Arachnida: India
Contents |
Faunal Diversity in India: Arachnida
This is an extract from FAUNAL DIVERSITY IN INDIA Edited by J. R. B. Alfred A. K. Das A. K. Sanyal. ENVIS Centre, Zoological Survey of India, Calcutta. 1998 ( J. R. B. Alfred was Director, Zoological Survey of India) |
Introduction
The members of the class Arachnida are arthropods, characterised by a number of features with the exception of some ticks and mites. The body is divided into two regions, cephalothorax or prosoma and abdomen or opisthosoma and antennae absent. The cephalothorax consists of six somites, each having a pair of appendages. This part is covered by a dorsal hard chitinous shield, the carapace. They have eight simple eyes which vary greatly in their position. One of the striking characteristic features of Arachnida is the absence of true jaws. The prey is crushed by the cephalothoracic appendages. Of the six pairs of cephalothoracic appendages, the first two pairs are used for capturing and crushing prey and the last four pairs are legs. The first pair of appendages are called the chelicerae consisting of two, rarely of three segments. The second pair of appendages are the•pedipalp. Arachnids have four pairs of legs each having seven segments, viz., coxa, trochanter, femur, patella, tibia, metatarsus and tarsus. The members of this class respire either by book¬lungs or tracheae or both. Sexes are separate and remarkable sexual dimorphism is found in some cases. They are viviparous or oviparous. The young ones resembling their parents are hatched from the egg but undergo a series of successive moults. The arachnids are generally terrestrial in habit. An outline idea about morphological features of the members of different arachnid orders reported from India is given below.
The order Scorpionida includes scorpions. These are considered as one of the oldest forms of life. The most striking features of scorpions are large sized pedipalps and abdomen divided into seve)} segmented pre-abdomen and five segmented post-abdomen or tail-the cauda having a large poison sting. Based on the nature of habitats scorpions are grouped into three categories, viz., burrowing (pelophilous), rock dwelling (lithophilous) and arboreal.
The members of the order Pedipalpida are commonly called as "whip¬scorpions" In their general forms, they show resemblances to scorpions but SANYAL & TANDON : Arachnida can be easily differentiated by the form of the pedipalps of the first pair of legs and of post-abdomen. There are both tailed and tailless forms of whip¬scorpions.
The members of the order Solifugae or Solpugida are known as 'false¬spiders' or 'wind-scorpions' They include a moderately large group of primitive forms of arachnids. These arachnids are nocturnal, exclusively predatory and carnivorous. The solifugae are principally desert forms but they also live in forests.
The members of the order Opiliones or Phalangida are commonly known as "Harvestmen" They can be recognised by their long and slender legs and segmented abdomen which is joined to the cephalothorax across the whole breadth and not by a narrow pedicel as in spiders. These arachnids are named as harvestmen because they are generally seen in the field during harvest time. The 'pseudoscorpions' or 'false-scorpions' are small, dorsoventrally flattened arachnids. They are found in dark, damp places, underneath the bark of tree, under logs or stones, rotten wood and leaves, in poultry houses and the nest of birds. They resemble scorpions in the general form of their pedipalps and body but the hind part of the abdomen is not narrow as in scorpions. The six segmented pedipalps are enormously developed which serve as prehensile organs to capture and kill the prey. The order Acari includes the mites and ticks and forms an important group under the class Arachnida. The size of mites range from 1.5-16 mm. The ticks vary in size from 1.7-12.7 mm. Engorged individuals of tick may attain 20-30 mm. The mites occur in soil, organic debris, arable land, pastures, salt and fresh water and all types of vegetation. The ticks are external parasites of invertebrates and vertebrates including man.
The members of the order Araneae are commonly known as spiders. All spiders are carnivorous. They feed usually on insects, sometimes on other spiders. The spiders are well-known for their webs or snares of different types which help in hunting prey. The social behaviour is unique among spiders which are mainly cannibals. It is known that most spiders have a pair of poison glands in the cephalothorax which contain venom to kill their prey.
