5 chapter morphology and anatomy of sw sps. commercially exploited breeds of sw

5- Chapter Morphology and anatomy of SW sps. Commercially exploited breeds of SW

1. Morphology and Anatomy of Bombyx mori & Commercially exploited breeds of Silkworms By, Nayana , B. P. Msc . (Agri.) SRF UAHS, Shivamogga

2. EXTERNAL MORPHOLOGY OF Bombyx mori L .

3. Eggs Eggs are placed in clusters on the leaves. The female covers the eggs by a gelatinous secretion which glues them to the surface of the leaves. The eggs are small, oval and silkworms producing white cocoons lay pale yellow eggs, while yellow cocoons producing silkworms lay deep yellow eggs

4. The egg contains a good amount of yolk and is covered by a smooth hard chitinous shell . The non - hibernating eggs are brighter than the hibernating eggs. An individual egg measure 1 to 2 mm in length and 0.9 to 1.2 mm in width. After laying the eggs the female moth does not take any food and dies within 4-5 days .

5. Pale yellow eggs Deep yellow eggs

6. Larva Larva is domesticated, glabrous, elongate Newly hatched larva is dark brown with large lustrous black head and dark brown body with more hairs hence called black ants. The body colour changes to bluish white in later stages The larva is Monophagous and feeds on mulberry, Morus indica , Morus alba, Morus rubra , Morus nigra , etc It forms dense cocoons

7. Head - Head is dark grey in color, formed by six body segments that are fused together with a cranium and can be identified in the embryonic stage. The epicranium forms the upper region of the head. An inverted 'Y' shaped epicranial suture separates frons from epicranium by its frontal sutures and divides epicranium into two by less distinct median suture. Frons carries clypeus, which is separated by fronto - clypeal suture. Clypeus carries labrum or upper lip.

8. Six pairs of ocelli are present with three pairs on each side of the head little above the base of five segmented antennae. Vertex is the dorsal region behind the frons and between ocelli on each side. Occiput is the hinder part of the epicranium between the vertex and the neck or cervix The lateral area below and posterior to the ocelli on each side is the gena .

9. Mouth parts are adopted for biting and chewing the leaves ( Mandibulate type) with two hard and powerful mandibles for crushing the food, maxillae with two or three segmented maxillary palpi Submentum is divided into sclerites and bears mentum , a very large and less chitinised part. Prementum is borne on mentum carrying spinneret and two segmented labial palpi on either side. The flexible and very narrow region between the head and thorax is cervix.

10. Thorax - It consists of three segments namely Pro, Meso and Meta thorax and carries a pair of thoracic legs ventrally. A pair of spiracles is present on lateral side of prothorax and a eye spot present on meso thorax.

11. It consists of 11 segments but only nine segments are clearly visible as the last three segments are fused to form ninth segment. The segments from first to eight bears a pair of spiracles laterally The abdominal segments from third to sixth and last possess a pair of prolegs with crochets ventrally Crescent shaped markings in second abdominal segment Star shaped marking in fifth abdominal segment Abdomen

12. Medio -dorsal caudal horn on eighth abdominal segment. Male worms can be identified by a milky white body called Herold's glands situated ventrally between 8 th and 9 th abdominal segments just above the caudal legs. Milky white spots on 8 th and 9 th abdominal segments ventrally called Ishiwata's glands can be seen by a very close observation in female worms.

13. Pupal sex marks are more prominent on the ventral side of 8 th abdominal segment Female has a conspicuous longitudinal slit (x mark) in the centre of 8 th And the male with a black round spot on ninth segment . The female pupae are heavier and bigger than the males. Besides this, female pupa is larger with a broader abdomen while the male is thinly built with narrower abdomen. Pupa – Obtect type.

14. Cocoon Cocoons vary In shape (spherical, oval, peanut, pointed and conical) and Colour (white, yellow, pink, golden yellow etc. ) according to race. The cocoon composes of a single continuous reelable thread ranging from 500- 1500 meters in length.

15. Different coloured cocoons

16. Different shapes of cocoons

17. Moths Moths are milky white in colour, medium sized , with smoky wavy lines on white coloured fore and hind wings. The apex of the forewing is slightly elongated and curved. Female moths are bigger and stouter with a knob-like projection at the last abdominal segment. This knob is protruded and retracted to expel the pheromone.

