by Dwight Bowman
The word “arthropod” means jointed body.
The most numerous organisms on earth, so in some respects, the
exoskeleton has been more successful than the backbone.
ortunately only a few are truly parasitic on humans.
Fortunately, only a few are truly parasitic on humans.
Primitive arthropods are suggested to have a form that consists of a
body composed of a series of segments where each segment had a
pair of jointed appendages, one on each side; down the center of
the body extended the tube representing the digestive tract.
The exoskeleton, that part which gives them that distinctive “crunch” if
you ever happen to crush one, covers the outside of the body and is
composed of chitin.
To grow or to change their internal and external morphology (the
process of metamorphosis), arthropods must change the external
cuticle that covers their body.
This they do through a process of laying down a new cuticle in a
process called molting, and then, the old cuticle is shed or ecdysed.
The body is composed of hollow spaces with a gut, heart (an open
circulatory system), and gills or tracheae for supplying air.
Four major groups of Arthopods:
The Trilobites comprise a group known only from fossils.
The Crustacea compose the group that includes the well
known shrimp, lobster, crab, and crayfish, along with the
less well known copepods.
The Cheliceriformes include the horseshoe crabs and the
arachnids (the spiders, scorpions, mites, and ticks).
The Uniramia include the Myriapods (millipedes),
Chilopods (Centipedes), and the Insects (Hexapods).
There are some obvious differences in the morphology
f insects and arachnids
of insects and arachnids.
Adult insects have 6 legs, adult arachnids have 8 legs.
Adult insects have three major body divisions (head,
thorax, and abdomen); adult arachnids have two basic
body divisions (cephalothorax and abdomen).
The medically important arachnids consist of
spiders (including the funnel webs, widows and recluse spiders),
subclass Acari (ticks and mites).
Within the subclass Acari, there are four orders of importance in
or Metastigmata (ticks)
And the mites:
Features that distinguish mites from ticks biologically
Mites are smaller than ticks
Have a short life span (usually living up to 3 months)
Often have reduced appendages, a body with long
, and possess a
ut this structure lacks the recurved teeth found on ticks
but this structure lacks the recurved teeth found on ticks
Mites may penetrate and reside below the host’s epidermis or in the
lungs and respiratory passages, ear canal and feather quills.