
on land to mate or burrows into the soft sand of the ocean bottom. Chelicerates have
two tagmata (Figure 3): the anterior prosoma and the posterior opisthosoma that may
be further divided into a mesosoma and a terminal metasoma. The horseshoe crab has
only a prosoma and an opisthosoma.
Figure 3 External anatomy of the dorsal surface of the Horseshoe carb Limulus.
Some authors use different names for the tagmata. The prosoma can be referred to as
the cephalothorax, and the opisthosoma as the abdomen that may be divided into preand postabdomen. The term cephalothorax is correct from the functional standpoint
because the anterior sensory appendages typically found on the head are combined
with the walking legs. The use of the term can be confusing particularly in reference to
developmental stages: for example, when the cephalothorax results from the fusion of
two tagmata; the head and some segments develop from the thoracic tagma. The first
tagma in the chelicerates is a single tagma, consisting of six paired appendages
(chelicerae, pedipalps, and four pairs of legs), and it is separate from the posterior
tagma that, for the most part, has lost its appendages; if still present these appendages
have become modified for special functions.
The long, spike-like structure at the posterior end of the body is the telson, with the
anus at its base. Why does the position of the anal opening identify this as a telson and
not a tagma?
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Prosoma
The most anterior appendages on the first tagma, the prosoma, are feeding
appendages, known as pedipalps and chelicerae, and four pairs of uniramous walking
legs (Figure 4). Unlike the other arthropod subphyla, chelicerates have no antennae,
and this is reflected internally by the absence of the deutocerebrum. In most
chelicerates, appendages on the opisthosoma have disappeared or been reduced to
structures adapted for gas exchange or spinning silk.
Figure 4 External anatomy of the ventral surface of the Horseshoe carb Limulus.
The dorsal surface of the prosoma (Figure 3) is an enlarged, horseshoe-shaped
carapace that covers the legs underneath and protects the internal organs. The carapace
is formed from the fused tergites of each segment, and its shape gives the origins of
the common name for Limulus, the carapace looks like a horse's hoof. When a
horseshoe crab digs its way into the soft sand, the leading edge of the carapace is
pushed into the sediments, like a shovel, by the last pair of walking legs. Examine the
surface of the carapace and identify the single medial and paired lateral ridges on its
surface. At the anterior tip of the medial ridge, locate the median eyes. These simple
eyes may be hard to see if there is debris or organic matter encrusted on the surface of
the carapace. At the outer edge of the lateral ridge, there is a pair of compound eyes;
their compound eyes may not be homologous to those found in the rest of the
Arthropoda, and this raises the question of whether or not the compound eyes of these
animals are an ancestral symplesiomorphy that is shared with other arthropods.
On the ventral surface of the prosoma (Figure 4), there are six pairs of appendages.
How does this arrangement of appendages on the opisthosoma differ from that of
most of the other chelicerates? The chelicerae are small, consist of only three joints,
and are located in front of the mouth. Behind them are the larger, six-segmented
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pedipalps. Starting at the base of the pedipalp, the segments are as follows: coxa,
trochanter, femur, patella, tibia, and tarsus, with the tibia and tarsus forming the
chelate distal end of the pedipalp. In mature horseshoe crabs, the tip of the male’s
pedipalp is modified into a clasper, formed by the segment becoming thicker and
hook-like, and used to hold onto the carapace of the female during mating. The base of
the pedipalps in both sexes has an inner margin lined with spines, a gnathobase. The
chelate tips of the appendage pass food to the gnathobase, which then grinds it up
before passing it to the mouth. The chelicerae keep food from escaping from the
gnathobases located on other appendages.
The segmentation of the first four pairs of walking legs follows the same pattern as
seen in the pedipalps. The tibia and tarsus forms the chelate tip of the legs, and at the
basal segments of the legs there is a spiny inner surface, the gnathobase. If your
specimen isn’t too brittle, try spreading the legs to see the mouth located at the base of
the legs. Although the fourth pair of legs has a gnathobase and the same number of
segments as the other three pairs, it also has two unique modifications. On the outer
surface of the base of the leg, there is a flattened, spatula-like structure used to clean
debris from the gill surface. The second modification can be found at the tip of the leg:
there are four flattened tarsal plates, the flabella, that help to push the horseshoe crab
as it burrows into soft sand. When a horseshoe crab burrows into the sand, the hinge
between the two tagmata flexes and the middle of the horseshoe crab rises in the
water; only the tip of the telson, the last pair of walking legs, and the front of the
carapace remain in contact with the substrate. While the telson anchors the posterior
end of the body, the fourth pair of walking legs spreads its flabella and pushes the
leading edge of the carapace into the sand. If a horseshoe crab is flipped over, how
does it right itself?
