PHASMIDS STUDIES VOLUME 17
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Specie M. Pfeifferae
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Phasmids 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. Where present, the first pair of wings is narrow and cornified, while the hind wings are broad, with straight veins along their length and multiple cross-veins. 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. The mouthparts project out from the head. 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. All phasmids possess compound eyes, but ocelli (light-sensitive organs) are only found in some winged males. Phasmids have an impressive visual system that allows them to perceive significant detail even in dim conditions, which suits their typically nocturnal lifestyle. They are born equipped with tiny compound eyes with a limited number of facets. As phasmids grow through successive molts, the number of facets in each eye is increased along with the number of photoreceptor cells. The sensitivity of the adult eye is at least tenfold that of the nymph in its first instar . As the eye grows more complex, the mechanisms to adapt to dark/light changes is also enhanced: eyes in dark conditions evidence less screening pigments, which would block light, than during the daytime, and changes in the width of the retinal layer to adapt to changes in available light are significantly more pronounced in adults. However, the larger size of the adult insects' eyes makes them more prone to radiation damage. This explains why fully grown individuals are mostly nocturnal. Lessened sensitivity to light in the newly emerged insects helps them to escape from the leaf litter wherein they are hatched and move upward into the more brightly illuminated foliage. Young stick insects are diurnal feeders and move around freely, expanding their foraging range.
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. In a seemingly opposite method of defense, many species of Phasmatodea seek to startle the encroaching predator by flashing bright colors that are normally hidden, and making a loud noise. When disturbed on a branch or foliage, some species, while dropping to the undergrowth to escape, will open their wings momentarily during free fall to display bright colors that disappear when the insect lands. Others will maintain their display for up to 20 minutes, hoping to frighten the predator and convey the appearance of a larger size. Some accompany the visual display with noise made by rubbing together parts of the wings or antennae.
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.
When threatened, some phasmids that are equipped with femoral spines on the metathoracic legs respond by curling the abdomen upward and repeatedly swinging the legs together, grasping at the threat. If the menace is caught, the spines can draw blood and inflict considerable pain.
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. The spray often contains pungent-smelling volatile metabolites, previously thought to be concentrated in the insect from its plant food sources. However, it now seems more likely that the insect manufactures its own defensive chemicals. Additionally, the chemistry of the defense spray from at least one species, Anisomorpha buprestoides, has been shown to vary based on the insect's life stage or the particular population it is part of. This chemical spray variation also corresponds with regionally specific color forms in populations in Florida, with the different variants having distinct behaviors. The spray from one species, Megacrania nigrosulfurea, is used as a treatment for skin infections by a tribe in Papua New Guinea because of its antibacterial constituents. Some species employ a shorter-range defensive secretion, where individuals bleed reflexively through the joints of their legs and the seams of the exoskeleton when bothered, allowing the blood (hemolymph), which contains distasteful additives, to discourage predators. 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.
Phasmatodea are recognized as injurious to forest and shade trees by defoliation. Didymuria violescens, Podacanthus wilkinsoni and Ctenomorphodes tessulatus in Australia, Diapheromera femorata in North America and Graeffea crouani in coconut plantations in the South Pacific all occur in outbreaks of economic importance. Indeed, in the American South, as well as in Michigan and Wisconsin, the walking stick is a significant problem in parks and recreation sites where it consumes the foliage of oaks and other hardwoods. Severe outbreaks of the walking stick, Diapheromera femorata, have occurred in the Ouachita Mountains of Arkansas and Oklahoma. The insects eat the entire leaf blade. In the event of heavy outbreaks, entire stands of trees can be completely ravaged. Continuous defoliation over several years often results in the death of the tree. Fortunately for control efforts, because the insects cannot fly, infestations are typically contained to a radius of a few hundred yards. Nevertheless, the damage incurred to parks in the region is often costly. Control efforts in the case of infestations have typically involved chemical pesticides; ground fires are effective at killing eggs but have obvious disadvantages. In New South Wales, research has been done into the feasibility of controlling stick insects using natural enemies such as parasitic wasps.