CK-12 Life Science
Page 28
Figure 12.16
A species of spider in its web.
Figure 12.17
A species of scorpion.
Figure 12.18
A centipede, from the subphyla of myriapods.
Figure 12.19
A species of millipede found in Hawaii.
Figure 12.20
The blue American lobster illustrates the segmented body plan of the arthropods.
Figure 12.21
Giant spider crabs.
Figure 12.22
The sessile barnacles shown here feeding.
Figure 12.23
A crayfish.
Arthropods are characterized by the possession of a segmented body with appendages on at least one segment. Arthropod appendages are used for feeding, sensory reception, defense, and locomotion. Their heart is on the dorsal side and the nervous system on the ventral. They are covered by a hard exoskeleton made of chitin, which provides physical protection and among terrestrial species resistance to drying out. In order to grow, arthropods shed this covering in a process called molting.
It is the largest phylum in the Animal Kingdom with more than a million described species making up more than 80% of all described living species. They are found commonly throughout marine, freshwater, terrestrial, and even aerial environments, in addition to various forms that are parasitic and symbiotic. They range in size from microscopic plankton (approximately ¼ mm) up to the largest living arthropod, the Japanese spider crab, with a leg span up to 12 feet (3.5 m).
Aquatic arthropods use gills to exchange gases. These gills have an extensive surface area in contact with the surrounding water. Terrestrial arthropods have internal surfaces that are specialized for gas exchange. Insects and most other terrestrial species have a tracheal system, where air sacs lead into the body from pores in the exoskeleton, for oxygen exchange. Others use book lungs, or gills modified for breathing air, as seen in species like the coconut crab. Some areas of the legs of soldier crabs are covered with an oxygen absorbing membrane. Terrestrial crabs sometimes have two different structures: one that is gilled, which is used for breathing underwater, and another adapted to take up oxygen from the air.
Arthropods have an open circulatory system with haemolymph, a blood-like fluid, which is propelled by a series of hearts into the body cavity where it comes in direct contact with the tissues. Arthropods have a complete digestive system with a mouth and anus.
Crustaceans
The crustaceans are a large group of arthropods, consisting of almost 52,000 species. The majority of them are aquatic, living in either marine or freshwater habitats. A few groups have adapted to living on land, such as terrestrial crabs, terrestrial hermit crabs, and woodlice (Figure below).
Figure 12.24
A terrestrial arthropod, a species of woodlice.
Crustaceans are among the most successful animals and are as abundant in the oceans as insects are on land. The majority of crustaceans are motile, although a few groups are parasitic and live attached to their hosts. Adult barnacles live a sessile life, where they are attached headfirst to the substrate and cannot move independently.
Various parts of the crustacean exoskeleton may be fused together, such as in the carapace, the thick dorsal shield seen in many crustaceans that often forms a protective chamber for the gills. The main body cavity is an expanded circulatory system, through which blood is pumped by a heart located near the dorsal surface. The digestive system consists of a straight tube that often has a gizzard-like gastric mill for grinding food and a pair of digestive glands that absorb food.
Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia (connections between nerve cells) close to the antennae and a collection of major ganglia below the gut. Most crustaceans have separate sexes. Many terrestrial crustaceans, such as the Christmas Island red crab, mate seasonally and return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, although in damp conditions. In other crustaceans, the females keep the eggs until they hatch into free-swimming larvae.
Six classes of crustaceans are generally recognized:
Class Information
Branchiopoda Includes brine shrimp
Remipedia A small class restricted to deep caves connected to salt water
Cephalocarida The horseshoe shrimp
Maxillopoda Includes barnacles and copepods
Ostracoda Small animals with bivalve shells
Malacostraca The largest class, with the largest and most familiar animals: crabs, lobsters, shrimp, krill, and woodlice
Centipedes and Millipedes
Centipedes and millipedes belong to the subphylum Myriapoda, which contains 13,000 species, all of which are terrestrial, and which are divided among four classes. They range from having over 750 legs (a species of millipede) to having fewer than ten legs. They have a single pair of antennae and simple eyes.
They are most abundant in moist forests, where they help to break down decaying plant material, although a few live in grasslands, semi-arid habitats, or even deserts. The majority are herbivores, but centipedes are chiefly nocturnal predators.
Although not generally considered dangerous to humans, many from this group produce noxious secretions, which can cause temporary blistering and discoloration of the skin. Centipedes are fast, predatory, and venomous. There are around 3,300 described species, ranging from one tiny species (less than half an inch [about 12 mm] in length) to one giant species, which may exceed 12 in (30 cm).
Most millipedes are slower than centipedes and feed on leaf litter and detritus (loose material, such as stone fragments, gravel, or sand, worn away from rocks). Around 8,000 species have been described, although there may be as many as 80,000 or more species actually alive.
