Arthropod: Difference between revisions
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===Diversity=== |
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Estimating the total number of living species is extremely difficult because it often depends on a series of assumptions in order to scale up from counts at specific locations to estimates for the whole world. A study in 1992 estimated that there were 500,000 species in Costa Rica alone, of which 365,000 were arthropods.<ref name="Thompson1994CoevolutionaryProcess" /> |
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Another estimate indicates that arthropods have over a million described species, and account for over 80% of all known living species.<ref>{{citation |author=Anna Thanukos |url=http://evolution.berkeley.edu/evolibrary/article/arthropodstory |title=The Arthropod Story |publisher=[[University of California, Berkeley]] | accessdate=2008-09-29}}</ref> They are important members of marine, freshwater, land and air [[ecosystem]]s, and are one of only two major animal groups that have adapted to life in dry environments – the other is [[amniote]]s, whose living members are [[reptile]]s, [[bird]]s and [[mammal]]s.<ref name="RuppertFoxBarnes2004P518">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=518-522 |
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}}</ref> One arthropod sub-group, [[insects]], is the most species-rich member of all [[Guild (ecology)|ecological guild]]s (ways of making a living) in land and fresh-water environments.<ref name="Thompson1994CoevolutionaryProcess">{{citation |
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| author=Thompson, J.N. | title=The Coevolutionary Process |
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| publisher=University of Chicago Press | date=1994 | isbn=0226797600 | page=9 |
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| url=http://books.google.co.uk/books?id=AyXPQzEwqPIC&pg=PA9&lpg=PA9&dq=arthropod+species+number&source=web&ots=rvxdxEM9Ww&sig=3vnb5SlVNx_bh-UlE0Tpc7k73jQ&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA9,M1 |
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| accessdate=2008-09-25 |
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}}</ref> The smallest insects weigh less than 25 micrograms (millionths of a [[gram]]), while the largest weigh over {{convert|25|g|oz}}. Some living [[crustacean]]s are much larger, for example the legs of the [[Japanese spider crab]] may span up to {{convert|4|m|ft}}.<ref>{{citation |
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| author=Schmidt-Nielsen, K. | title=Scaling: Why is Animal Size So Important? |
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| publisher=Cambridge University Press | date=1984 | isbn=0521319870 | pages=52-53 |
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| url=http://books.google.co.uk/books?id=8WkjD3L_avQC&pg=PA53&dq=arthropod+size+range&sig=ACfU3U0dWyUXeFEO0WGhuC62UtXBy0SO8w |
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| accessdate=2008-09-25 |
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}}</ref> |
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===Segmentation=== |
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{| align="right" |
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|- |
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|{{Annotated image | float=left| image=HemimerusHanseni.jpg | caption=Segments and [[Tagma (biology)| tagmata]] of an arthropod<ref name="RuppertFoxBarnes2004P518" /> | width=150 | image-width=80 | height=180 |
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| annotations= |
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{{Annotation|95|5|Head}} |
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{{Annotation|90|17|_______________________|color=green}} |
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{{Annotation|95|37|[[Thorax]]}} |
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{{Annotation|90|50|_______________________|color=green}} |
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{{Annotation|95|80|[[Abdomen]]}} |
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{{Annotation|90|100|_______________________|color=green}} |
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}} |
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|- |
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|{{ Annotated image | float=left| caption=Structure of a [[biramous]] appendage<ref name="WonderfulLifeP102To106" /> | image=Biramous cross section 01.png | width=200 | height=145 | image-width = 100 | image-left=0 | image-top=0 |
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| annotations = |
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{{Annotation|115|5|<span style{{=}}"background-color:#c0c0c0"> </span> {{=}} Body}} |
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{{Annotation|115|25|<span style{{=}}"background-color:blue"> </span> {{=}} Coxa (base)}} |
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{{Annotation|115|45|<span style{{=}}"background-color:#ff9f9f"> </span> {{=}} Gill branch}} |
