In this article we will discuss about Millipedes:- 1. Habit and Habitat of Millipedes 2. External Structures of Millipedes 3. Integumentary System 4. Digestive System 5. Circulatory System 6. Respiratory System 7. Excretory System 8. Nervous System 9. Reproductive System.
- Habit and Habitat of Millipedes
- External Structures of Millipedes
- Integumentary System of Millipedes
- Digestive System of Millipedes
- Circulatory System of Millipedes
- Respiratory System of Millipedes
- Excretory System of Millipedes
- Nervous System of Millipedes
- Reproductive System of Millipedes
1. Habit and Habitat of Millipedes:
Altogether 92 genera and 290 species of millipedes have been recorded in India; the most well-known millipede belongs to the genus Thyropygus. Ten species of Thyropygus are known and they range from southernmost part of India to the high altitudes of the Himalayas.
During rainy season Thyropygus lives in moist soil and during drought it lives within burrows under the upper layers of the soil. They love to stay in calcareous soil and have special attraction for sugar and manure. The secretion of stink gland from some large millipedes may produce wound on the human skin. The species described here is known as Thyropygus poseidon.
2. External Structures of Millipedes:
An adult Thyropygus may be of 6 to 7 cm in length and 7-8 mm in breadth. The body is dark-brown in colour with red patches in the mid-dorsal region.
The body is divisible into:
(1) A small head and
(2) Extended trunk (Fig. 18.40).
The head is curved anteroventrally and has powerful exoskeletal thickenings on its anterior, dorsal and lateral walls. The tergite of the first segment is called collum which fully covers the head anteriorly, dorsally and laterally to form a head capsule (Fig. 18.40).
Posteriorly the head capsule remains in contact with the tergite of second segment and ventrally with a plate-like gnathochillarium (Fig. 18.41B). The head capsule is constituted anteriorly by a median frons, a pair of clypeo-lateral lobes. Each lobe is formed of a clypeus and labrum.
The labrum is broad, freely movable and united with the ventral side of the clypeus. On the ventral side labrum constitutes a sinus. The clypeolabral lobe extends to form a concave epipharyngeal surface in the pre- oral cavity and sets freely on the anterior margin of the mandibles.
The ventral gnathochillarium is made up of fused maxillae and is divisible into following parts:
(1) Prementum—median, triangular plate,
(2) Mentum and
(3) Submentum—slender and elongated plates present posterior to prementum,
(4) Stipes—one pair, one on each lateral and anterior side of prementum. Each stipe bears posteriorly a small cardo and anteriorly a pair of palps, called malagnathochillaria, carrying sensory organs, called basiconic sensillae,
(5) Lingua— Present in between the apex of prementum and stipes. On each side another triangular plate, called lingua, is present with lingual lobe.
Head bears following structures:
Paired eyes occur one on each side of the frons. The middle of each eye is raised to a point. Each eye is an aggregation of simple eyes and is provided with skeletal ocular ridges.
B. Organ of Tomosvary:
Paired sensory organs, present one on each side and slightly ventral to the eye.
Paired antennae, originate one from each anterior side of the head. Each antenna is seven-segmented and placed within membranous antennal socket.
The trunk contains nearly 47- 48 segments. All the segments are doubled and such possession of diplosegments is regarded as the characteristic feature of millipeds. Each segment has both exoskeletal and endoskeletal coverings.
The cuticle in each segment is completely ring-like. The segments are attached with one another by a thin membrane. The terminal segment or anal segment or telson is more or less triangular and bears a short upwardly directed curved tail.
The trunk region contains following structures:
The first five segments superficially appear to be single and carry only one pair of legs in each segment, while remaining segments have two pairs of legs in each of them. Each leg has following parts—coxa, prefemur, femur, post-femur, tibia, tarsus, pretarsus and a terminal claw (Fig. 18.40D, E). In males, the legs of the seventh trunk segment are modified as intromittent organs and known as gonopods (Fig. 18.41C).
B. Stink glands:
The paired openings of the stink gland are present on the dorsolateral side of each trunk segment, excepting first five and last few segments. Inside the body cavity, each stink gland is globular and sac-shaped. It has a long neck called ejaculatory duct which possesses a cuticular tongue near its opening.
The ejaculatory duct and the tongue are provided with muscles and nerves. The secretion generally oozes out through the opening but may also be darted as a jet.
These minute opening are present ventro-laterally on each segment. The first (three segments bear one pair of stigmata while the Remaining segments have two pairs in each of them. These openings communicate with the inner tracheal pockets or pouches.
