Biology 625
Fall semester lecture note outline

Updated: 24 September 1999

The text below simply represents a crude lecture outline of one of the topics covered in class. It is not meant to substitute for attending lectures or ignoring the textbook. Additional material, including line drawings, kodachromes, and more extensive information on life-cycles and basic biology, will be supplied in the lectures.

TOPIC 39. Introduction to the phylum Arthropoda

  1. Approximately 1 million described species
  2. with exceptions, most are relatively small
  3. coelomate
  4. bilaterally symmetrical
  5. metameric (segmented); each segment termed a somite
  6. body segments often grouped into distinct body regions (tagmata)
  7. exoskeleton
    1. normally composed of chitin (N-acetylglucosamine linked by 1,4-alpha-glycosidic bonds), lipids, and tanned proteins (a type of cross-linking of adjacent amino acids); chitin may also be cross-linked to proteins
    2. often impregnated with inorganic salts, or coated with waxes
    3. constructed of plates (sclerites)
      1. tergites (dorsal sclerites)
      2. pleurites (lateral sclerites)
      3. sternites (ventral sclerites)
    4. muscles attached to inner cuticular ridges (apophyses) or even spines (apodemes)
  8. cuticular layers
    1. secreted by underlying hypodermis (epidermis)
    2. epicuticle (cuticulin) one micrometer or less in thickness, consisting of protein without chitin; sometimes coated or impregnated with lipid
    3. underlying the epicuticle is a procuticle
      1. in insects, divided into an outer exocuticle (heavily sclerotized) and an inner endocuticle (less sclerotization)
      2. in crustacea, procuticle impregnated with organic salts
        1. outer pigmented layer heavily sclerotized, and hardened with salts
        2. inner unpigmented layer hardened and outer portion calcified, but more flexible as proteins unsclerotized
  9. molting
    1. molting (ecdysis) controlled by a series of hormones that vary between groups
      1. synthesis of new proteins
      2. detachment of hypodermis from old procuticle (apolysis)
      3. enzymes produced by hypodermis begin to dissolve old procuticle; much of old cuticle salvaged and recycled
      4. new epicuticle forms and hardens
      5. new procuticle formed; protected from enzymes dissolving the old cuticle above by the new epicuticle
      6. old cuticle that is left splits (crustacea imbibe water; insects inhale air) and insect exits
    2. each stage between molts termed an instar
    3. various numbers of instars, dependent upon the species
  10. most species produce eggs that develop into larvae (life-cycle stage structurally distinct from the adult)
  11. some species have nymphs (life-cycle stage structurally similar to the adult)
  12. pupa (nonfeeding period where reorganization of structures occurs)
  13. complete gut; basic plan a foregut (stomadaeum), midgut, and hindgut (proctodaeum); midgut often has cecae
  14. respiration via tracheal system, gills, or book lungs
  15. excretion via Malpighian tubules
  16. neuroanatomy consists of dorsal ganglion (brain), nerves that run to the various sense organs, and a ventral nerve trunk with segmental ganglia
  17. hemocoel
    1. main body cavity (coelom) is technically remnant in a couple of internal organs. The hemocoel is actually a secondary body cavity with a limited amount of space
    2. filled with hemolymph, containing a variety of cell types
    3. enters heart through pores with one way valves (ostia)
    4. when heart pumps, forces blood through a series of open ended arteries so that hemolymph simply baths organs; hemolymph follows channels (sinuses) in body cavity
  18. external structure
    1. crustacea
      1. one or more somites of the thorax are fused with the head to form a cephalothorax
      2. cephalothorax followed by thorax and abdomin
      3. entire body sometimes covered by carapace
      4. median eyes present in larval forms (and adult copepods) and consist of 3-4 pigmented, light-sensitive ocelli
      5. most adults with compound eyes, which may be stationary or even mounted on stalks
      6. 1st pair of appendages termed antennules; 2nd pair of appendages termed antennae; most sensory (sometimes other functions)
      7. various anterior appendages have been modified into sets of feeding structures. Mandibles (1st pair), maxillules (1st pair of maxillae), and maxillae (2nd pair of maxillae)
      8. if appendages derived from the thorax are incorporated into the mouthparts, they are termed maxillules
      9. appendages biramous (2 branches)
        1. exopod or exopodite is the lateral branch
        2. endopod or endopodite is the medial branch
        3. each branch arises from a structure termed a basis; which is attached to a coxa
      10. thoracic appendages termed pereiopods (most for walking, swimming, or reproduction)
      11. abdominal appendages termed pleopods (most for walking, swimming, or reproduction)
      12. abdomin ends in a structure termed a telson, which may have associated abdominal appendages (pleopods) termed uropods
    2. insects
      1. head, thorax, and abdomin
      2. head is composed of 6 fused segments; 4 with appendages modified into feeding structures
        1. mandibles are the principle feeding structures
        2. posterior to the mandibles are the maxillae, which are used to help manipulate food
        3. labium is composed of fused appendages, posteriorly, and covers the posterior portion of the mouth parts
        4. labrum composed of fused appendages, anteriorly, covers anterior portion of the mouth
        5. maxillae and labium may also possess accessory food manipulatory or sensory structures termed palps
      3. head also with additional sensory structures
        1. 1 pair antennae
        2. 1 pair compound eyes
        3. 1 pair ocelli
      4. thorax composed of 3 segments, each with a pair of legs
        1. prothorax
        2. mesothroax (with pair of wings evolutionarily; may be reduced or absent in some)
        3. metathorax (with pair of wings evolutionarily; may be reduced, absent, or even modified into balance organs in some such as dipterans where they are termed halteres)
      5. many species with peritrophic membrane
    3. arachnids
      1. prosoma (cephalothorax) and opisthosoma (abdomin)
      2. somites highly fused
      3. acari (ticks and mites) with distinct modifications
        1. gnathosoma (basis capitulum, hyposome, and palps); has the feeding appendages
        2. idiosoma (remainder of body); often covered by a carapace
        3. hypostome (fused coxae of the pedipalps; extends anteriorly)
  19. taxonomy divides arthropods into 3 distinct subphyla
    1. Subphylum: Crustacea
      1. 2 pair antennae
      2. 1 pair mandibles
      3. 2 pair maxillae
      4. appendages, except for first pair antennae (antennules) biramous
      5. head often poorly defined from main body; cephalothorax normally with carapace
      6. most species aquatic; most species with nauplius larva
    2. Subphylum: Uniramia
      1. 1 pair antennae
      2. 1 pair mandibles
      3. 1-2 pair maxillae
      4. appendages all uniramous
      5. body with distinct head, thorax, and abdomin
      6. thorax usually with 2 pair of wings (sometimes reduced or absent, however) and 3 pair legs
      7. this subphylum and crustacea more closely related than chelicerates
    3. Subphylum: Chelicerata
      1. antennae absent
      2. with chelicerae (first pair appendages, modified into a variety of feeding structures)
      3. 1 pair pedipalps (2nd pair of appendages; various functions)
      4. most adults with 4 pair of legs
      5. body divided into prosoma (cephaltothorax) and opisthosoma (abdomin)

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