The phylum Arthropoda is the largest in the animal kingdom. More than 75% of all living organisms are arthropods. Like annelids they are characterized by metamerism, i.e. the body is segmented. In addition, they have a chitonous exoskeleton. The segmented body is divisible into functional units called tagmata. In some arthropods three 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 many arthropods the head and thorax are fused, forming a cepahalothorax.
The phylum contains three extant subphyla - Chelicerata, Crustacea, and Uniramia. The subphylum Chelicerata contains arthropods in which the first appendages are modified into chelicerae (pincer-like feeding structures). Well-known representatives of this subphylum include the class Arachnida (scorpions, spiders, ticks, etc.) and the class Merostomata (horseshoe crabs).
In the subphylum Crustacea, mandibles are the primary feeding appendages. All crustacean appendages are biramous i.e. they have two processes extending from the base. Gills are used in respiration. Shrimp, crabs, lobsters, and many microscopic species are included in this subphylum.
Organisms in the subphylum Uniramia also have mandibles as the primary feeding appendages, however their appendages are uniramous (having only one process extending from the base), and a tracheal system is used for respiration. This large subphylum includes the classes lnsecta (Insects), Diplopoda (millipedes), and Chilopoda (centipedes).
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 ocelli (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. The labrum forms an upper lip that is hinged at the base. Remove the labrum with a fine scissors and place it on a slide. Now grasp the labium (lower lip) with a pair of forceps. Holding it firmly, pull toward the neck, and remove the labium. Now remove the maxillae (one on either side) in a similar fashion, and place them on the slide. The remaining two hard, sclerotized triangular structures are the mandibles. Remove them and place them above the maxillae on the slide. The tongue-like structure that remains at the center is the hypopharynx. Cut it and place it at the center between the mandibles. You have now dissected the mouth parts of the grasshopper. Study the maxillae and the labium, including their palps which are sensory in function.
THORAX: The thorax consists of three segments. The anterior prothorax bears the first pair of legs. The mesothorax (middle segment) bears a pair of legs and a pair of leathery wings. The metathorax (third segment) bears a pair of highly modified jumping legs and a pair of membranous wings. The legs are jointed. Each is composed of a basal coxa, a small trochanter, a femur, a tibia and a many-segmented tarsus. At the tip you might see the pad-like puvilus and claws. Study the hind leg modification. What is its use?
ABDOMEN: The abdomen is simple, devoid of appendages, and 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. The spiracles are most prominent in the thorax region. They are the breathing pores of the elaborate network of the tracheal system.
If time permits, study the internal structures. Consult your instructor for this study.
Here is a student video of the grasshopper's external anatomy.
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