It is the posterior-most terminal opening of the alimentary canal and present in the last segment and directed ventrally.
Various structures which are present inside the trunk segments are shown in the figure.
3. Integumentary System of Millipedes:
The integument is represented by an outer cuticle and inner epidermis (Fig. 18.42). During summer cuticle may be distinctly separated into epicuticle and procuticle, but in rainy season epicuticle disappears. The procuticle is again divided into a thin homogeneous outer exocuticle and thick, lamella ted inner endocuticle. The epidermis includes one layer of large cells on a sheath of basement membrane.
The epidermal cells perform three important functions:
(1) Secrete juice which digests the old cuticle,
(2) Produce the cuticle and
(3) Release certain substances which stiffen the cuticle.
Numbers of unicellular and multicellular glands are present in the epidermis. Each gland has a duct of its own, which traverses through the cuticle and opens to the exterior through minute apertures.
4. Digestive System of Millipedes:
The alimentary system includes:
(A) Alimentary canal and
(B) Digestive glands (Fig. 18.43).
A. Alimentary canal:
It is more or less a straight tube which is divisible into three parts:
(2) Midgut and
(3) Hindgut. The fore and hind guts have inner cuticular lining.
(b) Mouth cavity and
This crescent-shaped opening is present at the ventral side of the head region and between epicranium and gnathochillarium. It leads into mouth or oral cavity.
(b) Mouth cavity:
The anterior end of the cavity is formed by the inner side of the epicranium and the posterior side by gnathochillarium. Mandibles with teeth and bristles bound its lateral sides (Fig. 18.41A). The cavity is divided into two chambers by the projection of the hypopharynx. The mouth cavity opens within the next part of the canal, called oesophagus.
This short tube, immediately after originating from the mouth cavity runs dorsally, then takes a turn to the posterior direction and passes ventral to the brain. It continues up to seventh or eighth segment. The inner wall of the oesophagus is folded to form villus-like structure like the small intestine of higher animals. These projections are more extensive in the anterior part.
The inner cuticular lining is also much thicker at the anterior part but at the posterior region numerous spiny processes are present. Beneath the inner cuticular lining lies a layer of columnar epithelial cells.
This is covered by distinct layers of circular and longitudinal muscles, which in turn are enclosed within a peritoneal membrane. The longitudinal muscles are broad and placed apart from one another. This gives the characteristic appearance of the fore gut.
It runs up to thirty to thirty- two segments. The terminal part of the mid gut is marked by a swelling of fatty tissue, from where originates the paired Malpighian tubules.
Following are the characteristics of the mid gut region:
(a) It is wider but thin-walled.
(b) Longitudinal muscular bands are narrow.
(c) Circular muscle layer is thinner.
(d) Presence of a fatty tissue beneath the peritoneal lining.
(e) Inner cuticular lining is absent.
(f) Epithelial cells are of various shapes and have an inner striated border.
(g) Epithelial cells are secretory and absorptive in function.
This is the last part of the alimentary canal which terminates in an aperture, called anus. The first part of the hindgut is of same thickness as the midgut, but after a couple of segments, it becomes wider. The beginning of this wider region is marked by a constriction. The posterior part is again narrow and has thick muscles in the wall.
The hindgut has the following features:
(a) Circular muscles are well-developed, specially at the posterior part.
(b) The wall of the hind gut is internally projected into folds.
(c) Epithelial cells are cubical and have no striated border.
(d) Inner cuticular lining is of much lesser thickness than the foregut.
B. Digestive glands:
Most important gland for digestion is the salivary gland (Fig. 18.44). One pair of lobular glands is present in the dorsal side of the oesophageal region. From each salivary gland, minute tubules drain into a salivary duct. The first part of the duct is sac-like and acts as reservoir.
It continues as slender and much coiled duct which runs anteriorly to open within the mouth cavity near the inner side of gnathochillarium. The details about the nature of enzymes in the saliva are not known but the products are obviously digestive in function.
In addition to salivary gland, the lining of mid gut contains secretory cells which also produce digestive juices.
Mechanism of Feeding and Digestion:
The food includes soft parts of plants or decaying animals. It likes calcium containing leaves and prefers glucose and sucrose more than starch. The midgut is the region where digestion and absorption take place. The lining of the hindgut absorbs water from the residual food.
5. Circulatory System of Millipedes:
The circulatory system of Millipedes includes:
(b) Blood vessels and
The heart is an elongated tubular organ which begins from the first segment and continues up to anal segment. It is present along the mid-dorsal line of the body cavity and is placed above the alimentary canal. It is enclosed within a pericardial membrane.