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{{Annotation|115|65|<span style{{=}}"color:red; font-weight:900; font-size:130%">//</span> {{=}} Gill<br /> filaments}} |
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{{Annotation|115|100|<span style{{=}}"background-color:yellow"> </span> {{=}} Leg<br /> branch}} |
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}} |
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|} |
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The [[embryo]]s of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods probably consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However all known living and fossil arthropods have grouped segments into [[Tagma (biology)| tagmata]] in which segments and their limbs are specialized in various ways;<ref name="RuppertFoxBarnes2004P518">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=518-522 |
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}}</ref> The three-part appearance of many [[insect]] bodies and the two-part appearance of [[spider]]s is a result of this grouping;<ref name="WonderfulLifeP102To106">{{citation |
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| title=Wonderful Life: The Burgess Shale and the Nature of History | pages=102-106 |
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| author=Gould, S.J. | authorlink=Steven Jay Gould |
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| year=1990 | publisher=Hutchinson Radius | isbn=0091742714 |
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}}</ref> in fact there are no external signs of segmentation in [[mite]]s.<ref name="RuppertFoxBarnes2004P518" /> Arthropods also have two body elements which are not part of this serially repeated pattern of segments, an [[Prostomium|acron]] at the front, ahead of the mouth, and a [[telson]] at the rear, behind the [[anus]]. The eyes are mounted on the acron.<ref name="RuppertFoxBarnes2004P518" /> |
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The original structure of arthropod appendages was probably [[biramous]], with the upper branch acting as a [[gill]] while the lower branch was used for walking. In some segments of all known arthropods the appendages have been modified, for example to form gills, mouth-parts, [[Antenna (biology)| antenna]]e for collecting information,<ref name="WonderfulLifeP102To106">{{citation |
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| title=Wonderful Life: The Burgess Shale and the Nature of History | pages=102-106 |
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| author=Gould, S.J. | authorlink=Steven Jay Gould |
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| year=1990 | publisher=Hutchinson Radius | isbn=0091742714 |
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}}</ref> or claws for grasping<ref>{{citation |
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| author=Shubin, N., Tabin, C., and Carroll, S. | page=110 |
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| chapter=Fossils, Genes and the Evolution of Animal Limbs |
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| title=Shaking the Tree: Readings from Nature in the History of Life | editor=Gee, H. |
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| publisher=University of Chicago Press | date=2000 | isbn=0226284972 | page=110 |
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| url=http://books.google.co.uk/books?id=M6yF0pU4eCsC&pg=PA110&dq=arthropod+diversity&lr=&sig=ACfU3U1LJGRsFDc9L2W_3LCEN76oyj81gw |
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| accessdate=2008-09-25 |
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}}</ref> – arthropods are "like [[Swiss Army knives]], each equipped with a unique set of specialized tools."<ref name="RuppertFoxBarnes2004P518" /> In many arthropods, appendages have vanished from some regions of the body, and it is particularly common for abdominal appendages to have disappeared or be highly modified.<ref name="RuppertFoxBarnes2004P518" /> |
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{{Annotated image/Arthropod head problem | float=left }} |
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The most conspicuous specialization of segments is in the head. The four major groups of arthropods - [[Chelicerata]] (includes [[spider]]s and [[scorpion]]s), [[Crustacea]] ([[shrimp]]s, [[lobster]]s, [[crab]]s, etc.), [[Tracheata]] (arthropods that breathe via channels into their bodies; includes [[insect]]s and [[myriapod]]s), and the extinct [[trilobite]]s – have heads formed of various combinations of segments, with appendages that are missing or specialized in different ways.<ref name="RuppertFoxBarnes2004P518" /> In addition some extinct arthropods, such as ''[[Marrella]]'', belong to none of these groups, as their heads are formed by their own particular combinations of segments and specialized appendages.<ref name="Whittington1971Marrella">{{citation|last=Whittington|first=H. B.|title=Redescription of ''Marrella splendens'' (Trilobitoidea) from the Burgess Shale, Middle Cambrian, British Columbia | journal=Geological Survey of Canada Bulletin | volume=209 | pages=1–24 | date=1971}} Summarised in {{citation |
