Abdomen
The
abdomen is a part of the body. In humans, and in many other
vertebrates, it is the region between the
thorax and the
pelvis (Separating the thoracic cavity and the abdominal cavity is the
diaphragm). In fully developed
insects, the abdomen is the third (or posterior) segment, after the head and thorax.
In vertebrates, the abdomen contains several
organs:
* Part of the
digestive system (
stomach,
liver,
pancreas, and
intestines),
* The entire
urinary system (
kidneys,
bladder).
* The internal
reproductive organs (
ovaries,
fallopian tubes and
uterus in women,
testis,
Vas deferens and
prostate in men)
The abdomen also contains some of the largest and most easily accessible
blood vessels in many animals, and is often used in
medicine and
experimentation for
catheterisation.
For various reasons, the abdomen is often coloured differently from the rest of the body. In animals with
furry or
hairy bodies, the abdomen is frequently hairless, or nearly so.
The abdomen is oval in shape and is the largest cavity in the body. It can be broken down into the lower and upper extremity. The lower extremity covers the inner surface of the bony pelvis. The Levator ani and Coccygeus are located on either side. The diaphragm forms the upper extremity and acts as a dome over the abdomen extending to the upper border of the fifth rib.
The invertebrate abdomen is built up of a series of concave upper plates known as 'tergites' and convex lower plates known as 'sternites', the whole being held together by a tough yet stretchable membrane.
The abdomen contains the insect's digestive tract and reproductive organs, it consists of eleven segments in most orders of insects though the eleventh segment is absent in the adult of most higher orders. The number of these segments does vary from species to species with the number of segments visible reduced to only seven in the common honeybee. In the 'Collembola' (Springtails) the abdomen has only six segments.
Unlike other Arthropods, insects possess no legs on the abdomen in adult form, though the 'Protura' do have rudimentary leg-like appendages on the first three abdominal segments. Many larval insects including the 'Lepidoptera' and the 'Symphyta' (Sawflies) have appendages called 'pseudo' or prolegs on their posterior abdominal segments as well as their more familiar thoracic legs which allow them to grip onto the edges of plant leaves as they walk around.