Heart is constricted into a number of chambers, each segment having one. In between the two chambers there is a pair of lateral openings, called ostia, through which blood enters within the heart. The wall of the heart is provided with muscles for both contraction and relaxation.
B. Blood vessels:
In each segment heart sends a pair of lateral arteries each of which bifurcates and each branch passes around the gut to open within a median ventral artery, called supraspinal blood vessel. Before opening into the latter, the lateral artery sends a complex of lateral vessels to supply blood to the different parts of the segments.
Anteriorly the first chamber of the heart continues as a median cephalic artery. The cephalic artery sends two pairs of lateral vessels which after sending branches to the parts around the mouth cavity finally open within the supraspinal vessel. The cephalic artery finally breaks into a number of branches to supply brain and other structures in the head region.
The ventral supraspinal vessel also sends number of segmental branches to vascularise the different parts of the body. The blood flows anteriorly through the heart and posteriorly through supraspinal vessel. Blood from arteries enters within the haemocoelomic space and bathes the tissues. From these tissue spaces blood returns through lacunae and enters within the heart through ostia.
Blood contains liquid part plasma and three types of floating corpuscles or haemocytes. Various substances like protein, fat, sugars, free amino-acids, etc., are found in the plasma. Most common types of haemocytes are small in size but they possess large nuclei.
The other type of haemocyte is large, oval, with distinct nucleus and granulated cytoplasm. The third category of haemocytes is spindle-shaped. Blood in millipedes is not involved in transport of respiratory gases. It is concerned with food storage, wound repair, defence against foreign bodies and excretion.
6. Respiratory System of Millipedes:
Respiratory organs of Millipedes are in the form of tracheae (Fig. 18.45). In Thyropygus, the tracheae are un-branched. A major tracheal trunk includes a large number of smaller tracheal tubes and each trachea is made up of several tracheoles. The trachea has cuticular lining but the spiral thickening is not distinctly visible.
The tracheae communicate with the exterior through minute valve-fitted apertures, called stigmata. Each segment bears two pairs of stigmata, excepting the first three which possess one pair per segment. Each stigmata leads into a tracheal pouch formed by epidermal invagination.
From each tracheal pouch, three sets of tubes originate:
(a) Transverse bundle directed ventrally between the opposite pouches,
(b) One lateral tube on each side running dorsolaterally to supply finer tubes to the different parts of the corresponding segment, and
(c) A pack of tubes which join with a common ventral tracheal trunk.
The two ventral tracheal trunks run along the entire length of the body and break up into finer vessels to supply the head region. Finer tracheolar tubules are immersed in haemolymph and open directly within the tissue. Same set of vessels convey both oxygen and carbon dioxide.
7. Excretory System of Millipedes:
The cuticle and Malpighian tubules (Fig. 18.46) are regarded as the organs for removing metabolic wastes. The waste products like uric acids are deposited in the cuticle and are shed at the time of moulting. Malpighian tubules are two in number, originating from the ventro-lateral side of the midgut near its junction with hindgut.
The region of origin is surrounded by a ring of fat tissue. Each tubule, immediately after its origin, is directed posteriorly and then it takes a turn to run anteriorly up to the oesophagus. From there the tubule bends posteriorly and comes up to the rectum. It then again turns upward and ends blindly.
The tubule is convoluted and capable of various movements. It remains immersed within haemolymph from where it collects excretory products in the form of uric acid and ammonia. The waste products are deposited inside the alimentary canal and are ejected with the faeces.
The transverse section of the tubule shows following structures from outside to inside:
(i) Outer peritoneal layer with numerous externally placed tracheal tubule,
(ii) Muscular layer with striated fibres,
(iii) A layer of large cubical cells having striations at the inner border.
8. Nervous System of Millipedes:
The nervous system of Millipedes includes:
(a) Central nervous system,
(b) Peripheral nervous system,
(c) Sympathetic nervous system and
(d) Sense organs.
A. Central nervous system:
It consists of:
(1) A large supra-oesophageal ganglion or brain,
(2) Paired circumoesophageal commissures,
(3) Paired sub-oesophageal ganglion and
(4) Elongated ventral nerve cord (Fig. 18.47).
1. Supra-oesophageal ganglion:
Though it appears to be a single large ganglion, yet it is formed by the fusion of several ganglia. It may be distinctly divided into three regions, protocerebrum, deutocerebrum and tritocerebrum. The protocerebrum contains two frontal and two lateral lobes. The deutocerebrum has a pair of antennal lobes, and tritocerebrum includes two lateral lobes.
