| Phasmatodea Temporal range: 55.8–0 Ma Eocene - Recent |
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| Leptynia hispanica | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Arthropoda |
| Class: | Insecta |
| Superorder: | Exopterygota |
| Order: | Phasmatodea Jacobson & Bianchi, 1902 |
| Suborders | |
The Phasmatodea (sometimes called Phasmida) are an order of insects, whose members are variously known as stick insects (in Europe and Australasia), walking sticks or stick-bugs (in the United States and Canada), phasmids, ghost insects and leaf insects (generally the family Phylliidae). The ordinal name is derived from the Ancient Greek φάσμα phasma, meaning an apparition or phantom, and refers to the resemblance of many species to sticks or leaves. Their natural camouflage can make them extremely difficult to spot. Phasmatodea can be found all over the world in warmer zones, especially the tropics and subtropics. The greatest diversity is found in Southeast Asia and South America, followed by Australia. Phasmids also have a considerable presence in the continental United States, mainly in the Southeast.
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Phasmids can be relatively large, ranging from 1 inch to over a foot in length. Females of the genus Phobaeticus are the world's longest insects, measuring up to 56.7 centimetres (22.3 in) in total length in the case of Phobaeticus chani, including the outstretched legs.[1] Females of the species Heteropteryx dilatata are the heaviest known phasmids, possibly weighing in excess of 65 grams.[2]
Some have cylindrical stick-like bodies, while others have a flattened, leaflike shape. The thorax is long in the winged species, since it houses the flight muscles, but is typically much shorter in the wingless forms. Where present, the first pair of wings are narrow and cornified, while the hind wings are broad, with straight longitudinal veins 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. Many species are wingless, or have reduced wings.[3] 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.[3] They have long, slender antennae that are as long or longer than the length of the body in some species.
All phasmids possess compound eyes, but ocelli are only found in some winged males.[3] Phasmids have an impressive visual system that allow 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 the insect grows through successive molts, the number of facets is increased along with the number of photoreceptor cells in the eye. The sensitivity of the adult eye is at least tenfold that of the first instar nymphs. As the eye grows more complex, the mechanisms to adapt to dark/light changes are 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 insect’s 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 illuminated foliage. Young stick insects are diurnal feeders and will move around freely, expanding their foraging range.[4]
Phasmatodea species exhibit mechanisms for defense from predators that both prevent an attack from happening in the first place (primary defense) and are deployed after an attack has been initiated (secondary defense).
The defense mechanism most readily identifiable with Phasmatodea is camouflage. Most phasmids are known for effectively replicating the forms of sticks and leaves, and the bodies of some species (such as O. macklotti and Palophus centaurus) are covered in mossy or lichenous outgrowths that supplement their disguise. Some species have the ability to change color as their surroundings shift (B. scabrinota, T. californica). In a further behavioral adaptation to supplement crypsis, a number of species have been noted to perform a rocking motion where the body is swayed from side to side that is thought to reflect the movement of leaves or twigs swaying in the breeze. Another method by which stick insects avoid predation and resemble twigs is by feigning death (catalepsy), where the insect enters a motionless state that can be maintained for a long period. The nocturnal feeding habits of adults also aids Phasmatodea in remaining concealed from predators.
In a seemingly opposite method of defense, many species of Phasmatodea will 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 E. 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.
When threatened, some phasmids that are equipped with femoral spines on the metathoracic legs (O. martini, Eurycantha calcarata, Eurycantha horrida, D. veiliei, D. covilleae) 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.[5]
A number of species are equipped with a pair of glands at the anterior edge of the prothorax that enable the insect to release defensive secretions, including chemical compounds of varying effect: from the production of distinct odors to the causing of a stinging, burning sensation in the eyes and mouth of a predator.[6] The spray often contains pungent-smelling volatile metabolites, previously thought to be concentrated in the insect from its plant food sources. However, based on recent research it seems more likely that they manufacture their own chemical defense substances.[7] Additionally, the chemistry of the defense spray from at least one species, Anisomorpha buprestoides, has been shown to vary[7] based on their life stage and/or population.[8] This chemical spray variation also corresponds with regionally specific color form populations in Florida who also have distinct behaviors.[9] The spray from one species, Megacrania nigrosulfurea, is even used as a treatment for skin infections by a tribe in Papua New Guinea because of its antibacterial constituents.[10] 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. Stick insects, like their distant relation the grasshopper, can also discharge the contents of their stomach through vomiting when harassed, a fluid that is considered inedible by some predators.
