Arthropods lec

• Two theories of arthropods where it has risen from Polychaete stock while another theory states that arthropods and annelids came from a common stock.
• The arthropods are animals with segmented/ jointed appendages covered with a chitinous exoskeleton.
• They are schizocoelomate, triploblastic, and bilaterally symmetrical.
• Metamerism - evident in the embryonic development of all arthropods and is conspicuous feature of many adults especially the higher primitive species
• Tagmatization- resulted from the fusion of metamere, with the last 5 segments fused into a head, the nest 3 into the thorax with most of the abdominal segments retained.
• variations in tagmatization size:

  cephalothorax- fusion of head and thorax

  opisthosoma- fusion of thorax and abdomen
• eight segments of jointed appendages of arthropods

  1. Coxa- proximal
  2. trochanter
  3. femur- usually longest
  4. patella
  5. tibia
  6. metatarsus
  7. tarsus
  8. pretarsus- usually modified for function
• Insects use tracheae (respiratory tubes) instead of hemocyanin.
• thin outer epicuticle - proteins & wax

  thicker inner procuticle
• 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
• Movement

  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 cuticles:

  1. dorsal tergum
  2. two lateral pleura
  3. 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:

  1. tracheal tube (insects, chilopods, diplopods)
  2. book lungs (scorpions and spiders)
  3. proctodeum and stomodeum
  4. certain portions of the reproductive tract
• color: depends on the 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
• ecdysis - molting which allows the arthropod to grow in size
• How does ecdysis or molting work?

  i.  hypodermis detaches from the cuticle and secretes a new epicuticle

  ii. hypodermis secretes chitinase and proteases that seep through the newly formed epicuticle and erode away the untanned endocuticle of the old skeleton

  iii. hypodermis secretes a new procuticle

  iv. old exoskeleton splits along certain predetermined lines usually along the mid-dorsal and mid-ventral lines, and the arthropod pulls out of the encasement
• 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
• instars - stages in between molts, the number of which may be fixed or continuous throughout life
• ecdysomes- hormone which controls molting
• Endoskeleton

  • in folding of the procuticle that produces inner projections or apodemes on which the muscles are inserted
• Endoskeleton

  • may involve the sclerotization of internal tissue forming free plates for muscle attachment within the body
• Muscular System

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

  innervated by neurons which may be:

  i. phasic (fast)

  ii. tonic (slow)

  iii. inhibitory (prevents contraction)
• schizocoelom - hemocoel that are sinuses or spaces in the tissue filled with blood, lined with cilia
• Coelom

  reduced to a cavity of the gonads or by the excretory organs.
• Circulatory System- open blood vascular system
• Circulatory System


  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)
• blood has:

  amoebocytes

  hemocyanin- respiratory pigment
• Blood flow:

  Heart --> arteries --> Sinusus (bathe the tissues) --> pericardium --> heart
• 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
• Digestive Sys.

  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
• Digestive Sys.

  hindgut - derived from the proctodeal region of the ectoderm - functions in the absorption of water and the formation of feces
• Nevous System:

  I. Brain- high degree of cephalization accompanied by well-developed sense organs (eyes and antennae).

  three major regions:

  1.   Protocerebrum

  2. Deutocerebrum

  3. Tritocerebrum
• Nervous Sys

  II. Sense organs

  1. hairs bristles, setae- chemoreceptors
  2. canals, pits, slits- chemoreceptors, cant detect vibrations or forces that change tension of skeleton
  3. Proprioreceptors- 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.
  4. Eyes
• Three major regions of brain:

  1.   Protocerebrum- with 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.

  2. Deutocerebrum- receives the antennal nerves and contains their association centers, arthropods without antennae lack this region.

  3. 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.
• Eyes

  1. simple with few photoreceptors
  2. compound composed of many long cylindrical units called ommatidium