2. Circum-oesophageal commissures:
From each lateral side of the supra-oesophageal ganglion a stout oesophageal commissure runs ventrally to unite with the sub- oesophageal ganglion.
3. Sub-oesophageal ganglion:
Paired sub-oesophageal ganglia appear to be single and the ganglionic swelling is not very prominent. It is connected posteriorly to the ventral nerve cord.
4. Ventral nerve cord:
The dorsoventrally flattened ventral nerve cord is formed by two separate cords which are completely fused to be one. It runs posteriorly along the mid-ventral line. In each segment it sends two pairs of peripheral nerves but does not possess segmental ganglion.
B. Peripheral nervous system:
Numbers of peripheral nerves are given off from the central nervous system to the different parts of the body. The optic nerves and nerves to the organ of Tomosvary arise from the lateral lobes of the protocerebrum. The nerves to the antenna originate from antennal lobes of deutocerebrum.
The brain also sends nerves to the gnathochillarium and mandibles. In each segment ventral nerve cord sends two pairs of nerves. In each side one nerve goes to the corresponding leg and other nerve sends branches to the muscles, trachea, stigmata and wall of the heart in the same segment.
C. Sympathetic nervous system:
It is represented by one unpaired and one paired nerve cords. All of them originate from the brain and run posteriorly. In the paired nerves swollen ganglia are formed.
D. Sense organ:
Following sense organs are seen in Thyropygus:
(1) Antennal sense organs,
(3) Organs of Tomosvary,
(4) Hydroreceptor organs and
1. Antennal sense organs:
Short, seven- segmented antennae bear three types of sense organs (Fig. 18.48A, B):
(a) Sensory bristles,
(b) Cone sensillae and
(c) Peg organs.
(a) Sensory bristles:
These bristles are scattered all over the antenna and contain sensory cells at their bases.
(b) Cone sensillae:
The two terminal segments of each antenna are transformed into conical structures containing sensory cells. These are called cone sensillae and they act both as chemoreceptor and taste organs.
(c) Peg organs:
The last segment bears hook-like structures, called peg organs. They also work as chemoreceptor and taste organ.
Numerous sensory structures called basiconic sensillae are present over the distal end of the gnathochillarium. These are also gustatory in function. They may also show taste discrimination.
3. Organs of Tomosvary:
Each of these appears as a cuticular depression with the lining of sensory cells which receive nerve supply from the optic nerve (Fig. 18.48C). This organ is believed to be responsible for determining vibration and also to react properly to the gravity.
4. Hydroreceptor organ:
These paired organs (Fig. 18.48D) are present one on each side near the base of gnathochillarium. It is responsible for determining the water or moisture content of the soil. Structurally it resembles the tarsal organ of the spider but functionally these two are completely different.
Each eye is made up of several simple eyes or ocelli. Each ocellus is formed by the modification of integument and in section it looks like a glass. The sides and the floor are made up of single-layer sensory epithelial cells. The cells lining the floor become pigmented and form retinal layer and the cuticles above are modified as lens and cornea.
The sexes are separate and the sexual differences are noted from the structure of legs in the seventh segment of male where these legs are modified into intromittent organs or gonopods.
Male reproductive system includes testes, sperm ducts, penis and gonopods. Each testis is tubular and extends from second to fortieth segment of the body. It is placed between the alimentary canal and ventral nerve cord. The posterior-most part of the testis is tube-like, the middle part is beaded and the anterior end is convoluted.
At the anteriormost part the two testes are transversely joined by small tubes. Sperm duct originates from each testis at the third segment and runs up to the second segment and finally communicates ventrally with a small penis. The gonopods collect the sperms from the sperm duct and at the time of copulation sperms are transferred to the female genital tract.
The female reproductive system consists of an ovary, a short uterus and paired vulvae. The ovary is placed mid-ventrally above the nerve cord and is conspicuous only during maturity. It contains ovisacs, each of which holds a single egg. The ovary continues as uterus which in the third segment bifurcates into two smaller tubes. Each small tube opens externally through a vulva which is placed laterally.
Fertilization and Oviposition in Millipedes:
During copulation sperms are transferred by the gonopods of the male within the vulvae of the female. The sperms remain viable for a considerable period in the vulvae and fertilization takes place at the uterus after certain period of copulation.
Eggs are laid in successive batches. Each egg is laid with a spherular wall formed by the secretion of the female. Eggs are generally laid at night within the chambers of burrows specially prepared for the purpose.