A comparative
study of ARTHROPODA is difficult, since its major elements became
diversified before the earliest clearly
recognizable fossils were deposited in
the Cambrian period, 500 million years ago, arthropod evolution is so
vast a
filed that innumerable examples
are required to get a real feeling for
patterns of change ad for groups that
have evolved during this long period.
The embryology of higher arthropods
lacks phylogenetically illuminating
early stages, which makes basic
relationships even more difficult to trace.
We will have time to study very few examples, but
these will provide a brief
introduction to
some major
patterns of arthropod adaptation. Check all names used here with the
classification outline in chapter 5 add
to the
outline generic names referred to here that are not listed there.
Two great group
(SUBPHYLA) of arthoropods are generally
recognized; the MANDIBULATA, jawed or mandibulate arthropods; and the
CHELICERATA, arthorpods whose first appendage bears clawlike pincers
A huge
assemblage is included in the sub phylum
MANDIBULATA, chiefly in the classes CRUSTACEA and INSECTA. In the subphylum
CHELICERATA are the large
class ARACHINIDA (scorpions, spiders,
mites, ticks, and relations); the extinct tribolites (TRILOBITA); The extinct giant eurpterids with the
still-existent horseshoe crabs (MEROSTOMATA);
and the peculiar marine sea spiders PYCNOGONIDA)
In the class CRUSTACEAN, subclass Malacostraca, is an assemblage of water
dwellers. Sometimes the pycnogonida, here considered a class of CHELICERATA, is
recognized as an additional subphylum of
arthropods. These “sea spoiders are strange, slow –moving, marine, somewhat
spider like creatures. The extinct
TRILOBITES, here listed as a class in the chelicerate line, also are sometimes
classified as a distinct subphylum. The phylum ONYCHOPHORA, a phylogenertically
important but rather rare and small group of caterpillear like animals, shoes important
evolutionary relationships with the arthropods
portent evolutionary relationships with the arthropods and the annelids (review your text discussion of per typified by Crayfishes
(cambarus), lobsters (HOMARUS), and shripmps, all of which are in cluded in the
order DECAPODA 9malacostracans with 10 pairs of walking legs). Mantis shrimps, beach fleas, and sowbugs
represent other major groups of the
subclass MALACOSTRACA
Other important
subclasses in the class CRUSTACEA
include barnacles (CIRRIPEDIA )and copepods (COPEPODA). In the subclass BRANCHIOPODA are found the most primitive crustacean, showing very
little specialization of the segments or appendages, examples are the fairy shrimps and brine shrimps (order ANOSTRACA).
Perhaps no animals can compare with the wide spread copepods (such as
the marine Calanus ) in overall abundance or importance in the ecological food
chain of the sea.
Our study of the crayfish as a single example of the entire
class CRUSTACEA must
therefore cover much biological
territory, a great period of geological
time, an enormous area of the earths
surface (about four –sevenths –
all the water –covered surface of the globe), and a rich evolutionary divergence in numbers and kinds
The two remaining classes of MANDIBULATA, the INSECTA
(or hexapoda, meaning
six-legged) and myriapoda (entipedes
and millipedes. figure 12.1 ), contain many familiar examples. Despite the fact that INSECTA is by
far the largest class of all, we will have time to study only one or two species, perhaps 800,000
to a million species of insects have already been described. Some entomologists believe that this includes less than half of the actual number
of living insect species, we will examine the common lubber grasshopper,
romalea, and the American cockroach, periplaneta. (these examples are selected
chiefly because they are large and easily procured).
Whether there
are 1 or 2 million species,
insects represent a biological success
story. Insect history differs markedly from that of the class CRUSTACEA in one important ecological respect: insects have become terrestrial. They have bridged the gap between water
breathing and air breathing. (terrestrial
isopods still require a moist habitat for survival). Occupation of land
opened a wealth of new environments in
which insects spread rapidly, as early as the Pennsylvanian period (late
carboniferous), s 320 million years
ago, and most rapidly during the cretaceous period, about 100 million years ago. In doing so, however, their adaptations for air
breathing and resistance to desiccation
as well as for osmoregularoty limitations seem to have
prevented reoccupation of the marine environment from which their ancestors
presumably arose . 9although a small
number of larval insects live in fresh
water, how many adult insects have you seen inhabiting ponds and streams?