The life cycle of the stick insect begins when the female deposits her eggs through one of three methods of oviposition: she will either flick her egg to the ground by a movement of the ovipositor or her entire abdomen, carefully place the eggs in the axils of the host plant or bury them in small pits in the soil, or stick the eggs to a substrate, usually a stem or leaf of the food plant.[5] A single female lay from 100 to 1,200 eggs after mating, depending on the species. Most species of phasmid are parthenogenic, meaning that females lay eggs without needing to mate with males to produce offspring. However, some parthenogenic species retain the ability to mate and are bisexual depending on the presence and abundance of males. Eggs from virgin mothers are entirely female and exact copies of their mothers.
Phasmatodea eggs resemble seeds in shape and size, and have hard shells. They have a lid-like structure called an operculum at the anterior pole, from which the nymph emerges during hatching. The eggs vary in the hatching period, from 13 to more than 70 days, with the average around 20-30 days.[5] Some species, particularly those from temperate regions, undergo diapause, where development is delayed during the winter months. Diapause is affected by photoperiod on the egg-laying adults or can be genetically determined. Diapause is broken by exposure to the cold of winter, causing the eggs to hatch during the following spring. Among species of economic importance, diapause affects the development of 2-year cycles of outbreaks.
Many species' eggs bear a fatty knoblike capitulum that caps the operculum. This structure attracts ants because of its resemblance to the elaiosome of some plant seeds that is a sought after food source for ant larvae, and usually contributes to ensuring seed dispersal by ants, a form of ant-plant mutualism called myrmecochory. The ants take the egg into their nest underground and can remove the capitulum to feed to their larvae without harming the phasmid embryo. There, the egg hatches and the young nymph, which initially resembles an ant (another instance of mimicry among phasmatodea), eventually emerges from the nest and climbs the nearest tree to safety in the foliage.[5]
The Phasmatodea life cycle is hemimetabolous, proceeding through a series of several nymphal instars. The stick insect’s wings develop externally. As is the case with hatching, if the nymph is caught in the encasing of a rejected cast skin (or shell), it will likely die because it cannot free itself. Once emerged the nymphs will eat the cast skin. Adulthood is reached for most species after several months and many molts. The lifespan of phasmatodea varies by species, but ranges from a few months to up to two years for some tropical varieties.
Phasmids are herbivorous, feeding mostly on the leaves of trees and shrubs, and a conspicuous component of many neotropical (South American) systems. Phasmatodea has been postulated as a dominant light-gap herbivore there. Its 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. By lowering the net production of early successional plants by consuming them and then augmenting the nutrients in the soil available to later successional plants through defecation, the walking stick ensures that the tendency of early successional plants to swiftly immobilize soil nutrients in light-gaps does not stymie new substantial growth and the recycling of the tropical forest.[11]
Phasmatodea is recognized as a defoliator that is injurious to forest and shade trees. D. violenscens, P. wilkinsoni and C. tessulatus in Australia, Diapheromera femorata in North America and G. 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 preys on the foliage of oaks and other hardwoods. Severe outbreaks of D. 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. However, investigations have been made into natural enemies of phasmatodea such as birds and parasitic wasps (such as the cleptid wasp Myrmecomimesis).
The classification of the Phasmatodea is complex. There are many people, including amateur entomologists, studying the order, and revisions are commonplace. Furthermore, there is much confusion over the ordinal name. Phasmida is preferred by many authors even though it is incorrectly formed. Phasmatodea is correctly formed, and is gaining in popularity. The term "Cheleutoptera" is now considered outdated.
Phasmatodea is sometimes considered related to other orders, including the Blattaria, Mantodea, Notoptera and Dermaptera, but the affiliations are uncertain and the grouping (sometimes referred to as "Orthopteroidea") may be paraphyletic and hence invalid in the traditional circumscription. Phasmatodea was once considered a suborder of Orthoptera, although most authors now consider it to be an order of its own. There are anatomical features that separate them as a monophyletic group from the Orthoptera. One is the instance among all species of Phasmatodea of a pair of exocrine glands inside the prothorax used for defense. Another is the presence of a specially formed sclerite called a vomer that allows the male to clasp the female during mating.[12]
The order is divided into two, sometimes three suborders.[12] The most common division is into the suborder groups Anareolatae and Areolatae, which are distinguished according to whether the insect has sunken areola, or circular areas, on the underside of the apices of the middle and hind tibiae (Areolate) or not (Anareolate).[13]
There are in excess of 3,000[14] described species, with many more yet to be described both in museum collections and in the wild.
One Australian species, the Lord Howe Island stick insect, is now listed as critically endangered. It was believed extinct until its rediscovery on the rock known as Ball's Pyramid. There is a large effort in Australia to rear this species in captivity. The best known of the stick insects is the Indian or Laboratory stick insect (Carausius morosus). These insects grow to roughly 10 centimeters (3.9 in). They reproduce parthenogenically and males are unrecorded, although part male part female gynandromorphs are relatively common.