Opisthosoma
The opisthosoma is composed of nine segments. All tergites on the dorsal surface are
fused together, and a medial ridge runs along the midline and two lateral ridges,
which are aligned with the lateral ridges on the prosoma (Figure 3). Along the lateral
ridge, are six pairs of cuticular depressions known as apodemes; here the cuticle folds
internally for the attachment of the muscles. What do the muscles attached to these
apodemes move? Along the lateral margin of the opisthosoma are six pairs of spines.
In combination with the apodemes, these spines identify six of the nine segments of
the opisthosoma.
On the ventral surface (Figure 4) and at the base of the last pair of legs, there is a
moveable cuticular extension called the chilaria, and it's all that remains of the first
segment of the opisthosoma. Behind the chilaria, there are six flap-like plates,
opercula, which are the modified appendages of the tagma; there are no appendages
on the eighth and ninth segments. Lift each operculum and look at what is underneath.
Beneath all but the first, you’ll find book gills composed of gill filaments. The
flapping motions of the opercula pump water over the respiratory surface, and, when
the horseshoe crab is small, the opercular movement helps it swim. The first pair of
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plates is the genital opercula; the paired gonopores can be seen underneath the
midline, about halfway back from the margin of the plate. The anus is located at the
base of the telson.
Scorpion - Chelicerata
Similar to that of the horseshoe crab, segmentation in the scorpion is still visible, and
this feature reveals how long these chelicerates have existed. Scorpions appear in
ocean fossils from over 400 million years ago, and they were some of the first animals
to invade the terrestrial environment, where they fed on soft-bodied invertebrates
living in moist locations. Chronologically, this makes scorpions the first arthropod on
land. However, the insects were most successful at exploiting the terrestrial
environment and, as they diversified and conquered that environment, they became
prey for the scorpions.
Figure 5 External anatomy of the dorsal surface of a scorpion.
Scorpions feed primarily at night on insects and other arthropods, but some of the
larger scorpion species feed on small vertebrates. Their large pincers grasp the prey
and, if the prey is small enough, crush it before they consume it. They immobilize
larger prey by venom from their sting, and then they crush the prey and tear it apart.
Like all chelicerates, scorpions are fluid feeders, and they must liquefy their meal
externally before ingestion extracorporeal digestion. Digestive secretions from the
alimentary tract are regurgitated onto pieces of shredded food in the preoral cavity,
and the base of the mouthparts squeezes the liquefied nutrients from the crushed food.
External anatomy
Like other chelicerates, a scorpion’s body is divided into two tagmata: an anterior
prosoma and a posterior opisthosoma. The opisthosoma is further divided into an
anterior, seven-segmented mesosoma and, behind it, a five-segmented metasoma
with the sting at its tip (Figure 5).
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Some authors use different names for the tagmata. The prosoma can be referred to as
the cephalothorax, and the opisthosoma as the abdomen that may be divided into preand postabdomen. The term cephalothorax is correct from the functional standpoint
because the anterior sensory appendages typically found on the head are combined
with the walking legs. The use of the term can be confusing particularly in reference to
developmental stages: for example, when the cephalothorax results from the fusion of
two tagmata; the head and some segments from the thoracic tagma. The first tagma in
the chelicerates is a single tagma, consisting of six paired appendages (chelicerae,
pedipalps, and four pairs of legs), and it is separate from the posterior tagma that, for
the most part, has lost its appendages; if still present these appendages have become
modified for special functions.
Prosoma
There is no visible segmentation on the dorsal surface of the scorpion (Figure 5), and a
cuticular carapace with four pairs of simple eyes. A pair of medial eyes is located on
the dorsal midline and the other three, or sometimes four, paired lateral eyes are
located on the anterior and lateral edges of the carapace.
Figure 6 External anatomy of the ventral surface of a scorpion.
Segmentation is visible on the ventral surface (Figure 6), where six pairs of
appendages surround a small sternal plate: the chelicerae, pedipalps, and four pairs
of walking legs. The second pair of appendages, the pedipalps, is the largest and has
hardened pincers, called chelae, which are used to capture prey. Starting close to the
body and moving outward, the segments are coxa, trochanter, femur, patella, tibia,
and tarsus. The tibia and tarsus form the chela, and this appendage is described as
chelate because of the pincer-like structure. Don’t confuse this chelate condition with
the chelicera, the mouthpart that gives the group their name, Chelicerata. The chela
consists of an enlarged tibia that forms the manus, which has an immovable fingerlike extension parallel to the moveable finger of the tarsus.
