In this article we will discuss about:- 1. Simple-Cell Insect Colony 2. Social Organisation in Communal Insects 3. Characteristics.
Simple-Cell Insect Colony:
In those protozoa which reproduce sexually, the zygote or the cell individual formed by the union of the gametes divides by fission into two or more individuals like itself and these usually separate and lead their own independent lives. Probably in a few cases the cell individuals had failed to separate and remained attached together forming the simple or primitive colony.
The characteristics of the simple type of cell colony are:
(a) The colony is Small being composed of relatively few cell-individuals.
(b) The cell individuals of the colony are loosely attached.
(c) The cell individuals are similar and more or less unspecialised.
Highly-evolved cell colony:
In case of a higher animal the body is an example of highly-evolved cell colony. On analysis of the body, the following characteristics will be observed. (1) The colony is enormous, composed of myriads of cell individuals.
(2) The individuals differ greatly from one another. The constituent cells are highly specialized and organised for the performance of various functions. The superficial layer of cells is specialised to form a protective skin.
Certain tracts of cells in the interior are specialised to form a rigid skeleton to support and protect the softer parts, some cells are modified to form contractile muscles to move the skeleton and the movement of the body as a whole.
A layer of cells has been inwardly tucked from the surface to form a tube which looks after mastication, digestion and absorption of food. Some cells have been specialized to excrete poisonous waste materials constantly being produced by the metabolic activities of the cell community.
(3) Members of the cell colony are closely linked together by complex organisation so as to ensure their harmonious co-operation in the activity of the whole organism. The major part in this organisation is played by the nervous system and the blood vascular system.
The nervous system serves to gain information through the sensory cells regarding the world outside, to judge the merit of the impression and to react accordingly. The blood vascular system keeps the individual linked together by supplying oxygen and food and by removing carbon dioxide and waste materials from all corners of the colony.
In highly-evolved cell community the general ceils of the body have become more efficient for playing their part in the cell community.
Obelia—a Lowly-Organised Colony:
Between those two extremes we find a stage in the cell colony formation in obelia. Obelia colony consists of a family or clan of individuals, all descendants of a common ancestor and they remain more or less in complete organic continuity with one another. The individuals are fixed in relation to one another and the community as a whole remains attached to some solid objects.
On examining an obelia colony it will be seen that it is composed of a number of dissimilar individuals but each one is in continuity with the other member by a strand of living substance. The majority of the members are Polyps with large trumpet- shaped mouth encircled by a circle of slender tentacles. While there are some members (Blastostyles) which are devoid of tentacles and mouth.
Social Organisation in Communal Insects:
The social organisations of two orders of insects, viz., Isopteraand Hymenoptera are well known. They indicate sufficiently advanced communal forms.
Amongst various species of termites we encounter innumerable stages of social evolution—from small communities consisting of a few unspecialized individuals to communities of several millions showing a high degree of specialization.
The termites live within nests built of soil particles or masticated and partially digested wood firmly cemented together. By habit the termites live in complete seclusion from day light. Large termite nests often form ‘ant-hills’ which may reach a height of 20 feet. It has been observed that termites keep in their nest domesticated beetles and other insects.
These beetles and insects collectively known as ‘Termitophils’ have greatly modified abdomen. The abdomen bears projections and exudes food material that is licked off by the termites. In the nest of some of the old world species of termites a carefully tended ‘garden’ is maintained. Cultivation of the mycelium of certain mushrooms is done in these gardens. The mycelium provides food to young termites.
Members in a termite community:
In a typical termite community there live five different castes of individuals.
1. Chief males and females:
These individuals differ markedly from other individuals of the colony (Fig. 3.18). They are deeply coloured with large well-developed eyes. They are provided with wings. The chief males and females are the only sexually- matured individuals in the colony. In some species the female or the queen’s abdomen shows immense increase in size for the steady production of eggs.
The workers are un-pigmented, small and blind individuals. These wingless individuals may be males or females but in both, the reproductive apparatus is entirely degenerated and functionless. The workers are provided with well-formed jaws.
The soldiers too are un-pigmented, wingless, blind and sexually undeveloped. The head of the soldiers is swollen and is provided with jaws.
Young complemental males and females:
Innumerable young termites are found in the fungus garden nurseries. From these grow up recruits for the various sections of the community. Those that are going to be the ‘king and queen’ are dark coloured and are provided with fully-developed wings and reproductive organs.
While others are blind, wingless but with limited power of procreation. Their duty is to supplement the production of youngs in the event of failure on the part of the chief sexed individuals of the communities.
Formation of a new colony:
The would be ‘kings and queens’ are produced in great number. During spring they emerge from their underground nest and swam forth on their mating flight. They are assailed by birds and other insectivorous animals at this time.
