KINGDOM
ANIMALIA
PHYLUM
ARTHROPODA – JOINTED APPENDAGES
INTRODUCTION
The
phylum Arthropoda is the largest in the animal
kingdom: more than 75% of all living organisms are arthropods. Like annelids, arthropods are
characterized by metamerism,
i.e. the body is segmented. In
addition, they have a chitinous exoskeleton that is
shed in a process known as ecdysis. Body
segments are typically fused into functional units called tagmata. In many arthropods, 3 tagmata are present: a head
(involved in feeding and sensory functions), a thorax (involved mostly in locomotion), and an abdomen (which performs the visceral functions). In some arthropods, the head and thorax
are fused, forming a cephalothorax.
The
phylum Arthropoda contains 4 extant subphyla:
Subphylum Chelicerata: These
arthropods have chelicerae, pincer-like mouthparts that are formed by the first
pair of body appendages. They also
lack antennae. Well-known representatives of this
subphylum include the class Arachnida (scorpions, spiders, ticks, etc.) and the class Merostomata
(horseshoe crabs).
Subphylum Crustacea: These
arthropods have 2 pairs of antennae
formed from the most anterior appendages.
Their mouthparts are mandibles,
formed from head appendages arising posterior to the antennae. Crustaceans have biramous (two-branched) paired
appendages on both the thorax and abdomen, and these appendages often have
specialized functions, such as walking or swimming. Shrimp, crabs, lobsters, and many
microscopic species are included in this subphylum.
Subphylum Myriapoda: These
arthropods have 1 pair of antennae and mouthparts formed from head appendages
arising posterior to the antennae, including mandibles forming the primary feeding appendages. Myriapods are
strictly terrestrial, and their appendages are uniramous (having only one process extending from the base). They
also have a tracheal system is used
for gas exchange
This subphylum contains the classes Diplopoda (millipedes) and Chilopoda
(centipedes). Their bodies are
formed as a series of similar segments, with each segment bearing paired legs
(1 pair per segment in Chilopoda and 2 pairs per
segment in Diplopoda).
Subphylum Hexapoda: These
arthropods are similar to myriapods in having 1 pair
of antennae, mandibles as mouthparts, uniramous appendages, and a tracheal
system for gas exchange. However, their
bodies are divided into 3 distinct tagmata, the head, thorax, and abdomen. The hexapods get their name from their 3
pairs of legs that are attached to the thorax. Included in this group is the class Insecta as well
as several smaller and less well known groups. Many of the insects have 1 or 2 pairs of
wings that arise from the thorax.
In
this exercise, you will examine the external structure of a chelicerate
(the horseshoe crab), a crustacean (crayfish), and a hexapod
(grasshopper). In addition,
specimens of several other arthropod groups will be on display for you to
observe.
PROCEDURE
Subphylum Chelicerata- Class Merostomata
(horseshoe crabs)
Obtain
a preserved specimen of Limulus polyphemus, the horseshoe crab
of the Atlantic Coast. Identify the major body regions of the animal- the prosoma and the opisthoma, which includes the abdomen and telson.
The carapace is the shield-like
covering of the prosoma. Note the laterally located compound eyes and the median ocelli (simple
eyes).
Locate
the mouth on the ventral surface of
the specimen, the chelicerae closely
associated with the mouth, and the 5 pairs of walking legs. The spiny
bases (gnathobases) of the legs are used to macerate
food. Note the abdominal book gills
and the delicate gill lamellae. The anus is located at the base of the telson. The
spike-like telson is not used aggressively,
rather it helps the animal right itself if turned over.
Observe
the spider, scorpion, and tick specimens on display and note that they exhibit
the typical arachnid (Class Arachnida) feature of
possessing 4 pairs of legs.
Subphylum Crustacea
Obtain a living or preserved crayfish and identify the major body regions. The cephalothorax, the fused head and
thorax covered by a carapace, and
the abdomen. The gills are housed beneath the carapace
in the crayfish. Head appendages include two pairs of antennae and stalked compound
eyes. Crustaceans possess variable numbers of legs. The crayfish belong to
a group of crustaceans, the decapods, whose members have 5 pairs of walking
legs (pereopods)
on the thorax. The first pair of legs is modified as pincers (chelipeds). The
abdomen is equipped with swimming appendages called pleopods.
Subphylum Hexapoda- Class Insecta
Obtain a preserved grasshopper and observe its external features. The exoskeleton is divided by sutures into plates called sclerites.
HEAD: The head consists of fused sclerites forming a cranium and mouth
parts. A
pair of antennae arise in
front of the compound eyes. Three simple eyes can be seen - one in
the center, between the antennae, and two located above the base of the
antennae.
With the help of a probe, locate the position of the various mouth parts and note that they form posterior to the antennae. Note that the mouthparts are formed from
several pairs of specialized appendages, including the hard, sclerotized mandibles.
THORAX: The thorax is formed from the fusion of 3 body segments. The anterior segment
bears the first pair of legs, the middle segment bears a second pair of legs
and a pair of leathery forewings,
and the third segment bears a pair of highly modified jumping legs and a pair
of membranous hindwings. The legs are jointed and composed of a number of segments. Identify the
basal coxa,
large proximal femur and a smaller
more distal tibia.
ABDOMEN: The abdomen lacks appendages, and is
made up of 10 to 11 segments. Note
the terminal structures and use them to determine the sex of the specimen. Be sure to compare your grasshopper to one of the opposite sex. In the female, the ovipositor is made up of
four highly sclerotized valves for laying the eggs inside the earth. At the tip look for a pair of sensory
structures known as cerci.
On
either side of the first abdominal segment you might see a thin membrane,
called the tympanum - a hearing
organ. Spiracles are present on either side of most of the segments but
are most prominent in the thorax region.
They are the breathing pores of the elaborate network of the tracheal
system.