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In front of the pedipalps are the chelicerae, which are considerably smaller and three
segmented. The distal two segments form the chela that is used to rip and tear the food
apart before it is passed into the preoral cavity. The top of the preoral cavity is
formed by the base of the chelicera; the sides by the coxa of the pedipalps; and the
bottom by the base of the first two pairs of walking legs.
The eight-segmented walking legs are attached to the body by the coxa followed by
the trochanter, femur, patella, tibia, metatarsus, tarsus (basitarsus), and distal
pretarsus (telotarsus) with its claws. Take a close look at the coxa of the first two pair
of legs. There is no sternite between them, and the anterior extensions of the coxa
form the gnathobase of the bottom of the preoral cavity. As mentioned previously,
scorpions are fluid feeders and extracorporeal digestion of food occurs in the preoral
cavity, and movements of the gnathobase chew and squeeze the food.
Opisthosoma
Each segment of the mesosoma consists of a dorsal cuticular plate, the tergite, which
is connected by a pleural membrane to the ventral sternite. The most conspicuous
structures on the ventral surface are paired pectines on the second segment (Figure 6).
The pectines, which resemble a comb, have a series of cuticular teeth that are
embedded in a rod attached to the segment. The pectines have a rich nerve supply that
extends into each of the teeth, suggesting a sensory role for the structure; although just
what it detects is still not clear to zoologists. Anterior to the pectines, and on the first
mesosomal segment, are two cuticular plates, the genital opercula (Figure 6),
covering the openings to the reproductive system. You can identify the sex of your
specimen by the opercula: two separate plates in females and a single, fused
operculum in males. The remaining mesosomal segments have no appendages, but the
ventral surface of segments three to six have paired spiracules opening to the book
lung (Figure 6).
Each of the five segments of the metasoma is formed from the fused cuticle of the
dorsal tergite and ventral sternite; there is no pleural membrane. The sting, located at
the tip of the metasoma, is not a segment; it is the telson because of its position behind
the anus (Figure 5). The sting consists of a hollow bulb and barb, and the poison gland
located in the bulb releases the toxin through an opening at the tip of the barb. The
anus is located in the membranous region between the sting and the last metasomal
segment.
Tarantula - Chelicerata
Although insects dominate the terrestrial environment, their chelicerate cousins were
probably the first arthropods to live on land. Scorpion-like chelicerates preyed upon
the soft bodies of the first terrestrial animals that survived in damp environments, for
example, the early insects and worms. But one group, the insects, would solve the
challenges of the terrestrial environment, particularly the problem of water loss, and,
in combination with the acquisition of flight, the first explosion of insect diversity
occurred. The second explosion of insect diversity resulted from the coevolution of
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insects and flowering plants. It should come as no surprise that as the diversity of
insects increased, so too did the diversity of their predators, the chelicerates. The
earliest spiders used a form of silk to detect prey that bumped into silken trip lines laid
on the ground. Once insects took to the air, the spiders suspended their silk in air and
trapped flying insects. Primitive spiders, like the tarantula, don’t spin webs for
catching prey; they use their silk to line burrows or construct retreats where they wait
and pounce on their prey.
Unlike other arthropods animals in the subphyla Chelicerata don’t have antennae, and
the part of the brain, the deutocerebrum, that normally integrates sensory input from
the antennae is missing. With the exception of the primitive horseshoe crabs, most
chelicerates are also missing compound eyes. However, this does not mean that they
are blind; usually four pairs of simple eyes (ocelli) sense light levels, and in some
spiders these eyes are large enough to form images. Instead of vision, spiders use
touch and vibration as their primary sense. Special slit sensilla on their legs and
sensory setal hairs, trichobothria, detect all forms of vibration – from the footsteps of
prey moving across the ground, to air currents created by approaching prey, to sounds,
and, of course, to the struggle of prey trapped in their webs.
Figure 7 Dorsal view of the external anatomy of a spider.