Some water beetles carry bubbles of air
under water, an indication of the difficulty adult insects have in returning to an aquatic environment).
Concurrent with
the arthropod transfer to the terrestrial habitat was the development of flowering plants, which provided insects with
a great variety of shelter, food, and
protection. In turn, insects came to play
a significant role in the distribution and structural modifications
of these plants, many of which possess
highly specialized structures for pollination by a specific insect group a type of evolutionary
partnership, illustrating coevolution.
Insects demonstrate to an unusual degree the
evolutionary process called adaptive radiation, in which a basic structural or functional modification permits reapid occupation of a
new environment. This exploitation of
previously unavailable habitats is followed by diversification formation of new species and then a new spread into more specialided niches within the new
environment. If another evolutionary modification appears in one of
the economically or geographically isolated groups, a new wave of
habitat occupation may carry these animals into still different environments.
A succession of such major structural
and functional changes or adaptive
breakthroughs, each producing a
wave of spread and specialization, is
what is meant by adaptive radiation.
When arthropods
were transferred from an aquatic to a
terrestrial environment, their jointed exoskeleton – durable, impervious, and
light in weight determined a series of profound morphological and physiological
changes that introduced adaptive radiations into the numerous habitats afforded by land and air.
The first of the
major changes related to the exoskeleton, and influencing the evolution of
insects, was probably the tracheal system, an extensive network of air
conducting tubes branching from the exoskeleton
to every cell of the organism. The
presence of tracheae, along with the strength, lightness, extensive area
for muscle attachment, and other advantages of the exoskeleton, permitted early
scorpion like arthropods to become air-breathing land dwellers. The reduction of the number of walking legs to
six freed the head appendages for
specialization in feeding, chiefly by
their modification to form jaws. Flight
was made possible by membranous wings, a
highly organized muscular system, a
rapid metabolic rate, small size,
and relative in destructibility all features made possible by the
exoskeleton. Vast new ecological realms
were made available by this remarkable development. No other invertebrates
and only reptiles, birds, and certain
mammals (Chiefly bats) have evolved the power of flight).
Metamorphosis,
related to the mode of growth and
ultimately to the exoskeleton, provided still further specilaisation: the protected
egg stage for critical and delicate early development; the pupa, another protected stage for the transition from larva to adult, and the flying adult, adapted for reproduction, occupation
of a different ecological niche, and
distribution. Matamorphosis meant specialization of each body form to functional needs at each stage
of the life cycle, such as development ,
growth, reproduction, and distribution
of the species.
Each of these
major stages of insect evolution was marked by adaptive radiation,
made possible by important morphological
and physiological changes, and molded by the structural limitations and advantages afforded by the
chitinous skeleton
Although the ant
may one day inherit the world, it will
not, as pictured in science fiction tales become huge in size. Total sixe is restricted
in insects for two reasons. First, tracheae can work efficiently nly in small
volumes, as the air moves in and out of
them chiefly by diffusion. Very large
insects simple could l not exchange gases rapidly enough by tracheal respiration.
Second just after each molt the insect
skeleton is soft and flexible the period of discontinuous growth.
Being unsupported by exoskeleton during
this vulnerable period, the body must be
supported by he external medium. This is
a manageable problem to a lobster but what would happened to a lobster in air
during the molting period? Its body would be permanently distorted by its own
weight. Imagine what would happen to the monster grasshopper pictured in
science fiction! However, instead of becoming huge, the ant and his innumerable
counterparts might simply multiply to the
fantastic degree possible for their organisms and occupy the total available
environmental, in this event, insects
would prove an excessively burdensome competitor to man for
food and space far more dangerous
to us than 10fit man eating
insects would be.
Review the body
type of the phylum ARTHROPODA as exemplified first by crustaceans and then by
insects.
·
How
does these differences permit the occupation of distinct environments and
subsequent divergence of the two groups?