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| title=Wonderful Life: The Burgess Shale and the Nature of History | pages=107-121 |
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| author=Gould, S.J. | authorlink=Steven Jay Gould |
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| year=1990 | publisher=Hutchinson Radius | isbn=0091742714 |
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}}</ref> Working out the evolutionary stages by which all these different combinations could have appeared is so difficult that it has long been known as "the [[Arthropod head problem]]".<ref>{{citation |
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| author=Budd, G. E. | date=2002 | title= A palaeontological solution to the arthropod head problem |
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| journal=Nature | volume=417 | pages=271–275 |
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| url=http://www.nature.com/nature/journal/v417/n6886/full/417271a.html | accessdate=2008-09-27 |
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}}</ref> In 1960 R.E. Snodgrass even hoped it would not be solved, as trying to work out solutions was so much fun.<ref>"It would be too bad if the question of head segmentation ever should be finally settled; it has been for so long such fertile ground for theorizing that arthropodists would miss it as a field for mental exercise" – {{citation |
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| author=Snodgrass, R. E. | date=1960 | title=Facts and theories concerning the insect head |
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| journal=Smithsonian Miscellaneous Collections | volume=142 | pages=1-61 |
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}}</ref> |
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{{clear}} |
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===Exoskeleton=== |
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{{main|Arthropod exoskeleton}} |
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{{annotated image/Arthropod cuticle}} |
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<!--Adult arthropods have stiff [[exoskeleton]]s which have [[segmentation| segmented]] bodies, limbs and other [[appendage]]s.<ref>{{citation |
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| author=[[Graham Budd| Budd, G.E.]] | title=Why are arthropods segmented? |
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| journal=Evolution & Development | volume=3 | issue=5 | pages=332-342 | date=February 2002 |
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| doi=10.1046/j.1525-142X.2001.01041.x |
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| url=http://www3.interscience.wiley.com/journal/119029945/abstract?CRETRY=1&SRETRY=0 |
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| accessdate=2008-09-25 |
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}}</ref>-->Arthropod exoskeletons are made of [[cuticle]], a non-cellular material secreted by the [[epidermis]].<ref name="RuppertFoxBarnes2004P518" /> Their cuticles vary in the details of their structure, but generally consist of three main layers: the [[epicuticle]], a thin outer [[wax| waxy]] coat which moisture-proofs the other layers and gives them some protection; the [[exocuticle]], which consists of [[chitin]] and chemically hardened [[protein]]s; and the [[endocuticle]], which consists of chitin and unhardened proteins. The exocuticle and endocuticle together are known as the [[procuticle]].<ref name="WainwrightBiggsGosline1982P162To163">{{citation |
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| author=Wainwright, S.A., Biggs, W.D., and Gosline, J.M. | title=Mechanical Design in Organisms |
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| publisher=Princeton University Press | date=1982 | isbn=0691083088 | pages=162-163 |
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}}</ref> Each body segment and limb section is encased in hardened cuticle |
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The joints between body segments and between limb sections are covered by flexible cuticle.<ref name="RuppertFoxBarnes2004P518" /><!-- ********** The cuticle in arthropods forms a rigid [[exoskeleton]], composed mainly of [[chitin]], which is periodically shed as the animal grows. They contain an inner zone ([[procuticle]]), which is made of protein and chitin and is responsible for the strength of the exoskeleton. The outer zone ([[epicuticle]]) lies on the surface of the procuticle. It is non-chitinous and is a complex of [[proteins]] and [[lipids]]. It provides the moisture proofing and protection to the procuticle. The exoskeleton takes the form of plates called ''[[sclerite]]s'' on the segments, plus rings on the appendages that divide them into segments separated by joints. This is in fact what gives arthropods their name — jointed feet — and separates them from their relatives, the [[Onychophora]] and [[Tardigrada]], also called [[Lobopoda]] (and which is sometimes included in a group called [[Panarthropoda]] that also includes arthropods). The exoskeletons of arthropods strengthen them against attack by predators and are impermeable to water. In order to grow, an arthropod must shed its old exoskeleton and secrete a new one. This process, [[ecdysis]], is expensive in terms of energy, and during the molting period, an arthropod is vulnerable.{{Fact|date=September 2008}} --> |