External anatomy
The chelicerate body is composed of two tagmata: an anterior prosoma and a
posterior opisthosoma (Figure 7). One of the first things you will notice about a
tarantula is its hairy covering of chitinous setae. Like the fur of mammals or the
feathers of birds, the dense mat of setal hairs creates a dead pocket of air against the
body, which acts as an insulating layer and may also be important in preventing water
loss across the book lungs, a tarantula’s main respiratory organ. As mentioned earlier,
the setal hairs are also sensory and may be important for spiders to escape their
predators. A tarantula’s first line of defense is to withdraw into its burrow, but if
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captured, the setal hairs easily detach, leaving the predator with a mouthful of hairs
rather than a meal. Tarantulas in North and South America take this setal defensive
strategy a step further: they have barbed, setal urticating hairs on the abdomen that
they fire at their predators. What mammal throws barbed hairs as a defensive strategy
against predators? If the urticating hairs come in contact with, and embed in, sensitive
membranes of the eyes, nose, or skin, they can cause a reaction: this can be a mild itch
or a severe reaction, particularly if the setal hairs enter the eyes or respiratory
passages. Once fired, the urticating hairs can only be regenerated at the next molt, and
a “bald” tarantula is one that has used this defensive strategy. Tarantulas, like all
spiders, have venom; however, the venom doesn’t affect humans other than possibly
causing an allergic reaction, and its purpose is to subdue the small insects and
vertebrates that spiders feed on.
Prosoma
There is little or no evidence of the ancestral segmentation on the dorsal surface
(Figure 7) of the tarantula, and a dorsal carapace covers the prosoma. Beneath the
hairs on the carapace you may see depressions and indentations on the surface; these
identify where the underlying musculature of the sucking stomach is located. On the
dorsal surface in front of the carapace, locate the four pairs of simple eyes arranged in
two rows on the surface of the optical tubercle (Figure 8). Some of the eyes look
forward; others look up or to the side. The position and size of the eyes varies in
spiders, but there are nearly always eight. Are all eight eyes in the tarantula the same
size?
Figure 8 Anterior view of the chelicerae and eyes of a spider
The prosoma has eight pairs of appendages attached to it: chelicerae, pedipalps, and
four pairs of walking legs. The most anterior pair of appendages is the chelicerae,
which have two segments each: the fang and the large basal segment that attaches the
chelicera to the prosoma (Figure 8). The poison glands, which produce a mixture of
digestive enzymes and neurotoxins, are located in the basal segment. A duct carries
the venom to an opening at the tip of the fang. As already mentioned, tarantulas are
primitive spiders, but another reason for placing them at the base of the evolutionary
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tree for spiders is the way the chelicerae articulate with the prosoma. In the advanced
species the fangs swing out from the side of the swing back in to meet along the
midline of the body. In the tarantula the fangs swing forward, not to the side, and as
they come back towards the body they pull in their prey and squeeze it against the
prosoma.
Behind the chelicerae are the large six-segmented pedipalps. Examine the ventral
surface of the pedipalps where they are attached to the body. The large basal segment
is the gnathobasic coxa (Figure 9) that covers the mouth and is equipped with sharp
edges used to crush and chew the food. Chelicerates are fluid feeders, and, as they
chew their food they regurgitate digestive juices onto it. After a period of time the
food is squeezed, and the liquid digestive soup in the extract is consumed. The process
is repeated until there is nothing left to suck up and the remaining husk of indigestible
material is discarded. What is the name given to this type of digestion?
Figure 9 Pedipalp of a male spider.
The pedipalps are sexually dimorphic in spiders. In females they look like the other
legs, except for the missing metatarsal segment. In mature males the terminal end is
expanded into a complicated bulblike structure called the cymbium, which consists of
two parts: the bulb and the embolus, which function together like an eyedropper
(Figure 9 and Figure 10). Mature males spin a small sperm web on which they deposit
a drop of sperm from their genital aperture. They then dip the end of the embolus into
the drop of sperm and draw it up into the bulb for later use. Spiders mate face to face:
the male places the embolus into the female’s genital aperture and releases sperm into
the spermatheca (seminal receptacle) inside.
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A large sternal plate covers the ventral surface of the prosoma (Figure 11). Around its
margins is the pleural membrane that connects the dorsal carapace and ventral
sternum. The pleural membrane is hard to see because it surrounds the coxal joints of
the four pairs of walking legs. Each of the walking legs has seven segments: the coxa
Figure 10 Detail of the pedipalp of a male spider.
next to the body, the trochanter, femur, patella, tibia, metatarsus, the tarsus, and
tarsal claws at the tip of the leg.
Opisthosoma
Although the membranous cuticle of a spider hides them, the opisthosoma has twelve
segments consisting of dorsal tergites connected by pleural membranes to ventral
sternites. The first segment of the opisthosoma is modified into the waistlike pedicel
that connects the opisthosoma to the prosoma (Figure 11).
Figure 11 Ventral view of the external anatomy of a spider
Locate the openings to the two pairs of book lungs on the ventral surface (Figure 11)
of the opisthosoma; they appear as slits behind the second sternite. The book lungs are
formed from sheets of thin cuticle, lamellae, arranged like the pages of a book, which
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gives the origin of the structure’s name. Book lungs are not the most efficient gas
exchange surface for a terrestrial animal because the large surface area exposed to air
is a potential surface for loss of water. Housing the lungs inside a cavity and
underneath a dead layer of air created by the setal hairs helps to minimize water loss.