Stick insects, like praying mantises, show rocking behavior in which the insect makes rhythmic, repetitive side-to-side movements. The common interpretation of this behavior's function is that it enhances crypsis by mimicking vegetation moving in the wind. However, the repetitive swaying movements may be most important in allowing the insects to discriminate objects from the background by relative motion. Rocking movements by these generally sedentary insects may replace flying or running as a source of relative motion to help them discern objects in the foreground.[15]
Mating behavior in Phasmatodea is impressive because of the extraordinarily long duration of pairings. A record among insects, the stick insect Necroscia sparaxes, found in India, is sometimes coupled for 79 days at a time. It is not uncommon for this species to assume the mating posture for days or weeks on end, and among some species (Diapheromera veliei Walsh and D. Covilleae), pairing has been observed to last 3-136 hours in captivity.[16] Explanations for this behavior range from males guarding their mates against reproductive competitors to the view that the pairings are a defensive alliance.
Instances of overt displays of aggression between males over mates would suggest that the extended pairing behavior may have evolved to guard females against sperm competition. Fighting between competing males has been observed in the species D. veiliei and D. covilleae.[17] During these encounters, the approach of a challenger causes the existing mate to manipulate the female's abdomen, which he has clasped by means of the clasping organ, or vomer, down upon itself to block the site of attachment. Occasionally the consort will strike out at the competitor with the mid femora, which are equipped with an enlarged and hooked spine in both sexes that has been observed to draw the blood of the opponent when they are flexed against the body to puncture the integument.[17]
Usually a strong hold on the female's abdomen and blows to the intruder are enough to deter the unwanted competition, but occasionally the competitor has been observed to employ a sneaky tactic to inseminate the female. While the first mate is engaged in feeding and is forced to vacate the dorsal position, the intruder can clasp the female's abdomen and insert his genitalia. If he is discovered, the males will enter into combat wherein they lean backward, both clasped to the female's abdomen, and freely suspended, engage in rapid, sweeping blows with their forelegs in a manner similar to boxing. Usually when the intruder gains attachment to the female's abdomen, these conflicts resolve in the displacement of the original mate.[17]
Lengthy pairings have also been described in terms of a defensive alliance.[16] When cleaved together, the pair is more unwieldy for predators to handle. Also, the chemical defenses (secretions, reflex bleeding, regurgitation) of the individual stick insect are enhanced when two are paired together. Female survivorship of attacks by predators is significantly enhanced by pairing, largely because the dorsal position of the male functions well as a shield. This could indicate that manipulation by females is present: if females accept ejaculate at a slow rate, for instance, the males are forced to remain in copulo for longer and the female's chances of survival are enhanced. Also, evolution could have simply favored males that remained attached to their females longer, since females are often less abundant than males and represent a valuable prize, so that for the lucky male even the sacrifice of his own life to preserve his offspring with the female may be worth it.
Sexual dimorphism in the species, where females are usually significantly larger than the males, may have evolved due to the fitness advantage accrued to males that can remain attached to the female, thereby blocking competitors, without severely impeding her movement.
Many stick insects are easy to care for in captivity. Almost 300 species have been reared in labs or as pets.[18]
The most commonly kept, the Indian (or Laboratory) stick insect, Carausius morosus, requires a tall (25+ cm) vivarium (even a jar with a few holes punched in the top), some bramble, ivy, privet and an atmosphere at room temperature. Indian stick insects are almost all female with only a few half-males (gynandromorphs), and these are not needed for reproduction. The females reproduce by parthenogenesis and seem content to live on their own. By the sixth molt the Indian stick insect will be sexually mature and can lay eggs.
Many of the other species of phasmids kept in captivity will feed on bramble. However, some are very specialist feeders and are therefore more difficult to rear. Beginners often make the mistake of thinking all species will feed on privet (the plant most commonly used to feed the Indian stick insect). In fact few species feed on privet. Most of the privet feeders on the Phasmid Study Group's culture list belong to the family Pseudophasmatidae and are from South America. Several of these will also feed on hebe. The few members of the family Aschiphasmatidae that have been reared have to be fed on fuchsia, willow herb, or evening primrose. Some of the species in the subfamily Necrosciinae will only feed on hypericum.
The optimal diet of phasmatodea in captivity during the winter months, or in colder climates, when/where tropical vegetation is scarce, is organic lettuce. A diet of carrots and spinach do not lead to the production of viable eggs and often leads to premature death.[19]
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