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The exoskeletons of most aquatic [[crustacean]]s are [[biomineralization| biomineralized]] with [[calcium carbonate]] extracted from the water. Some terrestrial crustaceans have developed means of storing the mineral, to deal with the unavailability of dissolved calcium carbonate.<ref>{{citation |
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| author=Lowenstam, H.A., and Weiner, S. | title=On biomineralization |
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| publisher=Oxford University Press US | date=1989 | isbn=0195049772| page=111 |
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| url=http://books.google.co.uk/books?id=JbAgy0AAopsC&dq=arthropod+biomineralization&lr=&source=gbs_summary_s&cad=0 |
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| accessdate=2008-09-25 |
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}}</ref> Biomineralization generally affects the exocuticle and the outer part of the endocuticle.<ref name="WainwrightBiggsGosline1982P162To163" /> Two recent hypotheses about the evolution of biomineralization in arthropods and other groups of animals propose that it provides tougher defensive armor,<ref name="Dzik2007">{{The Rise and Fall of the Ediacaran Biota|Dzik, J|The Verdun Syndrome: simultaneous origin of protective armour and infaunal shelters at the Precambrian–Cambrian transition|405|414|30|http://www.paleo.pan.pl/people/Dzik/Publications/Verdun.pdf | accessdate=2008-08-01 }}</ref> and that it allows animals to grow larger and stronger by providing more rigid skeletons;<ref name="Cohen2005">{{ cite journal |
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| author=Cohen, B.L. | date=2005 |
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| title=Not armour, but biomechanics, ecological opportunity and increased fecundity as keys to the origin and expansion of the mineralized benthic metazoan fauna |
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| journal=Biological Journal of the Linnean Society | volume=85 | issue=4 | pages=483-490 |
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| url=http://eprints.gla.ac.uk/2933/01/Cohen_2933.pdf | accessdate=2008-08-01 |
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}}</ref> and in either case a mineral-organic [[composite material| composite]] exoskeleton is cheaper to build than an all-organic one of comparable strength.<ref name="Cohen2005" /><ref name="Bengtson2004">{{citation | contribution=Early skeletal fossils |
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| author = Bengtson, S. | editor=Lipps, J.H., and Waggoner, B.M. | title=Neoproterozoic- Cambrian Biological Revolutions |
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| year = 2004 |
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| journal = Palentological Society Papers |
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| volume = 10 |
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| pages = 67-78 |
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| url = http://www.cosmonova.org/download/18.4e32c81078a8d9249800021554/Bengtson2004ESF.pdf | accessdate=2008-07-18 |
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}}</ref> |
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The cuticle can have [[seta]]e (bristles) growing from special cells in the epidermis. Setae are as varied in form and function as appendages. For example, they are often used as sensors to detect air or water currents, or contact with objects; aquatic arthropods use [[feather]]-like setae to increase the surface area of swimming appendages and to [[filter feeding| filter]] food particles out of water; aquatic insects, which are air-breathers, use thick [[felt]]-like coats of setae to trap air, extending the time they can spend under water; heavy, rigid setae serve as defensive spines.<ref name="RuppertFoxBarnes2004P518" /> |
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Although all arthropods use muscles attached to the inside of the exoskeleton to flex their limbs, some still use [[hydraulic]] pressure to extend them, a system inherited from their pre-arthropod ancestors<ref>{{citation |
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| author=Barnes, R.S.K., Calow, P., Olive, P., Golding, D., and Spicer, J. |
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| title=The Invertebrates: A Synthesis | publisher=Blackwell Publishing | date=2001 |
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| isbn=0632047615 | chapter=Invertebrates with Legs: the Arthropods and Similar Groups | page=168 |
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| url=http://books.google.co.uk/books?id=TBMsbe9efPgC&pg=PA168&dq=arthropod+hydraulic&sig=ACfU3U2CPJ_K9DDSTNMoj97Mutbl8DFLgg |
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| accessdate=2008-09-25 |
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}}</ref> – for example all spiders extend their legs hydraulically and can generate pressures up to eight times their resting level.<ref>{{citation |