The third class, Symphyla, contains 200 species. They resemble centipedes but are smaller and translucent. Many spend their lives in the soil, but some live in trees. The pauropods are typically 0.5-2.0mm long and live in the soil of all continents except Antarctica. Over 700 species have been described, and they are believed to be closely related to millipedes.
Arachnids
Arachnids are a class of joint-legged invertebrates in the subphylum Chelicerata. They are mainly terrestrial, but are also found in freshwater and in all marine environments, except for the open ocean. They comprise over 100,000 named species, including spiders (Figure below), scorpions (Figure below), daddy-long-legs, ticks, and mites (Figure below) and there may be up to 600,000 species in total, including unknown ones.
Figure 12.25
A daddy-long-legs with a captured woodlouse.
Figure 12.26
Various diseases are caused by species of bacteria that are spread to humans by hard ticks, like the one shown here.
Figure 12.27
A female crab spider sharing its flower with velvet mites.
It is commonly understood that arachnids have four pairs of legs and that they may be easily distinguished from insects on this basis (insects have three pairs of legs). Arachnids also have two additional pairs of appendages, the first pair, the chelicerae, serve in feeding and defense. The next pair, the pedipalps, are adapted for feeding, locomotion, and/or reproductive functions. Arachnids are further distinguished by the fact they have no antennae and no wings. Their body is organized into the cephalothorax, derived from the fusion of the head and thorax, and the abdomen.
Arachnids are also well adapted for a terrestrial existence. They have internal respiratory surfaces in the form of trachea or a book lung. They also have appendages modified for more efficient locomotion on land, internal fertilization, special sensory organs, and structures for water conservation, such as more efficient excretory structures and a waxy layer covering the outer layer of the exoskeleton.
Arachnids are mostly carnivorous, feeding on the pre-digested bodies of insects and other small animals. Several groups are largely venomous and they secrete venom from specialized glands to kill p
rey or enemies. Several mites are parasitic and some of those are carriers of disease. Arachnids usually lay eggs, which hatch into immature arachnids that resemble the adults. Scorpions, however, bear live young.
The arachnids are divided into eleven subgroups. Table (below) shows the four most familiar subgroups, with a description of each.
Subgroup of Arachnid Representative Organisms Approximate Number of Species Description
Araneae Spiders 40,000 Nearly all spiders inject venom to protect themselves or to kill prey; only about 200 species have bites that can be harmful to humans
All produce silk, used for many functions, including trapping insects in webs, aiding in climbing, forming smooth walls for burrows, producing egg sacs, and wrapping prey
Found all over the world, ranging from tropics to the Arctic, some in extreme environments;
Opiliones Daddy-long-legs 6,300 Mostly nocturnal, colored in hues of brown; a number of diurnal species have vivid patterns of yellow, green, and black
Many are omnivores, eating small insects, plant material and fungi; some are scavengers, eating decaying animal and other matter
Known for exceptionally long walking legs; no silk nor poison glands
Scorpiones Scorpions 2,000 Unlike the majority of arachnids, scorpions produce live young, which are carried about on the mother’s back until they have molted at least once; they reach an age of between four to 25 years
Nocturnal; during the day find shelter in holes or under rocks
Predators of small arthropods and insects, they use pincers to catch prey, then either crush it or inject it with a fast-acting venom, which is used to kill or paralyze the prey; only a few species are harmful to humans
Characterized by a tail with six segments, the last bearing a pair of venom glands and a venom-injecting barb
Acarina Mites and ticks 30,000 Many are parasitic, affecting both invertebrates and vertebrates, and may be vectors of human and other mammalian disease; those that feed on plants may damage crops
Live in nearly every habitat, including aquatic and terrestrial
Most are minute to small (no more than 1.0 mm in length), but some ticks and one species of mite may reach lengths of 10-20 mm
Why Arthropods are Important
Many species of crustaceans, especially the familiar crabs, lobsters, shrimp, prawn, and crayfish, are consumed by humans, and nearly 10,000,000 tons were produced in 2005. Over 70% by weight of all crustaceans caught for consumption are shrimp and prawns, and over 80% is produced in Asia, with China producing nearly half the world’s total.
Some mites prey on undesirable arthropods and are used in pest control, while others control weed growth. Populations of whip scorpions are valuable in controlling populations of cockroaches and crickets. Finally, an unquantified, but major positive contribution of the mites and ticks, as well as the centipedes and millipedes, is their role in ecosystems, especially their roles as decomposers and the resulting enriching of the soil due to the release of the nutrients during decomposition.
