LECTURES FOR ZOO 1010—CHAPTER 13
Arthropods
BASED ON HICKMAN ET AL. ANIMAL DIVERSITY
3rd Edition
Chapter Prologue—A Winning Combination: Humans are in a ceaseless struggle with
members of the dominant group of animals on earth today. Insects, as the preeminent
group of arthropods, outnumber all other species of animals in the world combined
and individual numbers are enormous. It has been estimated that there are 200 million
insects for every human being alive today, a total of about one and one-quarter
quadrillion individuals. Insects have adapted to all land environments and to virtually
all climates. Insects evolved wings and were in the air 150 million years before flying
reptiles, birds, or mammals. Insects are almost completely absent from the seas, but
crustaceans are found in great abundance and diversity there. The success of
arthropods depends on the combination of a versatile exoskeleton, metamerism, an
efficient respiratory system, and highly developed sensory organs. Many also have
a waterproofed cuticle.
The phylum Arthropoda includes the spiders, scorpions, ticks, mites, crustaceans,
millipedes, centipedes, insects, and several smaller groups. The fossil record of the
phylum extends back to the mid-Cambrian period.
Arthropods are bilaterally symmetrical, metamerically arranged eucoelomate
protostomes with a chitinous exoskeleton, well developed organ systems, and
jointed appendages. In addition, they have somites combined or fused into
functional groups called tagmata (sing. tagma). Thus, they exhibit tagmosis or
tagmatization.
Arthropods range in size from less than 0.1 mm to 3.7 m in length, although most are
shorter than 60 cm in length.
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Arthropods are the most abundant and diverse of all animals. Although some
arthropods compete with us for food supplies and spread serious diseases, others are
essential for pollination of many crop plants, serve as food, yield drugs and dyes, and
create products such as silk, honey, and beeswax.
Ecological relationships—Arthropods are found from low ocean depths to very high
altitudes and from the tropics far into both northern and southern polar regions. Some
live on land, other in fresh, brackish, or marine waters. Others live on plants and on
other animals. Most insects are capable of flight. The majority of arthropods are
herbivorous, although carnivorous and omnivorous are also known. Parasitism is
widespread.
Why have arthropods achieved such great diversity and abundance?—The structural
and functional patterns that have assisted arthropods in achieving great diversity and
abundance, broad geographic and ecological distribution, and adaptive ability in
changing environments are as follows: (1) a versatile exoskeleton (that, however,
requires ecdysis); (2) segmentation and efficient locomotory appendages; (3) an
efficient tracheal respiratory system in which air is piped directly to cells; (4)
highly developed sensory organs; (5) complex behavior patterns; (6) reduced
competition through metamorphosis.
Subphylum Trilobita:
Trilobites are fossil arthropods that flourished during the Cambrian and Ordovician
periods and have been extinct for about 200 million years. They possessed a
trilobed body and were probably bottom dwelling scavengers.
Subphylum Chelicerata:
The chelicerates are an ancient group including the extinct eurypterids or giant water
scorpions, horseshoe crabs, spiders, ticks, mites, scorpions, and sea spiders. They
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are characterized by a lack of antennae and possession of chelicerae as their
principal feeding appendages. Additionally, they have a pair of pedipalps and
four pairs of walking legs. The arachnids comprise the largest group of
chelicerates (spiders, scorpions, harvestmen, ticks, and mites.
Spiders are chelicerate arthropods in which the cephalothorax and abdomen are
unsegmented externally and connected by a slender pedicel. They possess poison
glands and can spin silk.
Scorpions are chelicerates with a short cephalothorax bearing the locomotory
appendages and one to six pairs of eyes. The abdomen is clearly segmented and
ends in a stinging apparatus. Most scorpions are not dangerous to humans.
Scorpions bear living young, which the female parent carries on her back.
Ticks and mites have completely fused cephalothorax and abdomen and are the most
numerous of any arachnids. Some are important disease vectors, others are serious
plant pests.
Subphylum Crustacea:
Crustaceans are primarily aquatic. They possess two pair of antennae, mandibles, and
two pairs of maxillae on the head. Their appendages are primitively biramous.
The major tagmata are a head, thorax, and abdomen. Many respire by means of
gills. Some crustaceans (some isopods) are terrestrial.
Crustaceans, as with all arthropods, must periodically molt in order to grow in size.
The old cuticle is cast off in a process called ecdysis, followed by growth and the
formation of a new cuticle. This process is under hormonal control.
Crustaceans include many predators, scavengers, filter feeders, and parasites.
Respiration is accomplished through the body surface or by use of gills.
Circulation, as in other arthropods, is through an open system of sinsuses
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comprising the hemocoel and a dorsal tubular pumping heart. Most arthropods
have compound eyes composed of ommatidia.
Subphylum Uniramia:
Uniramians have unbranched or uniramous appendages. The group includes insects
and myriapods. Myriapods have a body consisting of a head and trunk and include
the centipedes, millipedes, pauropods, and symphlyans. Insects have three
tagmata, a head, thorax, and abdomen, and three pairs of thoracic legs.
Uniramians have a single pair of antennae, mandibles, and one or two pairs of
maxillae. Respiration is accomplished by use of the body surface or tracheal
systems, although some juveniles, if aquatic, may have gills.
The class Insecta is the largest in the world’s largest phylum. Their success is due to
the combination of the same factors that account for the success of arthropods in
general combined with the usual presence of two pairs of wings and the ability to
remain dormant during adverse conditions. Respiration is by use of a tracheal
system and excretion is accomplished by use of Malpighian tubules. Sexes are
separate and fertilization is usually internal. Insects undergo either
hemimetabolous or holometabolous, both of which are under hormonal control.
Phylogeny and Adaptive Radiation:
Phylogeny— Until recently, zoologists believed that annelids and arthropods were
closely related and derived from a near common ancestor. More recently,
molecular evidence suggests that the two were derived independently from a
protostome ancestor and belong to different superphyla. If this hypothesis proves
supportable, then we have to conclude that metamerism would have evolved
independently in each superphylum.
Adaptive radiation—The monophyletic status of the phylum Arthropoda is supported
by a preponderance of morphological and molecular evidence. Adaptive radiation
among the arthropods has been enormous.