Status Of The Taxon
The class Arachnida was erected by Chevalier De Lamark, the great French philosophical biologist, in 1815, when he split Linne's heterogeneous group
Insecta into three classes. Lamark's class Arachnida included scorpions, spiders and mites together with the Myriapoda and Thysanura. The Thysanura and Myriapoda are now treated as separate classes. The living members of Arachnida are grouped into nine orders, viz., Scorpionida, Pedipalpida, Microthelyphonida, Solifugae, Ricinulei, Opiliones, Pseudoscorpionida, Acari and Araneae. The orders Microthelyphonida and Ricinulei are not recorded so far from India. The earliest record of arachnid from India was made as far back as 1758 when Linnaeus described ticks from India. After about a century in 1868 Peal discovered red-spider mite on tea in Assam. Nearly 30,000 species of Acari under 1,700 genera are known from the world (Krantz, 1978). A total number of 2079 species distributed over 631 genera and 205 families of mites and 107 species under 12 genera and 2 families of ticks are known from India.
The spiders are very common all over India. The studies on Indian spider was initiated by Blackwall (1864) and later gained momentum through the works of Tikader (1960-1987). A total of 1035 species belonging to 240 genera under 46 families are known from India. The Indian subcontinent is rich in variety as well as in abundance of Opiliones. But in India only a few attempts have so far been made to explore this group. Roewer in 1929 made some studies on Indian harvestmen and described nearly 167 species under 40 genera of the subfamily Gagrellinae of the family Phalangodidae.
The members of the order Scorpionida, Pedipalpida and Solifugae were studied in our country for the first time by Pocock about a century ago (1900). Later Birula (19.13-1928), Handerson (1913), Hirst (1915) and Mani (1959) studied these arachnids. In recent times Tikader and Bastawade (1983) have made very comprehensive study of Indian scorpions. At present a total of 104 species and subspecies distributed in 27 genera and 5 families are known from this country. The pedipalpids are found throughout the country. But after Pocock no comprehensive study was done and there is a need for a wide revisionary work. They are so far known by 25 species distributed over 10 genera and 4 families.
The Indian forms of false-spiders are so far represented by 15 species distributed over 3 genera and 2 families. After Pocock, till today, unfortunately no attempt has been made to explore this group in India.
The Indian pseudoscorpions were first taken up for extensive study by Murthy and Ananthakrishnan (1977). More than 100 species distributed over 47 genera and 13 families are so far known from India.
Biological Diversity And Its Special Features
The range of biological diversity in the class Arachnida is very wide. The class has been separated from other arthropod classes on the basis of its diversity in morphological features, habit and habitats. The major orders, viz., Acari, Araneae, Opiliones, Scrpionida and pseudoscorpionida have been divided into a number of families, subfamilies, genera, species and subspecies. The diversity in other two orders, pedipalpida and solifugae is not remarkable due to the fact that these groups have not yet been thoroughly studied in India (Table 1). The diversity in habitat preference is well-marked in the Arachnida. The members are found to live in almost all possible habitats. They may be plant associates, soil associates, storage associates, nest associates, water associates, etc. Maximum number of genera and species are found to occur in association with plants. Table -1 Number of arachnid taxa and species endemic to India and discovered as new to science from India
Endemicity
From the foregoing account it would be clear that though there is great diversity in the Arachnida, the fauna of majority of the states of this subcontinent still remain unexplored (except acari, spiders, scorpions and pseudoscorpions) and whatever distributional data available are insufficient. However, the data so far available indicate that the Indian arachnid fauna include a number of endemic taxa as shown in Table 1. The Table shows that nearly 50% of the total species known from India and nearly all the species discovered as new to science from India are endemic to this country.
Introduced Species
Though there is no record that arachnid species has been introduced in India, there is every possibility of introduction during transportation of material from outside India. The discontinuous or restricted distribution of many generalspecies or description or record of species based on solitary specimen support the possibility of introduction of species in India.
Value
The arachnids are of little economic importance except spiders which are highly beneficial and acarines which are harmful as well as beneficial. The majority of scorpions are neither beneficial nor harmful to human beings. Only a few species are known to give painful stings causing swelling and sometimes fever. The whip-scorpions, false-spiders and pseudoscorpions are of no economic importance. However, they play an important role in maintaining ecological balance. The harvestmen are primarily carnivorous and usually feed on dead animal tisues. In view of their habits they are considered to be an agent in biological control of insects. The spiders are considered to be one of the best friends of mankind, because they feed insects which are generally harmful to man and domestic animals. They play a significant role as predator in controlling pests of cotton, apple, banana and rice. Spider silk is also of inunense economic importance as reticles of a variety of optical devices. Silk webs are used to stop bleeding, to control recurring fever and as antipyretic agent. This group of arachnids have recently gained attention of scientists as a possible source of homeopathic and allopathic medicines. Spider silk and venom are used in preparation of several drugs. There is a belief that spiders are venomous and their bites are dangerous. But except the black-widow spider or large spiders like tarantula, others are practically harmless.