18. Following characters can be used to separate male and female moths: Character Female Male Colour Paler Darker Activity Less active More active Antennae Small Large Body size Large Small Abdomen Large and flat with seven visible segments Long, narrow, eight visible segmnets External genitalia As mentioned above As mentioned above The males are smaller and slender with a pair of hooks ( Harpes ) at the tip of the abdomen. Frenulum is almost atrophied or absent. Maxillary palpi and tymphanal organs are absent Antennae are bipectinate

19. Female Paler Male Darker

20. Anatomy of Bombyx mori . L.

21. Digestive system It is more or less a straight tube from the mouth to the anus. It is divisible into anterior foregut or stomodeum , middle midgut or mesentron and posterior hindgut or proctodeum . Foregut consists of narrow pharynx and oesophagus , which gradually widens towards posterior end and opens into the midgut . Between foregut and midgut there is a cardiac or stomodeal valve. Midgut is a long, wide cylindrical tube narrow at the posterior end and protected by peritrophic membrane form inner side.

22. DIGESTIVE SYSTEM

23. DIGESTIVE PROCESS

24. Pylorus valve is situated between midgut and hindgut. The hindgut consists of small intestine, colon and rectum. Rectum is provided with six rectal muscles for pressing the excrements. Digestion and assimilation of food occurs in midgut , while hindgut absorbs excess moisture through rectal muscle from the undigested food material and eliminates the undigested food.

25. Excretory system Malpighian tubules are the primary organs of excretion. They arise at the junction of the midgut small intestine and colon in the hindgut of the digestive tract. The tube arising on each side is enlarged into a excretory chamber or urinary bladder from which two branches arises. One of the tubes situated on the dorsal side branches further into three tubes All the six tubules stick to the midgut and run towards the anterior side, turn back and ultimately open into the rectum. This condition is called as cryptonephridal condition.

26. The tubes passing towards the anterior side from the urinary bladder are referred as ascending limbs The same tube turns back and pass towards rectum are called descending limbs (tubes) . The nitrogeneous compounds like protein are metabolized by these tubes and excreted principally as uric acid. They also excrete calcium oxalate. The malphigian tubes in the mature larvae are light yellow and their walls contain large quantities of yellow vitamin B2.

27. Respiratory system The respiratory function is carried out through the spiracles by the movements of the body wall. In the larva there are nine pairs of spiracles, one on each side of the pro-thorax from the 1 st to 8 th abdominal segments. Peritreme , sieve plate and atrium are the parts of the spiracles. The peritreme forms a ring round the spiracle. A slit can be seen along the median line of the sieve plate inside the ring.

28. The external opening of the spiracle leads into a cavity, the atrium which contains the closing apparatus . There are two closing membranes and their movement can be observed if the sieve plate is taken off. The sieve plate prevents the entry of dust and air. A number of tracheal branches start radially from the spiracular cavity. There are also two large tracheas called main or lateral tracheal trunks connecting all the tracheal bushes . The ventral transverse tracheas connect the two tracheal bushes in each segment. The trachea ramifies repeatedly throughout the body and organs and ultimately form tracheoles .

29. When the diameter of the tracheae is reduce d to one micron, these finer tubules are called tracheoles They carry oxygen directly to the cells and tissues. Through such a network of tracheae and tracheoles oxygen is carried to the various parts, tissues and cells of the body and carbon dioxide is removed, through the same channels. Tracheae are similar in structure to the integument and also consist of a supporting thickened ring-like structure called the taenidium which keeps the tubes distended and prevents them from collapsing.

30. Diagram showing the trachea, tracheoles and movement of air

31. CIRCULATORY SYSTEM OF SILKWORM LARVA The circulatory system is an open one with a dorsal vessel, extending from the head to the last abdominal segment along the dorsal median line of the body. The dorsal vessel is associated with the circulation of the blood and is closed posteriorly while the front end opens in the head. The anterior portion of the dorsal vessel is a finely drawn-out tube and constitutes the aorta , while the posterior portion forms the heart. Each of the segments from the second thorax to the ninth abdominal segment contains a pair of Ostia . There are eight pairs of alary muscles which are attached to the body wall along the lateral parts of the dorsum.

32. Systole (contraction) and diastole (expansion) of the heart are effected by the 8 pairs of alary muscles and the musculature of the heart. The alary muscles function more or less like an elastic band and cause diastole and systole. As the heart is filled with blood, a steady wave of contraction progresses in a posterior - anterior direction . Hence the blood is carried to the head and discharged. Blood leaves through the dorsal vessel not only by the anterior end but also by the Ostia in the meso and meta-thorax and sometimes by the Ostia in the first and second abdominal segments.

