Caste System And Reproductive Behaviour Of Termites

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Termites, bees and wasps fall in the category of social insects.  Also called ‘white ants’, termites are terrestrial polymorphic[1] eusocial[2] insects.  Phylogenetic studies indicate that they have evolved from a common ancestor along with the sub-social Cryptocercid[3]cockroach in the late Jurassic of Mesozoic era.  They have been placed in the order Isoptera along with other cellulose-eating insects.  However, because of their common ancestry with cockroaches, they are also placed in the order Dictyoptera that also contains mantids.Members of the Termitidae family are called higher termites while those of the families Mastotermitidae, Hodotermitidae, Termopsidae, Kalotermitidae, Rhinotermitidae and Serritermitidae are called lower termites. 

 Nests of termites consist of well-organized colonies that provide shelter to several generations living together.  They are distributed throughout the temperate, tropical and subtropical regions of the world with the maximum diversity appearing in tropical forests.  Termites are good scavengers and decomposers.  They feed on a wide variety of living organisms, faeces, dead or decaying plants material and soil rich in humus and therefore, recycle organic waste material. 

Termite mounds are rich in minerals and contribute towards soil fertility when they get demolished and mix with the soil.  Burrowing activities of termites also modify soil structure and composition, improve drainage and aerate the soil.  Termites can, therefore, behave as ecosystem engineers[4] by changing physical properties of soil, such as texture, water infiltration rates and nutrient content over long periods of time. 

Types of termites

Termites are classified according to their distribution, feeding habit and requirement of moisture.  Lower termites are divided into three broad categories- drywood, dampwood and subterranean.

Drywood termites (Kalotermitidae) can survive in dry conditions and have little requirement for extra moisture source; Dampwood termites (Termopsidae) ravage damp and decaying wood and are more commonly found in wood in contact with the ground, although they do not need soil contact to sustain; and Subterranean termites thrive in soil and require a continuous supply of moisture and attack moist wood. 

Brief Description

Termites typically measure between 4 to 15 mm in length.  Queens of the species Macrotermesbellicosus are the largest of the living species and can measure up to 10 cm in length.  Most workers and soldiers among termites are blind as they lack eyes.  Alates (winged males and females) bear eyes along with lateral ocelli[5]

 The termite thorax consists of three segments similar to other insects – prothorax, mesothorax and metathorax.  There are two legs in each segment.  In alates, wings are located at the mesothorax and metathorax.  Reproductive organs are similar to cockroaches but more elementary.  The non-reproductive castes of termites lack wings and depend on their six legs for locomotion.

Life cycle

 Dispersal of winged reproductives to conquer new resources marks the onset of the life cycle of termites.  The mating pair sheds their wings and builds a new nest.  Subsequently, the queen oviposits, the eggs hatch and nymphs develop into different forms.  It is during the post-embryonic development that castes are determined and each larva (first or second instar[6] termite) may develop into a worker, soldier or reproductive. 

King and Queen are the primary productives responsible for founding the nest and the colony.  They are pigmented and bear fully-developed wings.  The role of the King is to go on nuptial flights, copulate with the queen and fertilize her by insemination.  The Queen stocks the sperms in the spermathecae after copulation and utilizes them to fertilize eggs.  The primary function of the Queen is to produce eggs and she lays more than 3000 eggs per day.  Eggs are incubated for 5-60 days following which, nymphs hatch out in the form of neonates and undergo a number of moultings prior to becoming adults.  Being hemimetabolous[7], termites undergo incomplete metamorphosis.  A termite colony reaches its maximum size in 4-5 years with more than 60,000-200,000 workers.  Workers further modify into sterile soldiers or remain as sterile workers.  Both workers and soldiers are wingless and generally are marked by the absence of eyes.

Different Castes of Termites

A termite colony has a well-developed caste system.  The caste of each individual termite is regulated by pheromones, called social hormones, during post-embryonic development.  These pheromones are secreted by the reproductive castes, primarily by the King and Queen.  Caste system among termites is determined by division of labour and each caste is distinguished by unique traits.  A termite colony consists of two major castes- sterile and reproductive

 

  1. Sterile castes:

These castes are incapable of reproduction and consist of workers and soldiers.

  1. Workers:Workers can be true or false (pseudergates).  True workers emerge early and irreversibly from imaginal[8] development, while pseudergates separate late from imaginal line.  Workers of both sexes are sterile and evolve from fertilized eggs.  They constitute 80-90% of the entire termite colony.  They measure 6 to 8 mm in size, possess chewing mandibles[9]but typically lack eyes and wings. 

                Workers discharge all domestic, foraging, feeding and defence duties of the colony with the exception of reproduction.They take care of the eggs and young; gather food; store food and feed nymphs, King and Queen; take part in building of nests; cultivate fugus-garden in special chambers of subterranean channels; maintain moisture in the interior of a nest of a colony; clean other castes; and defend the colony.

