The following points highlight the thirteen main excretory organs found in arthropods. The excretory organs are: 1. Nephridia 2. Coxal Glands 3. Green Gland 4. Shell Glands 5. Malpighian Tubules 6. Hepatopancreas 7. Fat Body 8. Exoskeleton 9. Intestinal Caeca 10. Midgut Epithelium 11. Pericardial Cells 12. Nephrocytes 13. Oenocytes.
Excretory Organ # 1. Nephridia:
These are present in the Peripatus and are situated on the lateral side of the segmented body cavity. Numbers of these paired organs correspond to the number of the segments of the trunk.
Each nephridium consists of a terminal vesicle which opens to the exterior through one end and remains connected to a coiled loop with the other. This loop is known as nephridial canal and it opens inside the body cavity. Its internal lining is ciliated.
Excretory Organ # 2. Coxal Glands:
These glands are present only in Arachnida and their structures and positions vary (see Table 18.18—Arthropoda). Each coxal gland consists of convoluted tubules, called labyrinth and a sac called labyrinth sac. It opens externally by a short tube.
Excretory Organ # 3. Green Gland:
It is also known as the antennal gland or maxillary gland. In some species, the organs are also called green gland or antennal gland because of their colour and location (e.g., Astacus). In some freshwater crustacean species, the organs are situated near the maxillary segments and are called maxillary glands.
It is found in Malacostraca (excepting Isopods) and larval forms of all Crustaceans, specially in Entomostracan larvae. It is present in the proximal segment of the second antenna or adjoining regions of the head. Each gland has three parts—end sac, labyrinth and bladder. The labyrinth is the proper excretory gland.
Excretory Organ # 4. Shell Glands:
These glands are also known as maxillary glands and are present in the coxopodites of second maxillae in Branchiopoda, Ostracoda, Copepoda, Cirripedia and larval forms of all Crustaceans.
Excretory Organ # 5. Malpighian Tubules:
These are long filamentous bodies with or without lumen and are made up of ciliated or cubical epithelium. These tubules usually originate from the region of the gut which denotes the beginning of hindgut. Among the Crustaceans, the Amphipods possess one pair of tubules which originate as diverticula of the alimentary canal.
In Insecta these develop from the undifferentiated region between the midgut and hindgut and the number varies from 2 to 150. Malpighian tubules occur in all insects except in Collembola, some of the Thysanura and the Aphids. The tubules often occur in two’s or multiples of two.
The Malpighian tubules may be divided into 4 main types:
(a) In the simplest type the tubules join at the junction of midgut and hindgut. The distal ends of the tubules remain free and terminate blindly. The contents of the tubules are usually fluid and sometimes found the crystals when the insects are found in arid condition, e.g., Orthoptera, Dermoptera and Coleoptera.
(b) In the second type the distal ends of the tubules are attached to the hindgut. The condition of these is known as cryptonephridial or cryptonephridic or Cryptosolenic. This condition is seen among many Coleoptera and most Lepidoptera. Cryptonephridial condition is seen in the insects when they live in the drier environments and that helps the insects to conserve water by absorbing it from the faeces.
(c) The Malpighian tubules of the third type remain free at the distal ends and connect proximally with the gut through ampullae, and are found in Hemiptera.
(d) The fourth type of Malpighian tubules are found in the Lepidoptera and this type is a combination of second and third types. Two to four pairs of tubules are found in Myriapods and Arachnids and in the later the tubules are endodermal in origin.
Excretory Organ # 6. Hepatopancreas:
In Limulus, absorptive cells are present in the hepatopancreas. These cells shed large amount of calcium phosphate as excretory product into the intestine through which it is eliminated along with faeces.
Excretory Organ # 7. Fat Body:
In Insecta, Myriapoda and Onychophora, the fat bodies are made up of polygonal cells. The cells, as they grow old, become filled up with minute urate crystals.
Excretory Organ # 8. Exoskeleton:
In Crustaceans and Insects, the cells of the hypodermis secrete nitrogenous substances which remain deposited within the exoskeleton. These are eliminated at the time of ecdysis.
Excretory Organ # 9. Intestinal Caeca:
In Squilla, the rectum bears a pair of intestinal caeca having a comb-like internal wall. These are believed to be excretory in function.
Excretory Organ # 10. Midgut Epithelium:
In Nauplius larvae of Crustacea, the cells surrounding the midgut carry out excretory function.
Excretory Organ # 11. Pericardial Cells:
In Insects some cells around the heart and the pericardial membrane are excretory in function.
Excretory Organ # 12. Nephrocytes:
These are migratory cells, present in groups within the haemocoel of insects. These are regarded as modified fat body cells and are said to absorb unwanted colloidal particles from the blood.
Excretory Organ # 13. Oenocytes:
In Insects and Myriapods, certain cells are found in groups around the abdominal spiracles. These cells originate from surface epithelium and are believed to be both excretory and circulatory in functions. The excretory system is well-developed in land-living arthropods, which are concerned with the problem of water loss. In them the excretory organs work in such a way that very little water is lost from the body.