PHASMIDS STUDIES VOLUME 17
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Species H. Dehaani
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(also known as Phasmida or Phasmatoptera) are an order of insects, whose members are variously known as stick insects (in Europe and Australasia), stick-bugs or walking sticks (in the United States and Canada), phasmids, ghost insects and leaf insects (generally the family Phylliidae). The group's name is derived from the Ancient Greek phasma, meaning an apparition or phantom, referring to the resemblance of many species to sticks or leaves. Their natural camouflage makes them difficult for predators to detect, but many species have a secondary line of defence in the form of startle displays, spines or toxic secretions. The genus Phobaeticus includes the world's longest insects.
Members of the order are found all over the world except for the Antarctic and Patagonia, but they are most abundant in the tropics and subtropics. They are herbivorous with many species living unobtrusively in the tree canopy. They have a hemimetabolous life cycle with three stages : eggs, nymphs and adults. Many phasmids are parthenogenic, and do not require fertilised eggs for female offspring to be produced. In hotter climates, they may breed all year round; in more temperate regions, the females lay eggs in the autumn before dying, and the new generation hatches out in the spring. Some species have wings and can disperse by flying, while others are more restricted.
Phasmids can be relatively large, ranging from 1.5 centimetres to over 30 centimetres in length. Females of the genus Phobaeticus are the world's longest insects, measuring up to 56 centimetres in total length in the case of Phobaeticus chani, including the outstretched legs. The heaviest species of phasmid is likely to be Heteropteryx dilatata, the females of which may weigh as much as 65 g .
Some phasmids have cylindrical stick-like bodies, while others have flattened, leaflike shapes. Many species are wingless, or have reduced wings. The thorax is long in the winged species, since it houses the flight muscles, and is typically much shorter in the wingless forms. The body is often further modified to resemble vegetation, with ridges resembling leaf veins, bark-like tubercles, and other forms of camouflage. A few species, such as Carausius morosus, are even able to change their pigmentation to match their surroundings. Chewing mandibles are uniform across species. The legs are typically long and slender, and some species are capable of limb autotomy . Phasmids have long, slender antennae, as long as or longer than the rest of the body in some species. Phasmids have an impressive visual system that allows them to perceive significant detail even in dim conditions, which suits their typically nocturnal lifestyle.
Stick insects have two types of pad on their legs: sticky "toe pads" and non-stick "heel pads" a little further up their legs. The heel pads are covered in microscopic hairs which create strong friction at low pressure, enabling them to grip without having to be peeled energetically from the surface at each step. The sticky toe pads are used to provide additional grip when climbing but are not used on a level surface.
Phasmatodea can be found all over the world except for the Antarctic and Patagonia. They are most numerous in the tropics and subtropics. The greatest diversity is found in Southeast Asia and South America, followed by Australia, Central America, and the southern United States.Over 300 species are known from the island of Borneo, making it the richest place in the world for Phasmatodea.
Phasmatodea species exhibit mechanisms for defense from predators that prevent an attack from happening in the first place (primary defense), and defenses that are deployed after an attack has been initiated (secondary defense).
The defense mechanism most readily identifiable with Phasmatodea is camouflage, in the form of plant mimicry. Most phasmids are known for effectively replicating the forms of sticks and leaves, and the bodies of some species are covered in mossy or lichenous outgrowths that supplement their disguise. Remaining absolutely stationary enhances their disguise. Some species have the ability to change color as their surroundings shift . In a further behavioral adaptation to supplement crypsis, a number of species perform a rocking motion where the body is swayed from side to side; this is thought to mimic the movement of leaves or twigs swaying in the breeze. Another method by which stick insects avoid predation and resemble twigs is by entering a cataleptic state, where the insect adopts a rigid, motionless posture that can be maintained for a long period. The nocturnal feeding habits of adults also help Phasmatodea to remain concealed from predators.
Some species, such as the young nymphs of Extatosoma tiaratum, have been observed to curl the abdomen upwards over the body and head to resemble ants or scorpions in an act of mimicry, another defense mechanism by which the insects avoid becoming prey. The eggs of some species such as Diapheromera femorata have fleshy projections resembling elaiosomes (fleshy structures sometimes attached to seeds) that attract ants. When the egg has been carried to the colony, the ant feeds the elaiosome to a larva and the phasmid egg develops in the recesses of the nest in a protected environment.
Some species are equipped with a pair of glands at the anterior edge of the prothorax that enables the insect to release defensive secretions, including chemical compounds of varying effect: some produce distinct odors, and others cause a stinging, burning sensation in the eyes and mouth of a predator. Another ploy is to regurgitate their stomach contents when harassed, repelling potential predators. Phasmids are herbivorous, feeding mostly on the leaves of trees and shrubs, and a conspicuous component of many neotropical (South American) systems. Phasmatodea have been postulated as dominant light-gap herbivores there. Their role in the forest ecosystem is considered important by many scientists, who stress the significance of light gaps in maintaining succession and resilience in climax forests. The presence of phasmids lowers the net production of early successional plants by consuming them and then enriches the soil by defecation. This enables the late succession plants to become established and encourages the recycling of the tropical forest.