These structural adaptations to prevent water loss are enhanced by the tarantula’s
behavioral patterns of nocturnal hunting and daytime resting in a burrow. Most spiders
have a single book lung and a tracheal system, and it is assumed that the use of trachea
helps to overcome the problem of respiratory water loss. Tarantulas retain the original
two pairs of book lungs, and this is another reason they are considered primitive
species. The epigastric furrow in the cuticle follows the opening of the first pair of
book lungs. A cuticular plate, the epigynum – located between the openings to the
lungs and in front of the epigastric furrow – contains the female genital pore. When
spiders mate the male inserts the tip of the embolous into the opening and fill the
seminal receptacles.
At the posterior end of the opisthosoma are the spinnerets and the anal papillae.
Unlike most spiders that have a set of six or more spinnerets, tarantulas have between
two and four depending on the species. The spinnerets origins are thought to be the
appendages that were originally on these segments. Locate the anal slit on the tip of
the anal papillae.
Crayfish – Crustacea
Crustacea are usually marine species. However, around the world crayfish are
common in unpolluted swamps, streams, and the edges of lakes. Although they are
small, crayfish are some of the biggest crustaceans in freshwater environments, where
they hide in crevices between rocks or under debris in the water. In their northern
range, they live in water that doesn’t completely freeze during the winter. Their
common name can be crayfish, crawfish, crawdad, or mudbug, depending on where
they are found around the world, but they have similar morphology and are included in
three taxonomic families. The family Cambaridae, which includes the animal you are
most likely looking at, is one of more than 300 species in North America. Crayfish are
raised as food or bait on farms in the southern United States, and this is the source of
your specimens. Crayfish, like their marine cousins the lobsters, are omnivorous
scavengers that feed on any form of organic material. This observation guide can be
used for the dissection of either a crayfish or lobster.
External anatomy
The present arrangement of the tagmata in crayfish is different from the ancestral
crustacean pattern. The earliest crustacean had a head, with antennae and feeding
appendages, and a trunk with legs on each of its segments. Each leg was identical and
used for swimming and filtering particulate food from the water; movement of the legs
facilitated gas exchange across the surface of the leg. This ancestral pattern changed
when the legs became specialized for different functions, and this resulted in
modifications to the tagma to which they are attached. One of the early changes was
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the division of the trunk into a thorax with legs and an abdomen without. In some
crustaceans, the crayfish for example, the anterior walking legs on the thorax
combined with the original feeding appendages on the head to assist with feeding.
This created a division in the thorax: the part fused with the head became the
cephalothoracic tagma, the remainder of the thorax became the pereon, and the legs
attached to the pereon became the pereopods. The abdomen is also referred to as the
pleon, and the small appendages on it, the pleopods.
General anatomy
As in all arthropods, the outer covering of the crustacean body is a hard, chitinous
cuticle secreted by an underlying epidermis (hypodermis). Identify the two tagmata of
the body: the anterior cephalothorax and the abdomen (Figure 12).
The cephalothorax is covered by the carapace, a sheet of cuticle that covers the dorsal
and lateral sides of the body. A sharp rostrum extends between the eyes, and the
cervical groove on the dorsal surface of the carapace marks the division between the
head and thorax (pereon). Behind the cervical groove, the carapace extends over the
gills to form the branchial chamber. Locate the compound eyes on stalks; the head
appendages, including two pairs of antennae and the mouthparts; and five pairs of
walking legs (pereopods or thoracopopds)), the first of which is modified into
pincer-like chelipeds (Figure 12). You will look at the appendages in more detail in
the next part of the lab.
Figure 12 External anatomy of the crayfish or lobster
The structural organization of the abdomen is simple. Each of the six segments
consists of two cuticular plates: the overlapping curved dorsal tergites, and the ventral
sternites connected to each other by articulating membranes. Swimmerets (pleopods)
are found on the first five abdominal segments, and the first two pairs are dimorphic
and modified in the male. Locate the anus in the center of the telson.
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Anatomy of the appendages
As mentioned earlier, in the ancestral crustacean every segment had an appendage,
and all those behind the head looked the same. However, the appearance of the
appendages changed when they took on more specialized functions. As a result, the
appendages on each segment share a similar embryological origin, but have different
functions. This is the definition of homology, and because the homologous
appendages are arranged in a linear sequence along the length of the animal, this is
referred to as serial homology.
