Excretory Systems
What is homeostasis?
(And why does it matter?)
Homeostasis - the tendency toward a relatively stable
equilibrium between interdependent elements
• Cell activity is carried out most efficiently within a narrow
range of conditions
• The environment within cells and in insects (organisms)
should be maintained near optimal
• What is maintained?
constant level of salts and water, plus osmotic pressure in the hemolymph
1. elimination of toxic nitrogenous wastes from protein and purine metabolism
2. elimination of other compounds absorbed from the environment
Nitrogenous excretion
• ammonia is the primary nitrogenous end
product of protein and purine metabolism
• it is highly toxic
• safe elimination requires that it be present at
very low concentration
• ammonia is excreted in large quantity primarily
by insects living in fresh water, or very moist
environments
Nitrogenous Excretion
(Terrestrial insects convert ammonia to uric acid)
• uric acid is relatively harmless; but expensive to produce (in fatbody)
-
>80% of total nitrogenous material that is excreted
• it conserves water use because less water is needed for excretion
• it contains less hydrogen per atom of nitrogen than any other
nitrogenous end product produced by animals
-
this means less water needed for its production (transported by a protein?)
Examples of purines
A pyrimidine ring fused to an imidazole ring; purines are the most widely
occurring nitrogen-containing heterocycle in nature
imidazole
pyrimidine
The excretory system
(What does it involve and how does It work?)
• It involves the production of fluid urine that carries potentially
toxic materials from the body
• What parts of the body are involved?
1. Malpighian tubules (MT)
2. hindgut (HG), specificallly the ileum and rectum
• It occurs in two stages:
1. the formation of ‘primary urine’ (MT)
2. the selective modification of primary urine to form
‘secondary urine’ (HG)
Insect excretion (the big picture)
The name Malpighian tubules is derived from Marcel Malpighi, who made
reference to them as “vasa varicose” in 1669 in his work with the silkworm.
principal cells (pc) from ectoderm
stellate cells (sc) from mesoderm
(exist in a 5:1 ratio)
Malpighian tubules
(What about em?)
Marcello Malpighi
(1628-1694)
Founder of Comparative
Physiology
• long, thin, blindly end tubes
• lie freely in the body cavity
- variable in number (2 to 250)
- variable in length (2 to 100mm)
- variable in diameter (30 to 100 µm)
• absent in Collembola & aphids
Chapman (Fig 18.1)
Urine production
(How is primary urine formed?)
principal cells have
lots of microvilli
and mitochondria;
Na, K, and H
(plus fluids)
stellate cells have
less microvilli and
mitochondria;
mostly Cl
Urine production
(How is primary urine formed?)
1. cations move into lumen (via ATPase)
2. anions follow (down their electrochemical
gradient)
3. water follows (transcellularly)
Chapman (Fig 18.5)
Modifying primary urine
(How is secondary urine formed?)
The cuticle lining of the rectum limits the
size of the molecules that can be absorbed
(e.g., glucose yes, trehalose no)
Chapman (Fig 18.7)
Rectal pads line the rectum
(specialized cells involved in ion transport)
Urine production (an overview)
Water gain via the rectum
• in tenebrionid beetles, Malphigian tubules
are in close contact with the rectum,
enclosed by layers of flattened cells that
form the perinephric sheath
• involves the active transport of potassium,
sodium and hydrogen ions; chloride ions
follow passively
• air enters the anus
• water taken up via osmotic pressure
Additional functions of MTs
• produce silk
- lacewings
- antlions
- chrysomelid beetles
• produce material that when
combined with air forms
spittle
- spittlebugs
• they act as luminous organs
- fly larvae (Arachnocampa
luminosa)