116 2nd lec

• What type of appendages do arthropods have?

  Segmented/jointer appendages
• skeletal system?

  Chitinous exoskeleton
• Coelom?

  Shizocoelomate
• germ layers?

  Triploblastic
• Symmetry?

  Bilaterally symmetrical
• External features of arthropods

  metamerism, tagmatization
• Variations in tagmatization

  Cephalothorax, opisthosoma
• Eight segments of joint appendages of arthropods

  Coxa, trochanter, femur, patella, tibia, metatarsa, tarsus, pretarsus
• chitinous exoskeleton - secreted by the hypodermis

  • composition; thin outer epicuticle - proteins & wax thicker inner procuticle made up of:
  • outer exocuticle - made up of chitin & protein forming a complex glycoprotein, can be impregnated with salts & tannins (tanned)
  • inner endocuticle- same as above but not tanned
  • makes movement possible by the division of the cuticle into separate plates
  • articular membrane - primitively connects the plate of one segment to the plate of the adjoining segment; a region in which the cuticle is very thin and flexible
• four primary plates of the cuticle of each segment

  dorsal tergum, 2 lateral pleura, ventral sternum
• joints – tube-like segments or sections of the cuticular skeleton of the appendages, connected by an articular membrane; enable the segments of the appendages and body to move
• pores - generally penetrates the body, where secretions pass through
• structures originating from the ectoderm and lined by the exoskeleton:

  i. tracheal tube (insects, chilopods, diplopods)

  ii. book lungs (scorpions and spiders)

  iii. proctodeum and stomodeum

  iv. certain portions of the reproductive tract
• color: depends on deposition of brown, yellow, orange or red melanin pigments within the cuticle; also present are iridescent greens, purples and other hues due to striations of the epicuticle
• subcuticular chromatophores, blood and tissue pigments are other sources of pigments
• after molting, the new cuticle is soft and pliable, and is stretched to accommodate the increase in size of the arthropod due to internal pressure, which in turn is due to tissue growth, and sudden uptake of air and/or water

  additional procuticle may be added
• • instars - stages in between molts, the number of which may be fixed or continuous throughout life
  • ecdysomes (hormone which controls molting)
• IV. Muscular System

  striated muscle bundles - attach to the inner surface of the procuticle by tonofibrils

  • innervated by neurons which may be:

  i. phasic (fast)

  ii. tonic (slow)

  iii. inhibitory (prevents contraction)
• V. Coelom

  • schizocoelom - hemocoel that are sinuses or spaces in the tissue filled with blood, lined with cilia
  • reduced to a cavity of the gonads or by the excretory organs
• VI. Circulatory System

  open blood vascular system

  i. heart consists of one or more chambers with vascular walls arranged in a linear fashion

  ostia - pairs of lateral openings perforating the wall; enable the blood to flow into the heart from the pericardium (large surrounding sinus, a part of the hemocoel)

  ii. blood

  amoebocytes

  hemocyanin - respiratory pigment

  • direction of blood flow: heart ===> arteries ==> sinuses (which bathe the tissues) ==> pericardium => heart
• VII. Digestive System

  foregut - derived from the stomodeal region of the ectoderm

  chiefly concerned with ingestion, trituration and storage of food; its parts are variously modified for these functions depending upon the diet and mode of feeding

  midgut - derived from the endoderm; site of enzyme production, digestion and absorption

  commonly the surface area is increased by out pocketing forming pouches of large digestive glands

  • hindgut - derived from the proctodeal region of the ectoderm - functions in the absorption of water and the formation of feces
• VIII. Nervous System

  I. Brain

  -high degree of cephalization accompanied by well-developed sense organs (eyes and antennae).
• Three major regions of the arthropod brain

  • Protocerebrum
  • Deutocerebrum
  • Tritocerebrum
• Protocerebrum


  • One to three pairs of optic centers (neuropiles) which function in integrating photoreception and movement and are probably the centers for the initiation of complex behavior
• Deutocerebrum


  • Receives the antennal nerves and contains their association centers, arthropods without antennae lack this region
• Tritocerebrum


  • Gives rise to nerves that innervate the labium (lower lip), the digestive tract (stomatogastric nerves), the chelicerae of chelicerates; commisure is postoral, i.e. located behind the foregut
• Arthropod brain has a ventral nerve cord and segmental ganglionic swellings
• Sense Organs


  • Usually associated with some modification of the chitinous exoskeleton which otherwise would at a barrier to the detection of external stimuli
• Sense Organs include

  • Hairs
  • Bristles
  • Setae that may carry chemoreceptors
  • Canals
  • Pits
  • Slits that may house chemoreceptors or the opening may be covered by a thin membrane to the underside of which is attached a nerve ending, detect vibrations or other forces that change the tension of the skeleton
  • Proprioreceptors that attach to the inside of the integument or to tendons and muscles, for deletion of movement and position of muscles and appendages relative to each other
  • Eyes
• Simple Eyes


  • Few photoreceptors
• Compound Eyes


  • Composed of many long cylindrical units called ommatidia
• Cornea


  Translucent, covers outer part of ommatidium; functions as lens
• Facet


  External surface of cornea is square or hexagonal
• Crystalline cone


  Long cylindrical or tapered element behind the cornea, functions as a second lens
• Retinula


  Forms basal end of the ommatidium the center of which is occupied by rhabdome (translucent cylinder)
• Rhabdomere


  Area of tubules projecting centrally to make up a major part of the rhabdome; functions s a single photoreceptor unit and transmits a signal that represents a single light point
• Apposition Eyes


  Best in bright light
• Superposition Eyes


  Mostly in dim light but may also function in brighter conditions
• X. Reproduction and development

  -mostly dioecious;

  -fertilization: generally internal in terrestrial forms; external in aquatic forms

  -eggs: rich in yolk, centrolecithal (nucleus surrounded by a small island of non-yolky cytoplasm

  -cleavage: intralecithal or superficial

  -development: stereogastrula