33. The blood flows backward through the body cavity . The blood circulates throughout the body cavities and appendages transporting nutrients and removing waste products of the metabolism and filling most of the space not occupied by the internal organs. In the silkworm, the blood cells or haemocytes do not enter the dorsal vessel through the ostia . Only cell-free plasma is circulated. The heart pulsates rhythmically and the frequency of pulsation varies with age, temperature, movements and the race of the larvae.

34. CENTRAL NERVOUS SYSTEM OF SILKWORM LARVA The central nervous system of the silkworm larva is divided into the brain or supra oesophageal (cerebral) ganglion, the sub oesophageal ganglion and the ventral nerve cord. The brain lies in the head, dorsal to the oesophagus and consists of two pear-shaped lobules. It is formed by the protocerebrum , deutocerebrum and tritocerebrum and is the dorsal ganglionic centre of the head.  In the silkworm larva the ventral nerve cord is formed by a series of ganglia on the floor of the thorax and abdomen below the alimentary canal.

35. These are connected in a longitudinal chain by paired connectives arising from the posterior end of the sub oesophageal ganglion. The thorax consists of three ganglia, one in each thoracic segment, and eight ganglia in the abdomen, one each in the first to the eighth abdominal segment. From each thoracic ganglion two pairs of principal nerves arise of which one pair innervates the general musculature and the other pair innervates the leg muscles. Each abdominal ganglion has a pair of principal nerves that innervate the muscles of the corresponding segment.

36. Visceral or sympathetic nervous system The visceral or sympathetic nervous system is formed by an oesophageal sympathetic and a ventral sympathetic system. The oesophageal sympathetic system is directly connected with the brain innervating the fore and mid-gut, dorsal vessel and other parts. It consists of a median triangular frontal ganglion above the oesophagus The ventral sympathetic nervous system is formed by a pair of transverse nerves linked with the ventral nerve cord ganglia. A median longitudinal nerve connects each pair of transverse nerves with the preceding ganglion. The transverse nerves innervate the spiracles of the corresponding segment. The caudal sympathetic system innervates the reproductive system and the posterior portion of the gut and consists of nerves emerging from the composite (the last) ganglion of the abdomen.

37. It is fine network of sensory bipolar neurons which have connections with the sensory hairs and the central nervous system and multipolar neurons which make an elaborate plexus beneath the integument. The multipolar neurons have distal connections branching on the surface they supply and centripetal connection to the central nervous system ganglia. Peripheral sensory nervous system

38. REPRODUCTIVE SYSTEM In the larva, the sexual phase develops in the late instars when it may be possible to note in the female, a pair of triangular ovaries (gonads) situated on the dorsal side of the sixth abdominal segment; from each of which a blind tube arises and terminates at the eighth abdominal segment. The pair of translucent spots seen ventrally on the eighth abdominal segment is the Ishiwata's foreglands and the pair on the ninth abdominal segment is the Ishiwata's hind-glands.

39. These four ventral spots are prominent in the fifth instar and are used to identify the female larva. The male larva consists of a p air of kidney-shaped testes (gonads) on the dorsal side of the fifth abdominal segment and a blind tube which is connected with the Herold's gland - a milky white body-at the centre of the ventral side between the eighth and ninth abdominal segments.

40. Female moth Reproductive system In the female moth, paired ovaries occupy most of the abdominal space. A pair of lateral oviducts arises from the ovaries and unites to form a single, wide common oviduct that ends in the oviporus at the posterior end of the body. Each of the ovaries carries four egg tubes or ovarioles where the eggs are formed.

41. The ovarioles are polytrophic and an alternating succession of nurse cells and oocytes (developing eggs) are found in the egg chambers particularly in the pupal stage. In the adult stage, the four pairs of ovarioles look like coiled strings of beads containing eggs. The bursa copulatrix is a sac-like structure situated ventrally to the common oviduct and continues as a tube to the outside posteriorly on the eighth abdominal segment. At the time of mating, the aedeagus is received by the opening, ostium bursae , of this tube.

42. The bursa copulatrix also opens into the common oviduct by a narrow tube known as the seminal duct. Paired accessory glands occur dorsally to the common oviduct. The accessory glands produce an adhesive material which coats the eggs when they pass down the common oviduct and glues them to the substratum when they are laid. The spermatheca is a sac-like structure for the reception and storage of spermatozoa and opens into the common oviduct anterior to the accessory glands opening.

