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.
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
uticle that covers their body
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).
Given that name, because they have two wings (other groups of
insects that fly often have four wings such as the Lepidoptera
(butterflies) and Hymenoptera (wasps)).
The second set of wings on a dipteran is actually replaced with
(one haltere on each side) which function to
maintain the balance of the flies.
The Diptera is a huge group containing many species (more than
85,000), and a lot of these species are beneficial in that they assist
among other things in the process of decomposition of various organic
matter such as carcasses.
Unfortunately, some of the diptera have developed life cycles that
require the use of a vertebrate host, this may be for the purpose of
obtaining a blood or tissue fluid to nourish eggs developing in the
female or just adult flies in general, or they may use vertebrates as sites
of larval development.