Figure 13 Appendages of the head and maxillipeds
The appendages are all formed according to a biramous plan, which is an
autapomorphy for Crustacea. Each appendage is attached to the body by a basal
protopodite and has two branches: the inner endopodite and the outer exopodite. The
protopodite may have cuticular extensions on the inner surface, endites, or on the outer
surface, exites. Each component of the appendage may have become enlarged,
reduced, added to, fused, or even lost as the appendage specialized for the different
functions of food gathering, locomotion, or respiration. Each function requires
different modifications, and as a result the appendages appear different; however, the
serial homology is maintained. In this part of the lab, you will compare the crayfish
appendages to the original biramous plan, and keep track of the different
modifications. The appendages overlap in certain areas of the body, but you won’t be
able to sequentially remove them in order from anterior to posterior. To successfully
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remove the appendages, use blunt-ended forceps to firmly grab the base of the
appendage, wiggle it, and pull. Don’t use fine-pointed forceps because they will cut
through the appendage, and the parts you want to look at will remain attached to the
body of the crayfish.
Cephalothorax
Figure 14 Pereopods and abdominal appendages.
The cephalic region of the head has five appendages (Figure 13 and Figure 14), and
the most anterior are the antennules (first antennae), followed by the large antennae
(second antennae). Behind the mouth are located a single pair of mandibles and two
pairs of maxillae. You won’t see the postoral appendages until you remove the
maxillipeds later on.
Remove the antennules, each consisting of two filaments attached to a threesegmented protopodite. The antennule has a statocyst, which can be seen externally as
a small triangular depression on the dorsal surface of the segment. Carefully remove
the antennae. The second antenna has a single sensory filament, a modified
endopodite, and a flattened plate formed from the exopodite. The opening from the
antennal gland is located on the ventral side coxopodite and appears as a small pimple
or bump on the surface of the cuticle.
Maxillipeds are thoracic legs involved in feeding, and they are stacked on top of the
two pairs of maxillae, and all of these cover the mandibles. The maxilliped at the top
of the stack is the third maxilliped, and it protects the appendages underneath.
Remove the third maxilliped and be sure you get the gill that is attached to its inner
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surface. The third maxilliped best demonstrates the structure of the crustacean limb
with its obvious exopodites, endopodites, and the epipodite, which is partly modified
into a gill. The large chelipeds pass food to the maxillipeds, and the inner endopodite
tears food into smaller pieces that are easier to swallow. The next two maxillipeds are
the second and first, respectively. The second maxilliped looks like a smaller version
of the third and functions in the same way; that is, the movement of exopodites helps
water move through the branchial chamber. The first maxilliped is much smaller,
with a small endopodite and large endites on the protopodite. First maxilla and
second maxilla are original head appendages, and their movement pumps water and
the small food particles created by the maxillipeds into the mouth. Only the second
maxilla retains a part of the exopodite, and it forms the gill bailer, known as the
scaphognathite, that moves water through the branchial chamber. The mandible is
hard to pull out; so be sure to get a good grip with your forceps. The mandible consists
of a three-segmented palp, the endopodite, which is attached to the toothed, and the
grinding base formed from the protopodite. This is referred to as a gnathobasic
mandible, an autapomorphy of the Crustacea.
There are five pairs of walking legs: the appendages of the posterior five segments of
the original thorax. The first three pereopods are all chelate with pincers at the distal
end of the appendage; the largest is on the first pair, the cheliped. The chelate
appendages are used for feeding and walking. All five walking legs have lost the
exopodite and appear uniramous, which is a derived or secondary evolutionary trait.
Why aren’t the chelate appendages of the anterior legs considered biramous?
Abdomen
Behind the walking legs are six pairs of abdominal appendages: five pairs of
swimmerets and the last pair of modified appendages, the uropods. In combination
with the telson, the uropods form the rudder-like posterior end of the animal. The
unpaired telson is not an appendage, but it is the terminal part of the body. The first
two swimmerets are sexually dimorphic and are modified as copulatory organs in the
male. What is the sex of your specimen? If you have a female, be sure to look at a
male specimen to see the difference. How is the abdomen used in locomotion?
Centipede – Atelocerata (Chilopoda)
Centipedes and millipedes are myriapods, and as their name implies, they have many
pairs of legs along the length of the trunk tagma. Centipedes live in moist, terrestrial
environments and are predacious, feeding on insects, worms, and small molluscs.
They range in size from 10 to 30 centimeters; some are large enough to prey upon
bats, birds, and small rodents! Unlike their insect cousins, the cuticle of a centipede
lacks waterproofing waxes. The absence of the waxes, combined with a tracheal
system with spiracles that can’t close, explains why centipedes are restricted to moist
environments.