43. Overall Female Reproductive System

44. Male moth Reproductive system In male moth a testis is present in the fifth abdominal segment dorso-laterally on each side of the ventral nerve cord. The vas efferens of each testis leads into a vas deferens which opens into the seminal vesicles from where an ejaculatory duct arises and ends in the aedeagus. A pair of accessory glands opens into the seminal vesicle by a common duct located between the two vasa differentia. The membranous sac or the spermatophore is formed by the secretion of male accessory glands.

45. Silk glands Silk glands are These are transformed labial glands, ectodermal origin, cylindrical and tubular with characteristically branched nuclei. It is the second largest part of the body and each gland is several times the length of the larva, occupying 2/5th body weight and volume, situated from the 4th to 8th abdominal segments They are situated on the ventrolateral sides of the mid intestine and the posterior ends are blind. Anteriorly the paired ducts unite and open in to the spinneret.

46. Silk glands may be divided into 3 distinct regions; anterior, middle and posterior. The anterior region is a straight tube opening at the fore end into the duct and posteriorly in to the middle region. The middle region is the largest of the three regions and has 3 definite curves / bends / flexion. The middle region is again divided into three functionally different sections: anterior, middle and posterior.

47. The beginning of the middle region is narrow but widens suddenly - the middle part is the widest and the end of the rear part is narrow. The posterior region is crooked and curved between the dermo-viseral muscles and the tracheae.   A pair of glands known as Filippi's or Lyonnet's glands is situated at the junction of the two anterior regions. A viscous fluid is secreted by these glands and their exact function is not clear. The wall of the silk gland is composed of 3 layers, the tunica propria , the gland cells, and the tunica intima enclosing the lumen of the gland.

48. The tunica intima is made up of thickly laid chitin throughout but the anterior portions are renewed at each ecdysis or moult. Fibrion and sericin are secreted in the fourth and fifth ages. Fibrion , the silk protein, is secreted from the posterior region. The middle region acts as a reservoir for the maturation of fibrin and also secretes sericin around the fibroin as below. Sericin I, the innermost sericin , is secreted from-posterior section of the middle region; Sericin II, the middle-layered sericin , from the middle section of the middle region; Sericin III, the outermost sericin from the anterior section of the middle region.

49. The anterior region of the silk gland does not secrete any particular substance-it is simply a passage and carries the silk substance from the reservoir (middle region) for freezing. The silk fibre is made up of inner fibroin and outer sericin .

50. SILKGLANDS

51. Classification of Mulberry Silkworms / races Mulberry silkworm is reared in more than 30 countries of the world They are differentiated into number of distinct races. A. Based on Geographical Distribution: Japanese race - Here is Japan B. Mori has 2000 genetically distinct races Chinese race European race - univoltines Indian race - In India >2000 races are maintained in its different centers

52. At present, these rearing stations involving commercial races of multivoltine and bivoltine varieties Multivoltine breeding centers – Mysore and Berhampore Bivoltine breeding centers – Mysore, Kalimpung , Pampore

53. B. Based on number of generations in a year Univoltine races:- Produce only one generation / year eggs laid remain in dormant or diapausing stage till next year / spring Larvae of univoltines are highly susceptible to temperature All European races are univoltines Are unsuitable for summer even if we break the dormancy Larval nevoid very long Cocoons produced are very superior

54. 2. Bivoltine races :-  The first generation adults develop from non-diapering eggs The second generation adults developing from these eggs lay eggs which remain dormant till next spring The larval durations also long as univoltines Larvae can to lacerate environmental changes Bivoltine cocoons are commercially superior Japanese and Chinese races have Univoltine and Bivoltine varieties Bivoltine races of B. mori produce white

55. 3. Multivoltines or Polyvoltines Have >3 generations / year Larval duration is short They can tolerate change in temperature and Trt There are well adapted to tropical condition Larval and Cocoons are smaller in size Commercially these cocoons are of poor malit Multivoltine races produce colored cocoons Greenish or Golden yellow Some Multivoltine cocoons are able to produce white cocoons Voltines - number of generations in a year

56. C. Based on number of moults during larval growth - moultinism Trimoultiers - 3 moults in a larval stage Tetramoultiers - 4 Pentamoultiers - 5

57. Commercial races of India The indigenous races are no longer used for commercial rearing because they are degenerated, have low productivity and produce silk of inferior quality. So the high yielding exotic races are crossed with hardy local races trough hybridization and selection Many commercial races are arrival in India.

