The Isoptera is a relatively small order of insects and related to the cockroaches and mantids. Over two thousand species are known from the world, where they are primarily tropical in distribution, with the greatest number found in Africa. More than two hundred species are described from the Indian subcontinent but many more are still to be found. Termites are soft-bodied, pale insects which avoid sunlight and live cryptically, well protected from light. The winged forms are relatively large (six to eighteen millimetres) whereas the soldiers and workers are three to fifteen millimetres long. Of the two hundred and seventy or so species occurring in India, only thirty or forty have been recorded as damaging crops or habitations and these belong to five of the nine known families: kalotermitidae, Hodotermitidae, Rhinotermitidae, Stylotermitidae and Termitidae. Among them, the genera Odontotermes, Macrotermes and Microtermes cultivate fungus gardens in their nests. The genus Odontotermes, with thirty eight species in India, is the dominant termite group.
Termites have a fascinating biology. The individuals are differentiated into various castes or morphological forms which exhibit division of labour. The castes exhibit both anatomical and physiological specialization. Four distinct castes are recognizable; primary reproductives (king and queen), supplementary reproductives, soldiers (sterile males and females with heavily sclerotized heads), and workers (again sterile males and females). Also, a colony will contain immatures of all the castes at any particular time. The reproductives consist of winged males and females which leave the nest at a particular time each year to find new colonies. The primary function of both primary and supplementary reproductives is to keep the colony at optimum population by producing eggs (female) and spermatozoa (male). The soldier caste is very specialized with a well developed large head, and is of two types: mandibulate with well developed jaws of characteristic shapes and nasute, with the head produced into a rostrum (genus Nastitermes) and the jaws small and vestigial. In some species, two types of soldiers may occur, differing not only in size but also in morphology. Their main job is to protect the colony from enemies. This is achieved either by physically biting, or by emitting the secretions of the frontal glands in the Rhinotermitidae and the Termitidae (including Nasutitermes) which are toxic, repellent or just gummy. Also, soldiers may block the entry of enemies into the nest by blocking the entrance hole with their specially formed heads. During construction or repair of the nest or when the reproductives fly out of the nest, the protective activities of the soldiers are most noticeable. Compared to the soldier caste, the workers are relatively unspecialized with a pale soft body and mandibles strongly hardened and similar in structure to those of the reproductive caste in each species. The compound eyes are lacking in most cases. In two families (Kalotermitidae and Termopsidae) the worker caste does not exist and the function is performed by the early stages of the reproductives. In any colony, the workers are the most numerous caste and they carry out chores like foraging for food (which may be eaten there itself or carried back to the nest), feeding the young, soldiers, reproductives and the nymphs of reproductives. They also tend the eggs and repair and enlarge the nest and the internal galleries.
The eggs are laid singly and carried by the workers to be dumped as masses in the nest. There are from four to ten instars and the immatures develop into any caste depending upon the requirements of the colony. One queen may lay as many as two thousand to three thousand eggs in a single day. The population within one nest or colony may be from 15,000 to over 500,000 individuals depending on the size of the nest and the species of termites. The life of a colony may vary from fifteen to fifty years under natural conditions.
New colonies are founded generally by the flight of numerous winged males and females from the nest at a particular time each year and, in some cases, at a certain hour of the day. In India, the main time is just after the start of the monsoon. There may either be a single flight from one colony or the winged reproductive may fly out in several batches for a few weeks at irregular intervals. After a short, weak flight, the reproductive drop to ground, shed their wings and find the other sex with the help of an odour released by the female. The pair then forms a tanden one behind the other and they search for suitable nesting sites in soil or in wood. They excavate a 'nuptial chamber' once a suitable site is located and copulation takes place. The queen then starts laying eggs soon after a week and the small early brood is tended by the royal couple, until after several weeks when the queen's production of eggs increases and a nucleus of a strong colony is formed. The proportion of soldiers to workers in a colony may depend on the species concerned, and vary from one to fifteen percent or more. The winged reproductive are not produced until the colony is several years old. Both the original king and queen that formed the colony may live for a long time, from 15 to 50 years in the higher termites. In some species there may exist more than one pair of primary reproductive in the same colony. Colonies may also be founded by a group of nymphs going out of the colony and forming supplementary reproductive, as is often the case in the more primitive termites.
The food of termites consists of wood, either unaffected or decaying, grass, fungi, humus, bark, dead leaves and even herbivore dung. The termites that eat wood feed primarily on the cellulose contained in it which is converted into a form assimilable by the symbiotic flagellate protozoa that occur in the hind gut of the termite. In the Termitidae, which do not have a protozoan community in their digestive system, the cellulose, if fed upon, is digested by cellulose secreted by them or by that produced by bacterial flora in their guts. The species of termites that do not feed on wood have no protozoa at all or a much reduced number. Much of the food is eaten while foraging, but species which feed on grass or littered leaves, etc., carry it back to the nests. Foraging is carried out at night or in the day under sheetings of mud, which are conspicuous as 'trails' to our eyes.
Termites build very striking or cryptic nests either invisible to us or of complicated architecture and very large mounds. The nests are homeostatic ally regulated units with a constant relative humidity maintained within. Like the shell of an animal, the nest provides protection against weather and natural enemies. The simplest type of nest is found in the primitive forms like the Kalotermitidae and Termopsidae where the entire colony lives in a gallery system with chambers excavated in wood. The subterranean species construct a complex central nest from which underground galleries or covered 'trails' emerge and go to food sources above or below ground. The mounds or termitaria built by most Termitidae are the most complex of all termite nests. The outer layer of the termite mound is very hard and composed of hard earth or clay. Inside, it is much softer and comprises a network of galleries and chambers. The royal pair is often found in a special chamber and the eggs and young in the others. The gallery system of one colony may exploit all the food source around the nest for one hectare or more and a single trail may extend for up to seventy five or hundred metres in length. Though most termitaria are found as mounds on the soil surface, the genus Nasutitermes builds carton nests (somewhat analogous to the tree ant, Crematogaster) on tree branches or even on poles or sawed-off tree-trunks.
In mound-building termites, we have either the fungus growers or the non-fungus growers. The fungus-growing termites are Odontotermes, Macrotermes and Microtermes and these cultivate fungus gardens within their nests for food. The non-fungus growing termites construct small mounds (Amitermes, Cubitermes, nasutitermes and Trinervitermes) and fill them with dry food.
Termite nests harbour an assortment of other organisms either within the nest (termitophiles) or in the outer wall (termitariophiles). The outer wall is often used for nesting by other termite species, birds or lizards. Within the nest and closely associated with the termites, several groups of animals occur, either feeding on the eggs or young termites or on the debris in the termitarium or involved in a symbiotic relationship with the host termite. Most common are beetles of many families and aphids. Also occurs several flies, isopods, Collembola, Thysanura, centipedes, shrews, geckos, rats, mongooses, cockroaches, scorpions and some other groups. The emergence of winged termites after the first rains is as regular as event and the numbers of these frail nutritious creatures is so large, that many kinds of animals have taken the opportunity to congregate near the flying swarm and partake of it. Many birds, frogs, lizards, snakes, and even bats feed on them, while many orders of insects also utilize this event to satiate their hunger - the dragonflies conspicuously in the air and several beetles on the ground, mainly. Special animals like the anteaters and a few leptodactylid frogs exist primarily because the termites are their principal or only food.
|More Articles in Indian Natural History (69)|