However, a few become successful. The successful couple alight at a suitable spot, excavate for themselves a burrow in the earth. They go inside it, drop off wings and lay the foundation of a new colony.
On the basis of habit and physiological development three types of bees can be encountered. Some of the bees are solitary, some are gregarious and some are communal. The study of different species of bee depicts in a clear fashion the gradational series from solitary to communal forms. The hive bees, however, have attained the highest development.
Anthophora stanfordiana shows the beginning of gregarious life. Anthophora is technically a solitary bee. But they show a marked preference for one another’s company. In a colony of this mining bees it has been seen that there are many vertical burrows. The burrows have been so dug that though they are close together there is no interference.
Each burrow is the property of a single female bee. The burrow is never a store-house of food. The mother bee brings food to the larvae during the larva’s entire period of helplessness.
Andrena, another small mining bee, forms similar colonies. Here the single tunnel is always branched. Within, these blind branches eggs are laid and the branches are then sealed after pouring some food into it. The mother after completion of these arrangements waits at the mouth of the tunnel for the emergence of the young.
Helictus is the smallest mining bee. Each mother makes her own cell and stores food there. But the mothers like each-others company. The cells of all the mothers, though retain their individuality lie close together and open to the air by common vertical passage. That means one entrance and corridor gives access to a number of cells.
Bumble-bees (Psithyrus) form colonies but the colony lasts for one season only. These bees pair in the fall. After copulation the males die and the impregnated females pass the winter sleeping in some hole. In the following spring the females become active.
They collect pollen and honey. These two are then mixed to make a pastry mass. Each female now selects a hole that of a mouse or mole or she digs one herself. She enters the hole, pours the pastry mass and lays eggs on the mass.
The resultant products of development are sterile worker bees. The worker bees now take care of the nest. They enlarge it and collect food. From the egg of the mother several sexually sterile, broods are formed. Males and females are produced at late summer or fall.
The males and females come out, fly and pair. The males and the workers of the colony now die leaving the pregnant females only. These females become the potential founders of next year’s colonics.
The honey bees (Apis mellifica) make permanent colony. The number of individuals in such colonies may reach up to 80,000. The bee hives are made in a hollow tree (in wild state) or in deserted houses in those bees which form domestic swarms. The number of members in a colony becomes less in winter and becomes more in summer.
Social ranks and functions: The colony of honey bees is made up of three social ranks —a single queen, about 100 drones and thousands of workers. All these types can be anatomically distinguished. The queen and the stingless drones are fertile, their main functions are reproduction. The smaller- bodied workers are all sterile females. They build the hive, collect food, feed the queen and the drones and nurse the youngs.
A worker bee has a life span of about six weeks. A worker bee does not do the same type of work throughout its life. The age of a bee determines what work it will have to do. The house-keeping work is done by young workers while food collection is done by older workers.
The worker bees gather pollen, rich in protein and nectar, a sugary solution. The pollen is collected in ‘pollen baskets’—a cavity on each hind leg and nectar is swallowed into the ‘honey crop’, a specialized part of the alimentary canal where the sugar part of nectar is partly digested by saliva.
On returning to hive these workers unload the pollen in one chamber and the nectar in another chamber. Other members now look after the collection. Some start packing the pollen tightly in the chamber. Some start converting the nectar into honey. Nectar-filled cells are sealed with wax because it is the principle food store for the winter. The pollen being perishable is not usually stored.
Mode of development of different members:
The queen lays eggs individually into honeycomb cells. Some of these eggs may escape fertilization even in a young queen. None is fertilized in an old queen whose sperm store is exhausted. Unfertilized eggs develop into drones. Fertilized eggs have the potentiality to develop either into queen or into workers.
When a queen is to be raised the prospective larva is fed with ‘royal jelly’ containing pollen, honey and vitamins (Pantothenic acid). But new queens are not raised so long as the original queen remains in the hive, active and healthy. Larvae that are to be raised into workers are fed with pollen and honey by the nurse workers.
When the queen lays more eggs that can be accommodated in the hive its food supply is cut down. Conversely when the queen slows down in egg laying more food is supplied to it.
New hive formation:
When a hive becomes overcrowded, the queen, a few drones and thousands of workers come out of the hive and ultimately raise a new hive. In the older hive the workers pick up a small batch of that old queen’s eggs and raise them in specially built honeycomb cells.
The eggs develop into new queens. The first one that emerges from the cell immediately traces out the cells of other queens and kills them to death. In case two queens emerge at the same time they fight with each other until one dies. The victorious queen now mates with one of the drones. In the nuptial fight she receives millions of sperms and stores them in the receptacle.