  1. Soldiers: Soldiers of selected species across various genera exhibit diversity in characteristics.  They develop from unfertilized eggs.  They are wingless, larger than workers, more or less pigmented and possess large head and stout, powerful mandibles.  They are mostly blind, although eyes occasionally appear in some species, e.g.Mastotermes, Hodotermes.  They comprise of only 5-8% of the colony.  Soldiers are responsible for defence of the colony and depend on workers for food.
  1. Reproductive castes: They are fertile and are denoted by the presence of eyes.  Depending on the roles they play, this caste is divided into three types.
    1. Macropterous or large winged or primary reproductive forms: This caste consists of sexually mature females and males with two pairs of wings longer than the body.  They also have widely separated large compound eyes and paired ocelli.  The royal pair or sexually active male and female belong to this caste and are the founders of the colony.
    2. Brachypterous neotenics or short winged or substitute reproductive forms: Sexually mature male or female nymphs replace the primary reproductive in case of old age or death.  Substitute reproductives form by special feeding of the appropriate sex to attain maturity without further moulting.  These forms are denoted by short-pad like wings, ocelli and rudimentary compound eyes and small reproductive organs. 
    3. Apterous neotenic reproductive or wingless forms or replacement reproductives: If the Queen dies in adverse conditions or of old age, apterous neotenic reproductives are formed.  They are typically without wings and have less developed eyes.  However, reproductive organs of these forms develop after one or two moults. 

                The main difference between the sterile and reproductive castes is vision.  Workers and soldiers are blind as they live deep in the soil of the mound and stay in close proximity to food sources.  Reproductive adults, on the other hand, have well-developed eyes because they need vision for flights and quest for suitable nesting sites.

Reproductive Behaviour

Mating behavior in termites has been well studied.During mating, termites form pairs and establish colonies during and after nuptial flights.Termite kings can furnish huge numbers of sperms to the queen continuously for several decades.

Termite Queen

The pair of ovaries in female reproductive or queens grow considerable in size after swarming.The increased demand of eggs among termites leads to the development of ovaries of basal termite queens and causes the queen’s abdomen to swell significantly such that it becomes physogastric[10].In most Termitidae (e.g.Nasutitermescorniger), physogastry is extreme such that a mature physogastric queen can be 50-60 times larger than that of the initial female.

Queens are equipped with a spermatheca, a blind pouch to store sperms injected by the King.Cells in the spermathecal wall secrete mucopolysaccharides.The spermathecae primarily stores sperms needed to fertilize eggs in a later phase of life and protects their viability.Spermathecae of immature females or swarming alates do not contain sperms.This strongly points towards lack of mating in the parental nest before dispersal.

Nuptial Flight andSexual Act

A termite colony must reach a certain size before alates(mature reproductive with wings and functioning eyes that can produce kings and queens) are produced, which may take several years.Over time, alates become harder and darker to enable swarming termites to withstand exposure to light and less humid air.At the appropriate time, alates depart from the colony in a large swarm consisting of thousands of termites.Such mass swarming events, an essential part of a termite life cycle, last for a few days, once or twice a year.

Nuptial flights have two phases- a dispersing phase and pair formation phase.The dispersing phase is when both mating partners fly away from the nest.The pair formation phase is when individuals release pheromones to attract each other.Prior to mating, a courtship occurs also called ‘tandem running’.This involves the male following the female and touching her abdomen with his antennae as she runs to search for her nest.In lower termite families, only one king and queen inhabit the colony.If other sexual individuals invade the nest for mating, they are driven away or killed.In selected higher termite families, multiple primary kings and queens are found in a single nest.Although both monogamy and polygamy are attributes in termite colonies, polygamy is facultative not mandatory.Nuptial flights enable mate selection and pair formation but do not necessarily result in mating or copulation.Copulation only takes place in the security of the nest.Reproduction may also occur via inbreeding depression and parthogenesis.The former refers to reproduction among neotenics in the parental nest, while the latter refers to reproduction from an ovum without fertilization.

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  1. Termite. Encyclopædia Britannica [Online]; Encyclopædia Britannica, inc., Posted March 30, 2017. https://www.britannica.com/animal/termite (accessed Aug 11, 2019).
  2. Shah, R. Termites (The White Ants): Habitat, Life History and Control. http://www.biologydiscussion.com/invertebrate-zoology/the-white-ants/termites-the-white-ants-habitat-life-history-and-control/27684 (accessed Aug 11, 2019).
  3. Pervez A. Termite Biology and Social Behaviour. InTermites and Sustainable Management. Sustainability in Plant and Crop Protection; Khan M., Ahmad W. Eds.;Springer, Cham, 2018.

 

 


[1]Polymorphic:occurrence of variation among species or genes

[2] Eusocial:exhibits a high level of social organization

[3]Cryptocercia:Cryptocercus is a genus of Dictyoptera and the sole member of its own family Cryptocercidae; Species are known as wood roaches or brown-hooded cockroaches.

[4]Ecosystem engineers: any organism that creates, significantly modifies, maintains or destroys a habitat

[5]Ocelli:are simple eyes that detect light and consist of a single lens and several sensory cells. Theyare merely used to detect movement rather than forming a complex image of the environment.

[6] Instar: isa stage in the life of an arthropod (e.g. an insect) between two successive moults.

[7]Hemimetabolous: (of an insect) having no pupal stage in the transition from larva to adult.

[8]Imaginal:relating to an adult insect.

[9]Mandibles: in insects, one of the two parts of the mouth used for biting and cutting food.

[10]Physogastric: having a greatly distended abdomen

 

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