Development and castes

Termites are social insects, but their incomplete metamorphosis, with egg, nymph, and adult phases, distinguishes them from other social insects, such as ants, bees, and wasps, which develop through egg, larva, pupa, and adult stages. Another difference is the presence of a functional male in the termite community. In other social insects, only the female survives after the nuptial flight, and total egg production is limited by the usually single mating. Termite eggs are small and laid singly, though sometimes in quick succession, as in the case of Macrotermes with an average of one every 2 s over long periods. In M. darwiniensis the eggs are laid in batches of 16-24 and cemented together in two rows, similar to oothecae of some cockroaches and grasshoppers. The nymph phase consists ofa series of instars separated by molts, which may be recognized by the size of the individual (larva, worker, or nymph), the number of antennal segments, or the presence of wing pads. The reproductive organs become functional after the final molt, which may result in primary or secondary reproductives.

Colonies function with individuals that have particular roles and functions: defense and protection of the nestagainstpred-ators, foraging for food, or reproduction to expand the existing colony or establish a new colony. Each of these functions is accomplished by a caste, such as soldiers, workers, and reproductives. A typical termite colony contains a functional male and female. This pair of primary reproductives (called king and queen) originally established the colony, and they were at one time winged alates. A colony may have wingless, secondary, or neotenic reproductives (males and females) that remain in the colony and produce eggs. The colony has numerous workers and soldiers; these forms are wingless, have large mandibles, and are sterile. The proportion of workers to soldiers in a colony depends on the species, and usually changes with the increasing age and size of the colony. The ratio of workers to soldiers differs between termite family and species, butcolonies generally consistof8o-go% workers. In Reticuliter-mes,soldiers typically comprise about 2% of the colony individuals, butin Coptotermesformosanus, they comprise 10-15% of the colony.

Larvae hatching from eggs are essentially identical until the first molt. They have no eyes or wing buds, and they develop into other castes after one or more molts. In the primitive termites, nymphs develop from older larvae and they may become workers (called pseudergates) or soldiers with large heads and prominent mandibles. Older nymphs may also become reproductives, including winged males and females, and brachypterous or wingless neotenics. In the evolutionary advanced termites, workers develop from larvae after the second or third molt, and they may spend their entire 14-year life in this caste. However, a worker can molt twice (first to become a pre-soldier) to become a soldier; the soldier castis a terminal caste. Workers that pass through the nymph stage can develop into winged reproductives. Nymphs have external wing buds and, as they progress toward the winged adult stage, they gradually develop eyes, full-size wings, and functional reproductive organs. Nymphs may undergo a regressive molt, lose their wing buds, and revert to the worker stage. Under certain conditions, secondary reproductives develop fromnymphs or worker termites; they are called second- and third-form reproductives.

Workers are wingless individuals with well-sclerotized mandibles, lightly sclerotized heads, and, except for a few species, very lightly sclerotized bodies. In most cases, compound eyes and ocelli are lacking, and the number ofantennal segments is fewer than the other castes. The size of the worker in the colony may vary, depending on their stage of development, and they are considered major and minor workers. Their task in the colony may be related to age and evident in morphological and physiological differences. The young workers tend to remain in the nest caring for the brood, and the old workers conduct nest repair and forage for food. In termite species that do not have a soldier caste, workers assume some nest defense responsibility. Workers in most of the African sol-dierless termites have a line ofweakness in the cuticle behind the metanotum. When threatened by predators the abdominal muscles contract, the cuticle opens, and the intestines burst, scattering their contents on the intruder. The use of intestinal contents for defense has been observed in several genera, including Skatititermes and Speculitermes.

Soldiers differ from other castes in the colony by having modified heads and prominentmandibles. There are two types: mandibulate soldiers, which have a large head and mandibles, and nasute soldiers, in which the frontof the head is elongated and the mandibles are reduced. Mandibulate soldiers occur in nearly all families. Defense against invaders of the nest is achieved by biting or a snapping action of the mandibles. The head in these individuals is usually elongated to accommodate the muscles necessary to move the large mandibles. The enlarged head is used by some species, such as Cryptotermes, to seal galleries and entrance holes against predators. Mandibulate soldiers use salivary secretions as a chemical defense. Saliva is an irritant discharged into wounds created by the mandibles, or functioning as a glue to entangle intruders. Nasute soldiers have reduced mandibles, and they rely entirely on chemicals to deter predators. Soldiers in Nasutitermes and other related genera have a large rounded head with a pointed snout, at the end of which the frontal gland opens. When disturbed, the frontal gland releases an irritating, sticky substance, which can be expelled up to 15 mm. They are able to disable intruders without physical contact. When soldiers of some species are disturbed, they exhibit considerable agitation. They frequently jerk the body and head back and forth, and may produce an audible knocking as the head strikes the sides of galleries. Such action by one soldier often stimulates others to similar agitated knocking.

Secondary reproductives or neotenics are individuals that develop functional reproductive organs without becoming alates. They are produced in colonies thathave lost one or both of the primary reproductives; they are usually not produced in intact colonies. They may develop in portions of a colony that have become isolated or distant from the parent group. Neotenics develop from brachypterous workers and they are considered second-form reproductives. They may also develop from brachypterous nymphs and they are considered third-form reproductives. The primary reproductives in a colony may be replaced by a large number of secondary reproductives, and in this way the size and reproductive potential of the colony are increased. The original sexual pair in the colony is capable of chemically inhibiting development of the reproductive potential of all members of a large colony. The king and/or queen produce a pheromone that circulates through the colony and inhibits the formation of secondary reproductives. Several pheromones produced by the reproductives have been linked in the control of secondary reproductives. The inhibitory pheromone produced by the queen acts to prevent developmentoffemale secondary reproductives, but does notaffect the developmentofalates. This substanceis circulated through the larvae in the colony by anal trophallaxis. In Kalotermesflavicollis, several neotenics are developed following removal of the primary reproductives, but only one pair remains to serve the colony. Fights occur between neotenics of the same sex. Individuals thatare severely wounded during these battles are eaten by other nestmates. In Reticulitermes and Zootermopsis the primary pair may be replaced by numerous secondary reproductives, and the colony may become very large with the added productivity of many queens.

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