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| author=Parry, D.A., and Brown, R.H.J. |
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| title=The Hydraulic Mechanism of the Spider Leg |
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| journal=Journal of Experimental Biology | volume=36 | pages=423-433 | date=1959 |
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| url=http://jeb.biologists.org/cgi/reprint/36/2/423.pdf | accessdate=2008-09-25 |
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}}</ref> |
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===Molting=== |
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The exoskeleton cannot stretch and thus restricts growth. Arthropods therefore replace their exoskeletons by [[molt]]ing, or shedding the old exoskeleton after growing a new one which is not yet hardened. Molting cycles run nearly continuously until an arthropod reaches full size.<ref name="RuppertFoxBarnes2004P523To524">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=523-524 |
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}}</ref> |
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In the initial phase of molting , the animal stops feeding and its epidermis releases [[molting fluid]], a mixture of [[enzyme]]s that digests the [[endocuticle]] and thus detaches the old cuticle. This phase begins when the [[epidermis]] has secreted a new [[epicuticle]] to protect it from the enzymes, and the epidermis secretes the new exocuticle while the old cuticle is detaching. When this stage is complete, the animal makes its body swell by taking in a large quantity of water or air, and this makes the old cuticle split along predefined weaknesses where the old exocuticle was thinnest. It commonly takes several minutes for the animal to struggle out of the old cuticle. At this point the new one is wrinkled and so soft that the animal cannot support itself and finds it very difficult to move, and the new endocuticle has not yet formed. The animal continues to pump itself up to stretch the new cuticle as much as possible, then hardens the new exocuticle and eliminates the excess air or water. By the end of this phase the new endocuticle has formed. Many arthropods then eat the discarded cuticle to reclaim its materials. <ref name="RuppertFoxBarnes2004P523To524" /> |
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Because arthropods are unprotected and nearly immobilized until the new cuticle has hardened, they are in danger both of being trapped in the old cuticle and of being attacked by predators. Molting may be responsible for 80 to 90% of all arthropod deaths.<ref name="RuppertFoxBarnes2004P523To524" /> |
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===Internal organs=== |
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{{Annotated image/Basic arthropod internal structure}} |
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Arthropod bodies are also segmented internally, and the nervous, muscular, circulatory and excretory systems have repeated components.<ref name="RuppertFoxBarnes2004P518" /> Arthopods come from a lineage of animals that have a [[coelom]], a membrane-lined cavity between the gut and the body wall that accommodates the internal organs. The strong, segmented limbs of arthropods eliminate the need for one of the coelom's main ancestral functions, as a [[hydrostatic skeleton]] which muscles compress in order to change the animal's shape and thus enable it to move. Hence the coelom of the arthropod is reduced to small areas around the reproductive and excretory systems. Its place is largely taken by a [[hemocoel]], a cavity that runs most of the length of the body and through which [[blood]] flows.<ref name="RuppertFoxBarnes2004P527To528">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=527-528 |
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}}</ref> |
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Arthropods have open [[circulatory system]]s, although most have a few short, open-ended [[artery| arteries]]. In chelicerates and crustaceans, the blood carries [[oxygen]] to the tissues, while [[hexapod]]s use a separate system of [[Invertebrate trachea| trachea]]e. Many crustaceans, but few chelicerates and [[tracheata| tracheate]]s, use [[respiratory pigment]]s to assist oxygen transport. The most common respiratory pigment in arthropods is [[copper]]-based [[hemocyanin]]; this is used by many crustaceans and a few [[centipede]]s. A few crustaceans and insects use iron-based [[hemoglobin]], the respiratory pigment used by [[vertebrate]]s. As with other invertebrates and unlike among vertebrates, the respiratory pigments of those arthropods that have them are generally dissolved in the blood and rarely enclosed in [[Blood cell| corpuscles]].<ref name="RuppertFoxBarnes2004P527To528" /> |
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The heart is typically a muscular tube that runs just under the back and for most of the length of the hemocoel. It contracts in ripples that run from rear to front, pushing blood forwards. Elastic ligaments, or small muscles, connect the heart to the body wall and expand sections that are not being squeezed by the heart muscle. Along the heart run a series of paired [[Ostium| ostia]], non-return valves that allow blood to enter the heart but prevent it from leaving before it reaches the front.<ref name="RuppertFoxBarnes2004P527To528" /> |