58. Commercially Exploited breeds of Silkworm Though, sericulture is introduced as commercial venture way back in late 17 th century, silkworm breeding was initiated only in 1920s. Basically polyvoltine culture prevailed up to the 50’s and indigenous polyvoltine breeds like Pure Mysore and C. Nichi in South India, Kashmir Race Nistari in West Bengal, Sarupat and Moria in the North East were reared. Though, these breeds were very well adapted and popular, their productivity and quality of silk was strikingly low with very high renditta .

59. Indian Races Is a generations in a year this was imported from China in 1895. Indigenous race quite popular with the farmers of South India Low productive race Characterized by greenish yellow spindle shaped cocoons Mysore Local/ Pure Mysore Suitable for subsistence farming condition, fluctuating temperature and poor hygienic conditions

60. Demerit of the race is the long larval period (27-28 days). About 1200 cocoons makeup 1kg. The single cocoons weight is 0.826 gm. Shell ratio is 10-12% More floss percentage (lS-22 %) Very poor post cocoon quality parameters High renditta (1 l-12)

61. C. Nichi race Exotic race quite popular in rainfed areas Low productive race Characterized by typical dumb-bell shaped white cocoons with coarse grains Suitable for subsistence farming condition, fluctuating temperature and poor hygienic conditions Very poor post cocoon quality parameters High renditta (14-15)

62. Nistari race Popular in West Bengal Introduced from China Multivoltine Dominant commercial rearing in North India More cocoon weight Easy to handle Tolerant to diseases Low consumption rate Has ability to feed on poor quality leaves Withstands varied temperature and RH These indigenous races are not reared because of low production

63. Multivoltine White coloured cocoon HS-6 C1 C5 Multivoltine Yellow coloured cocoon MY-1 MY-2 NS- 4 Multivoltine Commercial breeds of Silkworm

64. Developed by CSRTI, Mysore during 80s Parentage : Pure Mysore x Nistari Higher cocoon yield than PM Plain larvae with shorter larval duration than pure Mysore Characterized by light greenish yellow elongated oval shaped cocoons with coarse grains Better post cocoon quality parameters than pure Mysore High renditta (IL-12) MY-1

65. Multivoltine hybrid for irrigated zone Developed by CSRTI, Mysore during 90s Parentage: BL24x BL2l Higher cocoon yield than Pure Mysore Plain larva characterized by light greenish yellow elongated oval shaped cocoons with medium to coarse grains Floss percentage less than Pure Mysore Post cocoon quality parameters better than Pure Mysore Recommended for irrigated areas of South India Renditta (8-9) BL67

66. Cauvery Parentage of Cauvery : BL67 x CSR101 Highly productive with survival > 90% cocoon shell ratio around 20% Renditta around 6.5 Overall silk grade A Recommended for irrigated areas of South India . Tolerant to high temperature and BmNpV .

67. Multivoltine hybrid for rain-fed zone Varuna (BL24 x C. Nichi ) with high survival and better productivity traits High shell weight, High silk content, Long filament length and Better neatness with higher reel ability Cocoons are light greenish yellow The average cocoon yield is 3l kg/l00dfls Renditta of 10-ll Cocoons of this hybrid is fetching Rs.8-l0/- more per kg compared to PM x C. Nichi Varuna

68. Kolar Gold PM X CSR2 Characterized by its robustness Higher reelability as compared to PM x NB4D2 Larvae are plain and bluish white in colour Cocoons are light greenish yellow Hybrid cocoons with oblong shape Gives higher yield than existing hybrid, PM x NB4D2

69. Bivoltine races from CSTRI – Mysore NB-4 D-2 NB-7 CC-1 CSR-2 CSR-3 CSR-4 CSR-17 New productive bivoltine breeds and hybrids

70. Developed at CSRTI, Mysore during 90s Parentage : Shunrei x Shogetsu Productive breed with better post cocoon parameters Plain larvae with bluish white body colour Bright white oval shaped cocoons with fine to medium grains Cocoon shell ratio : 24-26% Raw silk percentage : 19-20% Fibre quality : 2A-4Agrade CSR-2

71. CSR-4 Developed at CSRTI, Mysore during 90s Parentage : (BNl8 X BCS25)XNB4D2 Productive breed with better post cocoon parameters Plain larvae with bluish white body colour Bright white dumb-bell shaped cocoons Cocoon shell ratio : 22-23% Raw silk percentage : 17-18 % Fibre quality : 2A - 4A grade