This observation guide is based on specimens of Scolopendra or Lithobius, the species
usually available from commercial suppliers.
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External anatomy
The body of a centipede is divided into two tagmata: the head and trunk. The head,
like all animals in the subphylum Atelocerata (Tracheata), is formed from the fusion
of six segments and has only one pair of antennae. Each of the dorsoventrally flattened
trunk segments is identical, that is, homonomous, and a pair of legs extends from each
segment. The number of segments differs between different species and their common
name implies that centipedes have a hundred legs; in fact, the number can range from
15 to over 175 pairs. Although the number of legs may vary, there is always an uneven
number because the first pair of legs on the trunk has been modified into feeding
appendages, poison claws (forcipules).
Figure 15 External anatomy of the anterior region of a centipede.
Head
The head consists of the typical six segments found in all Atelocerata (Figure 15). The
appendages include the preoral labrum and antennae, and the postoral mandibles
and the first and second pair of maxilla. The antennae, which are the main sensory
organ of a centipede, have two parts: the scape that attaches the antenna to the head
and the flagellum with its numerous articulations. The typical compound eye of
Arthropods has either been replaced with a cluster of simple eyes (ocelli) or
disappeared entirely, and the centipede is blind. The reduction and loss of the visual
system and the sensory importance of the antennae is no doubt related to a centipede's
nocturnal lifestyle. When viewed from the front, a small labrum is located underneath
the cuticle of the cephalic shield that forms the head.
Use the dissecting microscope to look at the ventral surface of the head (Figure 16),
where the most obvious feeding appendages are the poison claws (forcipules).
Technically these are maxillipeds, since they are a trunk appendage used for feeding,
and in poisonous species the poison claws inject venom from the tip of the claw to
subdue prey. In non-poisonous centipedes the poison claws are still effective for
capturing a meal. The coxa and the sternite of the poison claw are fused into a
coxosternite that covers the remaining mouthparts; the anterior edge of the
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coxosternite forms a coxal process with teeth that hold onto the prey before it is
ingested. To see the other mouthparts, you will have to pull the poison claw back and
out of the way.
Figure 16 Ventral view of the centipede head.
As in all of the Atelocerata, the postoral segments include a mandible and two pairs
of maxillae, each with a jointed outer maxillary palp. Starting from the back and
working forward, the second pair of maxillae is fused at its base to form a broad
structure similar to the labium in insects; this forms the floor of the buccal cavity, and
a three-segmented sensory palp is attached to the fused basal segment. The first
maxilla has two lobes. Unlike the small fleshy inner lobe, the outer lobe is a modified
three-segmented palp with teeth on the distal segment for manipulating food before
passing it to the mandibles. Together, the first maxilla and the mandible form the sides
of the buccal cavity; the preoral labrum is the top. The mandible is easy to see with its
darkened teeth located where the two appendages meet along the midline.
Trunk
Each trunk segment is dorsoventrally flattened, and the dorsal tergite is connected to
the ventral sternite by a pleural membrane. The organization of the tergites does not
reflect the underlying segmentation of the body, and single tergites often cover more
than one segment. Only the sternites on the ventral surface reflect the true
segmentation of the trunk. Look closely at the pleural membrane and locate the
spiracles. In some species, these are located on alternating segments.
Seven-segmented legs extend from the pleural membrane, and the parts of the leg
include the proximal coxa, trochanter, prefemur, femur, tibia, tarsus with two
tarsomeres, and the distal tarsal claws. As mentioned already, the first trunk segment
is the maxilliped, consisting of four segments. The legs on the last segments, the anal
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legs, are longer than the walking legs and are not involved in locomotion (Figure 17).
Depending on the species of centipede, the anal legs may have sensory or defensive
functions. Gonopods, often located on the last segment, manipulate the eggs as they
are laid; these are present in Lithobius, but absent in Scolopendra. The very tip of the
trunk is the telson, which contains the genital pore and anus.
Figure 17 Terminal appendages on the trunk of a centipede
Millipede – Atelocerata (Diplopoda)
Millipedes and centipedes are myriapods, and as their name implies they have many
pairs of legs running down the length of the trunk segment. The diplopods are
commonly called millipedes, a name derived from what seems to be thousands of legs
that really number around 375 pairs. Like centipedes, millipedes are terrestrial and
restricted to moist soil, leaf litter, and rotting vegetation; this is because their cuticle
lacks waterproofing waxes and the spiracles don’t close. Unlike their centipede
cousins, millipedes are not predators; they are herbivores that feed on plants or
decomposing plant materials. On the head, the first maxillae form the gnathochilarium
and adjacent trunk segments are fused to form units called diplosegments with two
pairs of legs. Diplosegments and the gnathochilarium are autapomorphies of the taxon.