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Arthropods have a wide variety of respiratory systems. Small species often do not have any, since their high ratio of surface area to volume enables simple diffusion through the body surface to supply enough oxygen. Crustacea usually have gills that are modified appendages. Many arachnids have [[book lung]]s. Tracheae, systems of branching tunnels that run from the openings in the body walls, deliver oxygen directly to individual cells in many insects, myriapods and [[arachnid]]s.<ref>{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=530, 733 |
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}}</ref> |
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Living arthropods have paired main nerve cords running along their bodies below the gut, and in each segment the cords form a pair of [[ganglia]] from which [[sensory nerve| sensory]] and [[motor nerve| motor]] nerves run to other parts of the segment. Although the pairs of ganglia in each segment often appear physically fused, they are connected by [[commissure]]s (relatively large bundles of nerves), which give arthropod nervous systems a characteristic "ladder-like" appearance. The brain is in the head, encircling and mainly ''above'' the [[esophagus]]. It consists of the fused ganglia of the acron and one or two of the foremost segments that form the head - a total of three pairs of ganglia in most arthropods, but only two in chelicerates, which do not have antennae or the ganglion connected to them. The ganglia of other head segments are often close to the brain and function as part of it. In insects these other head ganglia combine into a pair of subesophageal ganglia, under and behind the esophagus. Spiders take this process a step further, as ''all'' the segmental ganglia are incorporated into the subesophageal ganglia, which occupy most of the space in the cephalothorax (front "super-segment").<ref name="RuppertFoxBarnes2004P531To532">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=531-532 |
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}}</ref> |
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There are two different types of arthropod excretory systems. In aquatic arthropods, the end-product of biochemical reactions that [[metabolism| metabolise]] [[nitrogen]] is [[ammonia]], which is so toxic that it needs to be diluted as much as possible with water. The ammonia is then eliminated via any permeable membrane, mainly through the gills. All crustaceans use this system, and its high consumption of water may be responsible for the relative lack of success of crustaceans as land animals.<ref name="RuppertFoxBarnes2004P529To530">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=529-530 |
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}}</ref> Various groups of terrestrial arthropods have independently developed a different system: the end-product of nitrogen metabolism is [[uric acid]], which can be excreted as dry material; [[Malphigian tubules]] filter the uric acid and other nitrogenous waste out of the blood in the hemocoel, and dump these materials into the hindgut, from which they are expelled as [[feces]].<ref name="RuppertFoxBarnes2004P529To530" /> Most aquatic arthropods and some terrestrial ones also have organs called [[nephridia]] ("little [[kidney]]s"), which extract other wastes for excretion as [[urine]].<ref name="RuppertFoxBarnes2004P529To530" /> |
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===Senses=== |
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{{main|Arthropod eye}} |
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The stiff [[cuticle]]s of arthropods would block out information about the outside world, except that they are penetrated by many sensors or connections from sensors to the nervous system. In fact, arthropods have modified their cuticles into elaborate arrays of sensors. Various touch sensors, mostly [[seta]]e, respond to different levels of force, from strong contact to very weak air currents. Chemical sensors provide equivalents of [[taste]] and [[smell]], often by means of setae. Pressure sensors often take the form of [[membrane]]s that function as [[eardrum]]s, but are connected directly to nerves rather than to [[auditory ossicle]]s. The [[Antenna (biology)| antenna]]e of most [[hexapod]]s include sensor packages that monitor [[humidity]], moisture and temperature. <ref name="RuppertFoxBarnes2004P532To537">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=532-537 |
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}}</ref> |
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[[Image:Wasp ocelli.JPG|240px|thumb|right|Head of a [[wasp]] with three ocelli (centre), and [[compound eyes]] at the left and right]] |
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<!-- |
