💩 Excretory System

• The process of elimination of nitrogenous waste (harmful) products from insect body is known as excretion. It maintains internal environment of insect body and ionic balance of haemolymph.
• These toxic materials are nitrogenous products of metabolism (mainly ammonia), pigments, salts etc.
• The principal excretory product in gaseous form is CO2, liquid form is honey dew, solid form is urea/uric acid and semi solid form is allantoin.

Excretion and Osmoregulation

• Insect faeces, either in liquid form or solid pellets, contains both undigested food and metabolic excretions. Aquatic insects excrete dilute wastes from their anus directly into water by flushing with water. But Terrestrial insects must conserve water. This requires efficient waste disposal in a concentrated or even dry form, simultaneously, avoiding the toxic effects of nitrogen.
• Both terrestrial and aquatic insects must conserve ions, such as sodium (Na⁺), potassium (K⁺) and chloride (CI^-), that may be limiting in their food or lost into the water by diffusion.

Excretory Organs

Malpighian Tubules

• In insects it is chief organ of excretion.
• The Malpighian tubules long, tubular structures which open proximally in between midgut and hindgut and closed distally, floating freely in the haemolymph, waving around in the haemolymph where they filter out solutes.
• It is present in all insects except collembolan and aphids.
• The primitive number of malpighian tubules is six.
• In Lepidoptera, MT are found only six while in isopteran 2-8, in orthoptera 2-200 in number and in collembolan and aphids these are absent.
• These are discovered by an Italian scientist, Marcello Malpighi in the year 1669, which were named after him by Heckel in 1820.
• Functions of Malpighian tubules:
  • Helps in the process of excretion or removal of waste products in order to regulate the internal body environment by maintaining ionic and water balance.
  • In case of glow worms, the distal ends of tubules produce light energy.
  • Also helps in the storage of Ca necessary for the processes such as hardening of puparium.

    

Cryptonephry

• In some of the insects such as caterpillars and coleopterans, the distal ends of the Malpighian tubules get reattached to the alimentary canal by opening into the rectum of hindgut. This condition is called ‘cryptonephridial condition’.
• The cryptonephridial arrangement is concerned with re-absorption of water from rectum.
• It is found in larvae of Lepidoptera and larva & adult of Coleopteran to enable to conserve moisture from rectum.

Nephrocytes

• These are the special cells that are distributed in the body cavity and scattered.
• Nephrocytes are cells that take up foreign chemicals of relatively high molecular weight which malpighian tubules may be incapable of dealing with.

Integument

• Through the process of moulting, insects remove the waste nitrogenous products, i.e., they are deposited in the form of exuviae.
• In some insects, where respiration occurs through body wall, CO₂ is removed through integument as waste product (cutaneous respiration).

Tracheal System

• The respiratory tubes, the trachea which are distributed throughout the body, function in elimination of CO₂ through spiracles.

Alimentary canal

• The gut of the insects also plays a major role in excretion by removing the unwanted material, dead cells formed during enzyme secretion (holocrine) and intima layer during moulting.
• Rectum plays an important role in excretion by reabsorbing the water from faeces.

Fat bodies

• A loose or compact aggregation of cells, mostly trophocytes, suspended in the haemocoel, responsible for storage and excretion.

Oenocytes

• These are specialised cells of haemocoel, epidermis or fat body with many functions.
• One of the functions is excretion.

Urate Cells

• Some of the fat body cells which store urea or uric acid in the form of granules are known as urate cells.
• Preserved uric acid can be utilized subsequently.
• This phenomenon of storage of urea / uric acid in the fat body cells is called ‘storage excretion’ which is useful for supply of nitrogen, when insect feeds on nitrogen deficient food.

Chloride Cells

• These are the cells distributed on the body of aquatic insects such as larva of mayfly or stone fly.
• They absorb ions from salt water (body) and then excrete into surrounding medium to compensate the changes in the ionic concentration of haemolymph.

- Insecta - Introduction: K.N. Ragumoorithi, V. Balasurbramani & N. Natarajan
- A General Textbook of Entomology (9th edition, 1960) – A.D. Imms (Revised by Professor O.W. Richards and R.G. Davies). Butler & Tanner Ltd., Frome and London.
- The Insects- Structure and Function (4th Edition, 1998) – R.F. Chapman. Cambridge University Press
- https://www.amentsoc.org/
- Researchgate
- Wikipedia