PHYLUM ARTHROPODA – JOINTED APPENDAGES
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.
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.
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.