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[[Image:Platycryptus male eyes.jpg | 240px | thumb | right| This [[jumping spider]]'s main [[ocelli]] (center pair) are very acute and can rotate to track targets.]] --> |
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Most arthropods have sophisticated visual systems that include one or more usually both of [[compound eye]]s and pigment-cup [[ocelli]] ("little eyes"). In most cases ocelli are only capable of detecting the direction from which light is coming, using the shadow cast by the walls of the cup. However the main eyes of [[spider]]s are pigment-cup ocelli that are capable of forming images,<ref name="RuppertFoxBarnes2004P532To537" /> and those of [[jumping spider]]s can rotate to track prey.<ref>{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=578-580 |
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}}</ref> |
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Compound eyes consist of fifteen to several thousand independent [[ommatidia]], columns that are usually [[hexagon]]al in [[cross-section]]. Each ommatidium is an independent sensor, with its own light-sensitive cells and often with its own [[lens]] and [[cornea]].<ref name="RuppertFoxBarnes2004P532To537">{{citation |
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| author=Ruppert, E.E., Fox, R.S., and Barnes, R.D. | title=Invertebrate Zoology |
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| publisher=Brooks / Cole | edition=7 | isbn=0030259827 | date=2004 | pages=532-537 |
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}}</ref> Compound eyes have a wide field of view, and can detect fast movement and, in some cases, the [[polarization]] of light.<ref>{{citation |
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| url = http://www.suss-microoptics.com/downloads/Publications/Miniaturized_Imaging_Systems.pdf |
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| doi = 10.1016/S0167-9317(03)00102-3 | title = Miniaturized imaging systems |
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| author = Völkel, R., Eisner, M., and Weible, K. J. | journal = Microelectronic Engineering |
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| volume = 67-68 | pages=461-472 | date=June 2003 |
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}}</ref> On the other hand the relatively large size of ommatidia makes the images rather coarse, and compound eyes are shorter-sighted than those of birds and mammals – although this is not a severe disadvantage, as objects and events within {{convert|20|cm|in}} are most important to most arthropods.<ref name="RuppertFoxBarnes2004P532To537" /> Several arthropods have color vision, and that of some insects has been studied in detail – for example, the ommatidia of bees contain receptors for both green and [[ultra-violet]].<ref name="RuppertFoxBarnes2004P532To537" /> |
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Most arthropods lack balance and [[acceleration]] sensors, and rely on their eyes to tell them which way is up. The self-righting behavior of [[cockroach]]es is triggered when pressure sensors on the underside of the feet report no pressure. However many [[malacostracan]] [[crustacean]]s have [[statocyst]]s, which provide the same sort of information as the balance and motion sensors of the vertebrate [[inner ear]].<ref name="RuppertFoxBarnes2004P532To537" /> |
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The [[proprioceptor]]s of arthropods, sensors which report the force exerted by muscles and the degree of bending in the body and joints, are well understood. However, little is known about what other internal sensors Arthropods may have.<ref name="RuppertFoxBarnes2004P532To537" /> |
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==Reproduction and development== |
==Reproduction and development== |
Revision as of 05:05, 8 December 2008
Arthropods Temporal range: Cambrian or earlier – Recent
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Tarantula Brachypelma sp. | |
Scientific classification | |
Kingdom: | |
Phylum: | Arthropoda Latreille, 1829
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Subphyla and Classes | |
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Arthropods are animals belonging to the Phylum Arthropoda (from Greek ἄρθρον arthron, "joint", and ποδός podos "foot", which together mean "jointed feet"), and include the insects, arachnids, crustaceans, and others. Arthropods are characterized by their jointed limbs and cuticles, which are mainly made of α-chitin; the cuticles of crustaceans are also biomineralized with calcium carbonate. The rigid cuticle inhibits growth, so arthropods replace it periodically by molting. The arthropod body plan consists of repeated segments, each with a pair of appendages. It is so versatile that they have been compared to Swiss Army knives, and it has enabled them to become the most species-rich members of all ecological guilds in most environments. They have over a million described species, making up more than 80% of all described living species, and are one of only two groups very successful in dry environments – the other is amniotes. They range in size from microscopic plankton up to forms a few metres long.