The first three segments of the trunk have a single pair of legs on each segment.
External anatomy
The body is hard, cylindrical, and composed of two tagmata, the head and the trunk.
Similar to all animals of the subphylum Atelocerata (Tracheata), the head is formed
from the fusion of six segments and has only one pair of antennae. While their
common name relates to their number of legs, the term Diplopoda refers to what
appears to be two legs on each segment of the trunk.
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Figure 18 External anatomy of the head region of a millipede
Head
The head of a millipede is bent forward, and the dorsal cuticle forms the head capsule
that is called the epicranium (Figure 18). Ateloceratans have two preoral appendages,
and the most anterior edge of the head capsule is fused to the bilobed, toothed labrum
that forms the roof of the buccal cavity. The most prominent preoral appendages on
the head are a pair of eight-segmented antennae and at the base of each antenna is a
cluster of simple eyes, the ocelli. The typical compound eyes of the Arthropoda have
been lost in the diplopods, and this is probably related to their nocturnal existence.
The postoral sequence of appendages in millipedes differs from that of the chilopods
and insects, which is, mandibles and then first and second maxilla. In millipedes, there
are two mouthparts only: the mandible and the gnathochilarium that forms the
ventral surface of the buccal cavity. There is some disagreement among zoologists
concerning the origins of the gnathochilarium. Some believe it is a modification of the
first maxilla; others think it is the fusion of the first and second maxillae. Each side of
the gnathochilarium has a central lingual plate and an outer stipes with a small palp at
its tip. The mandibles, which form the sides of the buccal cavity, are located between
the gnathochilarium and the anterior margin of the head capsule. Use a pair of forceps
to pull the gnathochilarium back and expose the inner surface of the mandible, which
is composed of two segments. A large, immovable basal segment forms the sides of
the buccal cavity, and a medial segment with teeth and grinding surfaces chews the
food before it is passed into the digestive system.
Trunk
Take a quick look at a typical trunk segment, a diplosegment, located near the middle
of the body. It is almost a perfect, solid circle of cuticle with two pairs of legs under
each of the dorsal tergites. This functional unit is referred to as a diplosegment and
represents the fusion of two ancestral segments. What other evidence is there that
each diplosegment is the fusion of two ancestral segments?
The first four segments of the trunk are not diplosegments. The first has no
appendages, and the tergite is enlarged to form the wedge-shaped collum that bends
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Figure 19 Terminal segements on the trunk of a millipede.
the millipede’s head forward. The following three segments each have a single pair of
legs, and pleural membranes separate the dorsal tergites from the ventral sternites.
Because there are three legs here and two on each of the remaining diplosegments,
millipedes always have an odd number of legs.
One unique autapomorphy of the millipedes is the repositioned genital opening near
the front of the trunk rather than at the posterior end. In the male and female, the
genital opening is located on the third segment, called the genital segment, which is
the second segment after the collum. The penis is usually retracted, and the medial slit
from which it everts is difficult to see. The gonopod, which is the most obvious male
feature, consists of the modified legs of the seventh segment, the third diplosegment.
The female genital opening, an almond-shaped piece of cuticle, is located in the vulva,
which is to the side and front edge of the tergum; but this will be hard to see.
Take a close look at a diplosegment near the center of the trunk where each “segment”
is almost a perfect circle composed of the tergite, two pleurites, and the sternite; the
dorsal tergite forms most of the circle. The four plates are completely fused, and there
is no membranous region of cuticle between them. You may have noticed how hard
the cuticle of the millipede is, and if you crush a segment it crunches. Unlike other
members of the subphylum, the cuticular strength in millipedes comes from a
combination of sclerotization and embedded calcium salts. The tergites overlap each
other, with the posterior margin overlapping the anterior margin of the one behind it.
This overlap is an important part of the millipedes’ defensive reaction. How does the
arrangement of the tergites help millipedes defend themselves from attack?
The legs are seven-segmented, including a basal coxa, trochanter, prefemur, femur,
tibia, tarsus with two segments, and the distal tarsal claw. Look closely at the lateral
surface of each tergite and locate the opening to the spiracle. If you have trouble
finding these on a diplosegment, you may find them easier to see on the first trunk
segments. The posterior end of the millipede consists of two plates, the upper anal
valve and lower anal valve, that meet along the midline and form the slit-like anus.
This last trunk segment lacks appendages and is the apodous diplosegment (Figure
19).
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