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.