In the next lesson, we will discuss the diversity of insects. As we will see, insects, also arthropods are beneficial in many ways, both to the ecosystems of which they are part, as well as to humans.
Lesson Summary
The phylum Arthropoda includes four living subphyla; chelicerates, including spiders, mites, and scorpions; myriapods, including centipedes and millipedes; hexapods, including insects; and crustaceans. Arthropods are characterized by a segmented body; appendages used for feeding, sensory structures, defense, and locomotion; a dorsal heart and a ventral nervous system; and a hard exoskeleton. Arthropods are the largest phylum in the Animal Kingdom with more than a million described species; they are found in all environments. There are a variety of respiratory systems in arthropods, including gills, tracheal system, book lungs, and oxygen absorbing membranes; arthropods have an open circulatory system and a complete digestive system.
Crustaceans consist of almost 52,000 species, the majority of which are aquatic; they are among the most successful animals. There are six classes of crustaceans, including brine shrimp, barnacles and copepods, and the malacostracans, including crabs, lobsters, and shrimp. Centipedes and millipedes belong to the myriapods, where they occur most abundantly in moist forests; they are chiefly nocturnal predators.
Arachnids are mainly terrestrial and comprise over 100,000 named species; adaptations for a terrestrial existence include specialized respiratory structures, appendages modified for locomotion on land, internal fertilization, special sensory organs, and structures for water conservation. Arachnids are divided into eleven subgroups, the most familiar being spiders; spiders produce silk, which is used in a variety of ways. Many species of crustaceans are used for food; some species of mites are used in pest control and for controlling weeds; and centipedes, millipedes, and the acarines play a valuable role as decomposers, enriching the soil as a result.
Review Questions
What are arthropod appendages used for?
What respiratory systems do terrestrial arthropods use?
Arachnids have several adaptations for living on land. For each adaptation you list, explain how it is beneficial for a terrestrial existence.
How does the scorpions’ method of producing young differ from most other arachnids?
Further Reading / Supplemental Links
http://cybersleuth-kids.com/sleuth/Science/Animals/Arthropods/index.htm
http://www.oceanicresearch.org/education/wonders/arthropods.htm
http://www.biokids.umich.edu/critters/Crustacea
http://www.nps.gov/archive/yell/kidstuff/Alphabet/a.htm
Vocabulary
acarina
The group of arachnids containing the mites and ticks.
araneae
The arachnid group containing the spiders.
arthropoda
The phylum meaning “jointed feet;” includes four living subphyla of arthropods.
book lungs
Gills modified for breathing air.
carapace
The thick dorsal shield seen in many crustaceans; often forms a protective chamber for the gills.
cephalothorax
The anterior part of the arachnid body, derived from the fusion of the head and thorax.
chelicerae
The first pair of arachnid appendages; used in feeding and defense.
chelicerata
An arthropod subphylum containing the arachnids.
ganglia
A compact group of nerve cells having a specific function.
gastric mill
A gizzard-like structure for grinding food.
haemolymph
A blood-like fluid, which is propelled by a series of hearts into the body cavity, where it comes in direct contact with the tissues.
molting
The process by which arthropods shed their hard exoskeleton in order to grow.
myriapoda
An arthropod subphylum containing the centipedes and millipedes.
opiliones
The arachnid group containing daddy-long-legs.
parasitic
Living on or in an organism of another species; harmful to the host species.
pedipalps
The second pair of arachnid appendages used for feeding, locomotion, and/or reproductive functions.
scorpiones
The group of arachnids containing the scorpions.
silk
A thin, strong, protein strand extruded from the spinnerets; most commonly found on the end of the abdomen of spiders.
symbiotic
The living together of two dissimilar organisms.
Points to Consider
Arthropods are characterized by the possession of a segmented body with appendages on at least one segment and they are covered by a hard exoskeleton made of chitin. How is the general arthropod body plan specialized in the insects?
Insects are the only group of invertebrates to have dev
eloped flight. Compare this mode of locomotion to those discussed in the groups of arthropods already discussed. What advantages might there be to using flight for a method of locomotion?
Lesson 12.4: Insects
Lesson Objectives
Describe the characteristics of insects.
Explain how insects obtain food.
Describe reproduction and the life cycle of insects.
Explain how insects are important.
Describe how insect pests are controlled.
Check Your Understanding
What is an arthropod?
Is a spider an insect? Why or why not?
Introduction
Insects, with over a million described species, are the most diverse group of animals on Earth. They may be found in nearly all environments on the planet. That would explain that no matter where you travel, you are bound to see representatives from this group and probably lots of different kinds as well. Even if you were not partial to bees, wasps, and ants perhaps, it would be difficult not to admire the beauty of a butterfly, moth, or even a dragonfly!