Total number of species of ants are nearly 5000 and all the species are communal. Ants are found almost everywhere on land. Ant colonies are very stable sometimes outlasting generations of men. Such stability is due to the longevity of individual ants, since the workers live from four to seven years and the queens live for thirteen to fifteen years.
Another important feature by which the ants are distinguished from all other social insects is the diet.
The ants do not restrict their diet to cellulose as do the termites or too few specialised substances like honey or pollen as do the bees. Another characteristic of the ant colony is that a single female is never the reproductive centre of the colony. A successful species always keeps a number of fertile females in the colony.
Against these there is one handicap for the colony. The ants are unable to move themselves or their brood to other localities easily in case the nest is disturbed, when moisture and temperature become unfavourable or food supply fails.
Categories of social ants:
On the basis of the social evolution ant colonies have been classed as:
a. Foraging or Marauding ants:
The genus Eciton illustrates a state of savagery. These ‘army ants’ make long marches in search of food. The ants invade the nest of other ants and insects, capture larvae and pupae and use them as food. They store some of the food in their temporary nest as they are nomadic and also carry the food during the march for another insect nest.
b. Slave-holding ants:
Some ants bring home the larvae and pupae of other ant colonies. When these larvae and pupae hatch out they are employed by the raider to do work. Some slave-holding ants (Polyergus) are so much dependent upon their captives that they have become unable to dig or care for their youngs. This over specialization will certainly lead to their extinction.
c. Herding ants or Dairy ants:
Some ants keep aphids or plant lice which serve them as domestic cattle do to mankind. The brown ant, Lasius, collect young larvae of the corn root louse in the fall and keep them in their nests till springs Now they are transported to the roots of certain weeds.
When corn seeds germinate the lice are taken to the roots of corn plant and herded there. In autumn these lice are allowed to pair and the offsprings are preserved as before.
d. Agricultural ants:
Members belonging to the genus Pogonomyrmix make nests partly below and partly above ground in the form of large symmetrica] heaps. The nests are usually located in open sunny places and where there is grass.
The nests are stored with seeds and grains garnered from the surrounding grasses which are cut away in the area close to the nest. From this bare area well-worn trails diverge into the surrounding grass. It is believed that harvestor ants actually harvest their favourite grasses.
Leaf-cutting ants prepare leaves upon which fungi are grown. The fungi are systematically pruned and cared for by gardening.
e. Honey ants:
Certain desert ants called Honeypot ants, (Myrmecocystus) collect nectars from the flowers and feed it to some of their fellow workers which are kept within the nest. These workers become greatly distended with honey to become the ‘Living bottles’. They hang by their feet from the ceiling of the dome-shaped chambers of the nest. During dry season they dispense drops of honey to their thirsty mates.
f. Thief ants:
Members of the genus Soleonopsis are small in size and live in close association with several different species of large-sized ants. These thief ants feed upon the larvae and pupae of the large ants.
g. Commensal ants:
These ants live in a commensal way and exchange mutual benefit. Such a relation is found between Myrmica and Leptothorax. The Leptothorax ants are small in size and live in the Myrmica nests. They make one or more small chambers from which very small tunnels enter the Myrmica nests.
The tunnels are so small that larger Myrmicas cannot go through them. When food is needed a Leptothorax enters the Myrmica chambers and mounts a myrmica ant. Then it starts licking the body first and then the lower jaw of the Myrmica and gets a minute drop of liquid. Thus Leptothorax gets food and Myrmicas get cleaned and ‘shampooed’.
Behaviour cycle of the ants:
In the behavioural cycle of colonies there is a nomadic phase alternated by statary phase. In the nomadic phase a large proportion of the adult workers go out on daily raids and collect food. The raids begin at dawn. The ants come out of the bivouac and form several columns, each column later dividing into a network of branches. In running along these trails the ants do not depend much on vision.
These ants (Eciton) have vestigial eyes consisting of only a single facet; they can detect changes in the intensity of light and almost certainly cannot reproduce an image of an object. The raiding ants stay together following a chemical trail laid on the ground by the other workers.
It has been demonstrated that the substance deposited by army ants originates in the hind intestine but it is not yet established whether the substance consists of undigested food or a glandular secretion or a combination of both.
By midmorning the raiding columns overrun an area extending a considerable distance from the nest—often more than 100 metres. At the front of the raiding columns the ants attack insects and other arthropods, biting and stinging the prey, pulling it apart and carrying the softer pieces back to the nest. Thus the column is actually a two-way stream, with some ants advancing and others returning with their prey.