The second major division of ARTHROPODA, the
subphylum CHELICERATA, is comprised of spiders, scorpions, ticks, mites, and allied forms. Here the general arthropod
pattern is usually modified for prediation as well as for existence on land. Many chelicerates are equipped with poison
claws or glands, and their mouthparts are usually adapted for sucking out the
juices or soft tissues of their prey. Although
chelicerates are highly resistant to desiccation, few except mites have spread to the variety
of habitats utilized by insects, perhaps
because of their more specialized food habits.
The two sectioned body consists of head (cephalothorax) and babdomen,
six pairs of jointed, segmented
appendages are hinged onto the cephalothorax. These include one pair of
clawed chelicerae (contianing the position
fangs and ducts, or claws), one pair of pediplapi (six jointed, leg like
structures to hold or crush prey, specialized for sperm transfer in males), and four pairs of seven jointed walking legs.
True antennae and mandibles, characteristic
of the MANDIBULATA, are entirely absent.
The respiratory system also has specialized features; book gills in kin crabs, book lungs in some
spiders, tracheae in the abdomen of other spiders, and direct diffusion through
the cuticle in mites.
The
class MEROSTOMATA includes horse shoe
crabs and extinct eurypterids (huge scorpionlike predators of Paleozoic seas). arachnida
includes nine somewhat familiar orders,
including many feared and often maligned examples SOCORPIONIDA scorpions(Figure 12.2) (divided into two suborders: trapdoor spiders and tarantulas in one, and black widow, funnel web spiders, hunting spiders, orb weavers, crab spiders
and jumping spiders in the other ); solpugida sun spiders, PSEUDOSCORPIONIDA
false scorpions, PHALANGIDA harvestmen or daddy
longlegs, and ACARINA ticks and mites
Chelicerate
mouth structure, absence of wings, and predaceous food habits in general restrict the distribution of members of this
subphylum, at least in comparison with that of insects. Spiders, ticks,
and mites, however, do occupy a vast
area and many types of terrestrial
habitats
Through marked
reduction of size and simplification
of body structure, mites have in
fact developed a different widespread
adaptive radiation . it has enabled them to become abundant
soil dwellers and to feed on a wide variety of materials, from organic debris to blood.
Although mites represent an extension of the arthropod type into numerous
ecological niches, these tiny creatures
are among the least –known groups of animals.
Other classes of
CHELICERATA are the extinct Trilobita, the pycnogonida or sea spiders, and the
merostomata . The latter group is
represented by an animal that is a good candidate for a crossward puzzl, THE
SUBCLASS Xiph OSURA. along the
east coast beaches, one can see these
bizarre creatures, with small bodies in an oversize armored shield ending in a
spiny tail. the
single surviving member is limulus (figure 12.3), the
horseshoe crab, a primitive reminder of ancient paleozoic seas once also
inhabited by trilobites and predaceous
10ft eurypterids, all related
merostomes.
Our detailed
study of the phylum arthropoda must be confined to just two or three examples:
the decapods malacostraca crustacean
cambraus, the crayfish, the grasshopper romalea, and it time permits, the
cockroach periplaneta Illustrated here
are common examples of some of
the more important arthropod groups
within the class CRUSTACEA. They
represent a wide range of types,
starting with one of the most primitive, artemia (figure 12.4), the brine
shrimp, with little specialization of its appendages,. The abundant water flea dapnia (figure 12.5) and Cyclops (FIGURE 12.6) are widespread members of nearly every aquatic environment,
part of the so-called microcrustacean
fauna. They are primary members of the
food chain supporting nearly every aquatic environment , part of the so –
called microcrustacean fauna. They are
primary members of the food chain supporting
nearly all larger aquatic animals, argulus (figure 12.7) is
specialized for attachment (note suckers
) and is a common fish ectoparasitee lepas (figure 12.8) and
balanus (figure 12.9) are
barnacles, specialized for permanent
attachment to a substrate, and the latter is well adapted for protection against
wave action and periodic exposure to
intertidal desiccation. the familiar sowbug (pill bug ) oniscus (fgure 12.10) is adopted for terrestrial existence.
homarus (figure 12.11), the succulent lobster, represents an
advanced crustacean with a high degree
of modification of its appendags, with
specialized mouthparts and powerful
pincers.