Mites and ticks are of immense economic importance. The phytophagous mites damage plants by desapping, causing reduction of growth and yield. Some are vectors of plant and viral diseases, while others cause malformations and deformations of plants. A good number of mites infest stored food grains in granaries. The parasitic mites and ticks attack domestic animals, apiary and poultry, causing either direct injuries due to blood sucking or heavy mortality by acting as vectors of viral, bacterial, rickettsial and protozoan diseases. The ticks are also known to cause viral diseases in human beings. Some aquatic mites live as ectoparasites of fish and other aquatic animals. The mites are also responsible for causing respiratory diseases and dermatitis in human beings.
Not all the mites are our enemies. There are many who are our friends too. Some species of mites who are known as efficient and useful predators of the plant feeding mites can successfully suppress the population of pest mites below economic injury levels. Some water mites are also known to feed upon mosquito larvae and thus check their population. Soil mites play a vital role in humification of organic matter resulting in the increase of soil fertility.
Threats
Indiscriminate use of pollutants and destruction of habitats particularly the species-rich tropical rain forests threaten the survival of numerous arachnid species as well as all the other animals especially insects inhabiting those areas. The loss of insect population ultimately endangers the survival of spiders in nature. The economically important mites are also in problem of survival due to habitat destruction. Similarly, the other Indian arachnids which play partly in maintenance of ecological balance are also in problem due to shrinkage of their habitat.
Conservation Strategies And Future Studies
As has already been mentioned that the beneficial arachnids are in great danger, this is the high time to formulate strategies for their conservation. The first and foremost task is to check destruction of habitat and use of poisonous chemicals. Later a system of regular monitoring of the management activities is to be ensured. It may not be an under statement that only about one-tenth of the arachnid fauna has been documented in our country. There is an urgent need to take up extensive surveys and studies on taxonomy, ecology and bionomics of Indian arachnids. The young scholars and students are to be encouraged to undertake studies on faunal taxonomy which is the basic tool for biodiversity conservation.
Selected References
Cook, D. R. 1967. Water mites from India. Mem. Amer. Ent. Inst., 9: 411 pp. Gupta, S. K. 1985. Handbook: Plant mites of India. Z. S. I., Calcutta: 520 pp. Gupta, S. K. 1988. Fauna of India (Acari: Mesostigmtlta) Family Phytoseiidae, Z. S. I., Calcutta: 350 pp. Pocock, R. I. 1900. Fauna of British India, Aracllnida, London: 279 pp. Prasad, V. 1974. A catalogue of mites of India. Indira Acarology Publishing House, Ludhiana : 320 pp. Prasad, V. 1982. The !listory ofAcarology. Indira Publishing House, Michigan, U. S. A.: 472 pp. Tikader, B. K. 1987. Handbook: Indian spiders. Z. S. I., Calcutta: 251 pp. Tikader, B. K. and Bastawade, D. B. 1983. Fauna of India, Scorpions, Calcutta, III :
i-xii +: 671 pp. Sanyal, A. K. and Bhaduri, A. K. 1986. Checklist of Oribatid mites (Acari) of India. Rec. zool. Surv. India, ace. Pap. No. 83 : 79 pp. Sanyal, A K. and Bhaduri, A K. 1988. The present state of knowledge of oribatid taxonomy in India. In : Progress in Acarology (eds. Channa Basavanna, G. P. & Viraktamath, C. A), Oxford & IBH Publishing Co. Pvt. Ltd., New Delhi, 1 : 295-300.
Savory, T. H. 1977. Arachnida. Academic Press, 340 pp. Sharif, M. 1928. A revision of the Indian Ixodidae, with special reference to the collection in the Indian Museum. Rec. Indian Mus., 30(3) : 217-344.
