Chapter 33 Introduction to Phylum Arthropoda

Arthropods as Vectors Mechanical vector - functioning merely as a passive carrier of the etiologic agent Biological vector - the arthropod is used by the disease producing organism not only as a means or transmission but also as a vehicle for the development and/or reproduction before it becomes infective

General Features of Arthropods

Metamerism Tagmatization Display metamerism (=segmentation) and their segments have appendages The evolution of the arthropods was witness to a reduction in metamerism Tagmatization The arthropods evolved modified groups of segments The fusion of groups of segments into functional groups is called tagmatization In so doing, various appendages on segments became specialized for functions other than locomotion

Exoskeleton and Molting The epidermis secretes the exoskeleton Advantages to possessing this structure: provides strong support; provides rigid levers that muscles can attach to and pull against; offers protection; serves as a barrier to prevent internal tissues from drying out; serves as a barrier to prevent infection The exoskeleton is composed of the polysaccharide chitin and protein bound together to form a complex glycoprotein The outer surface of the cuticle is called the epicuticle The thicker portion is called the procuticle; divided into the exocuticle and the endocuticle In the exocuticle the glycoprotein chains are cross-linked - the process is called tanning

Exoskeleton and Molting cont. In order to grow the arthropod must shed its exoskeleton, and secrete a new and larger one - molting or ecdysis.

Jointed Appendages The exoskeleton is divided into a number of plates and numerous cylinders around the appendages At the junction point between plates and cylinders, the arthropod exoskeleton remains thin and flexible - the joints - hinge-like areas of the legs and body The presence of jointed appendages allows arthropods to move efficiently and quickly without dragging the body Muscles are integral to arthropod movement The muscles of the arthropods attach to the inner side of the exoskeleton And they often function as a lever system

Post Embryonic Development

Insects There are two general types of metamorphosis: incomplete and complete 1. Incomplete Metamorphosis - Hemimetabolous Early developmental stages are very similar to the adults Only the wings and the reproductive structures gradually develop The immature stages are called nymphs Thus development is egg----> nymph ----> adult

2. Complete Metamorphosis - Holometabolous Each of the developmental stages is structurally and functionally very different The egg develops into an immature larva, that eats voraciously Larvae then forms a transitional stage called the pupa, that is often contained within cocoon Within the pupal exoskeleton a metamorphosis takes place and emerging from the cocoon is a sexually mature adult insect

Crustaceans The typical larva hatching from the egg is the nauplius larva It has three pairs of appendages: antennules, antennae, and mandibles; all of which tend to have a locomotor function The larvae undergoes several ecdyses and usually adds somites and appendages with each molt Crustaceans vary widely in their developmental patterns

Diversity in Crustacean Development

Mites and Ticks After the eggs there is usually an immature nymph, which is a tiny version of the adult The number of nymphal instars depends on the group in question Among the mites a six-legged larva becomes an eight-legged nymph after the first molt Most mites have 3 nymphal instars: protonymph, deuteronymph, and tritonymph Drawing of bulb mite development stages

External Morphology

Crustacea Possess 2 pair of antennae; first pair is homologous to those of insects; second pair are unique to the crustaceans Second pair of antennae have various functions: sensory, locomotion, feeding. The head usually bears a pair of compound eyes and 3 pairs of mouthparts for grinding, chewing or filtering, including a pair of mandibles, and 2 pairs of maxillae The trunk usually has 4 pair of walking legs

Crustacea cont. The abdomen is also highly variable; primitively there are usually six pairs of appendages Five pairs of structures called swimmerets (=pleopods); used in locomotion, generating water currents, brooding eggs The last pair of abdominal structures are called uropods; together with the terminal telson they form a tail fan than can serve as rudders during locomotion

Crustacea cont. Primitively many of the appendages of the crustaceans are biramous, two-branched: exopodite and endopodite); Both borne on a basipodite that is attached to the coxa (coxopodite) Possess a hardened exoskeleton, which is impregnated with calcium carbonate; forms a protective shell called the carapace

Insects Body is divided into 3 parts: the head, thorax and the abdomen. On the head are one pair of antennae and a pair of compound eyes and several sets of simple eyes

One pair of maxillae are fused together to form a lower lip - labium Insects cont. Mouthparts of primitive insects were adapted for chewing; consist of a pair of mandibles, and two pairs of maxillae One pair of maxillae are fused together to form a lower lip - labium There is an upper lip - labrum - formed from an extension of the head Throughout evolution other types of mouthparts have evolved: The cutting-sponging type - sharp-bladed mandibles and long, styletlike maxillae The mouthparts of most nonbiting dipterans are the sponging type The piercing-sucking type - mandibles, maxillae, and a hypopharynx modified into a long, thin, tubular, sharp-tipped stylet for piercing skin

Insects cont. The thorax is composed of 3 segments and each one has a pair of legs; he last two have a pair of wings. Each leg is usually divided into 5 segments; the segments include basal segment or coxa, trochanter, femur, tibia, and tarsus

Acari The main tagmata of this group are the cephalothorax (prosoma) and the abdomen (opisthosoma)

Acari cont. The gnathostoma or capitulum is a small anterior projection bearing the 3 structures that make up the mouthparts: the hypostome, a pair of chelicerae, and the pedipalps The hypostome is usually toothed, medially located and ventral to the mouth The chelicerae are used to pierce or tear the skin, then the entire captiulum of the toothed hypostome can be inserted into the opening During feeding, the pedipalps either bend outward (soft ticks) as the chelicerae and hypostome penetrate the flesh or remain rigidly and intimately associated with the hypostome (hard ticks) during skin penetration In either instance, the pedipalps serve as counter-anchors while the tick is attached to the host