At night fall the entire colony moves to a new bivouac typically immigrating along one of the principle raiding trails of that day. It may take the colony most of the night to complete the move to the new nesting site.
The daily routine of massive raids and emigrations from bivouac to bivouac is followed for 14—17 days. Then the colony settles down abruptly, it stops emigrating and remains at the same nesting place. This statary phase lasts for about weeks. At the end of the time the cycle begins again.
Social behaviour of army ants:
Colonies of Army ants (Family Dorylinae) typically consist of a single queen, a brood of developing youngs, and a large population of adult workers. The workers are of different sizes (Fig. 3.19). The queen is the colonies’ sole agent of reproduction, and she is responsible to a great extent for the colonies’ cohesion.
The queen secretes certain substances that are attractive to the workers and therefore holds the colony together. More important, the chemical secretions of the queen actually enhance the survival of the workers.
At regular intervals (about every five weeks) the queen’s large abdomen swells with fatty tissue and eggs and she may lay well over 100,000 eggs in the course of a week. The eggs then give rise to the successive stages of development: larva, pupa and adult (Fig. 3.20).
The newly-emerged adults are readily recognizable because of the light pigmentation on their bodies and are called ‘Callows or young workers. The workers are all females but sterile with underdeveloped ovaries.
In species of Eciton, the workers developing from a given batch of eggs vary in size structure—a condition known as polymorphism. The developmental basis for polymorphism is not clear, although two possibilities exist.
The first is that the developmental pathways leading to adult workers that differ in size and structure are determined by the amount and kind of food eaten by the larvae immediately after their hatch from the egg.
This mode of development exists in Honey bees. The second possibility is that the eggs laid by the queen are so different biochemically that the subsequent development of the larvae is altered by the quantity or quality of food consumed.
Whatever the differentiating mechanism may be, the adult ants that differ in size and structure also exhibit contrasting patterns of behaviour, with the result that there is a division of labour in the colony.
Small workers (size about 3 millimetres) spend most of their time in the nest feeding the larval broods. Intermedial- sized workers constitute most of the population, going out in raids as well as doing other jobs. The largest workers (more than 14 millimetres) have a huge head and long powerful jaws.
These individuals are the Soldiers. They carry no food but customarily run along the flanks of the raiding and emigration columns. An excited “soldier’ is a formidable animal, it rears up on its hind legs, vibrates its antennae and rythmicajly opens and closes its jaws.
Characteristics of the Insect Colony:
(a) Large colony:
The army ants live in large colonies. The Asian genus, Aenictus make colonics with not less than 100,000 individuals. Eciton of Central and South America has colony sizes approaching a million individuals. The African genus, Dorylus makes a colony with twenty million individuals.
(b) Periodic shifting of nesting sites:
The second characteristic of the colony is the ants’ periodic shifting of nesting sites. The colony movements or emigrations involve the entire colony—workers, brood and queen. At the end of each emigration the army ants settle down only in temporary bivouacs.
(c) Carnivorous in food habit:
The third characteristic is that the army ants are almost exclusively carnivorous. They feed on other arthropods particularly insects and some species have been observed to eat small vertebrates such as lizards and snakes. The raiding parties of certain species are so huge that wherever they live the ants rank as major predators in the ecological community.
(d) Firm cohesiveness:
The last characteristic of the army ants is their tight cohesiveness. Army ants do not. forage for food individually. Instead all immigrations and raiding to new bivouacs are conducted by groups of individuals that closely follow a chemical trail deposited continuously by all the ants as they run along the ground.
Regulating factors in the behaviour cycle:
At one time there were two con trasting hypotheses about the regulating factors. One suggested that the cycles of behaviour were influenced by physical conditions of the environment, such as temperature, humidity, air pressure or the phases of the moon. The other suggested that the stimulus for emigrations was due to the depletion of the food supply in the area around the bivouac.
Later on the work of Schneirla showed that both these hypotheses are incorrect. He found that generally within a given environment some of the colonies were in the nomadic phase and some were in the statary phase—that rules out environmental conditions as the determinant of whether or not a colony would make nightly emigrations.
Schneirla disposed of the second hypothesis by observing that a colony of army ants would sometimes move into a nesting site that had just been vacated by another colony and the new-comers would remain at this site even for a three-week statary period— clearly indicating that the food supply around the bivouac had not been exhausted.
The actual regulator in the ants’ nomadic and statary behaviour was not some external influence but the breeding cycle within the colony. It has been observed that the nomadic phase always coincides with the period when a larval brood is developing in the colony and the statary phase begins when the larvae start to spin cocoons and go into the pupal stage.
When these ants emerge from the pupal cocoon as young workers, the nest stirs with high activity.