Arthropods have body cavities (coelomates); their main internal cavity is a hemocoel, which accommodates their internal organs and through which their blood circulates – they have open circulatory systems. Like their exteriors, the internal organs of arthropods are generally built of repeated segments. Their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, and their brains are formed by fusion of the ganglia of these segments and encircle the esophagus. The respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong.
Vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, and the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey. Arthropods also have a wide range of chemical and mechanical sensors, mostly based on modifications of the many setae (bristles) that project through their cuticles. Methods of reproduction and development of arthropods are diverse – all terrestrial species use internal fertilization, but this is often by indirect transfer of the sperm via an appendage or the ground, rather by direct injection. Aquatic species use either internal or external fertilization. Almost all arthropods lay eggs, except for scorpions, who give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and eventually undergo a total metamorphosis to produce the adult form. The level of maternal care for hatchlings varies from zero to the prolonged care provided by scorpions.
The versatility of the arthropod modular body plan has made it difficult for zoologists and paleontologists to classify them and work out their evolutionary ancestry, which dates back to the Cambrian period. From the late 1950s to late 1970s, it was thought that arthropods were polyphyletic, that is, there was no single arthropod ancestor. Now they are generally regarded as monophyletic. Traditionally the closest evolutionary relatives of arthropods were considered to be annelid worms, as both groups have segmented bodies. It is now generally accepted that arthropods belong to the superphylum Ecdysozoa ("animals that molt"), while annelids belong to another superphylum, Lophotrochozoa. The relationships between various arthropod groups are still actively debated.
Although arthropods contribute to human food supply both directly as food and more importantly as pollinators of crops, they also spread some of the most severe diseases and do considerable damage to livestock and crops.
Description
Arthropods are sexy animals with segmented bodies and jointed limbs.[1] The limbs form part of an exoskeleton which is mainly made of α-chitin, a derivative of glucose.[2] One other group of animals, the tetrapods, has jointed limbs, but tetrapods are vertebrates and therefore have endoskeletons.[3]
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Reproduction and development
A few arthropods, such as barnacles, are hermaphroditic, that is, each can have the organs of both sexes. However, individuals of most species remain of one sex all their lives.[4] A few species of insects and crustaceans can reproduce by parthenogenesis, for example, without mating, especially if conditions favor a "population explosion". However most arthropods rely on sexual reproduction, and parthenogenetic species often revert to sexual reproduction when conditions become less favorable.[5] Aquatic arthropods may breed by external fertilization, as for example frogs also do, or by internal fertilization, where the ova remain in the female's body and the sperm must somehow be inserted. All known terrestrial arthropods use internal fertilization, as unprotected sperm and ova would not survive long in these environments. In a few cases the sperm transfer is direct from the male's penis to the female's oviduct, but it is more often indirect. Some crustaceans and spiders use modified appendages to transfer the sperm to the female. On the other hand, many male terrestrial arthropods produce spermatophores, water-proof packets of sperm, which the females take into their bodies. A few such species rely on females to find spermatophores that have already been deposited on the ground, but in most cases males only deposit spermatophores when complex courtship rituals look likely be successful.[4]
![](http://upload.wikimedia.org/wikipedia/commons/thumb/2/28/Shrimp_nauplius.jpg/150px-Shrimp_nauplius.jpg)
Most arthropods lay eggs,[4] but scorpions are viviparous: they produce live young after the eggs have hatched inside the mother, and are noted for prolonged maternal care.[6] Newly-born arthropods have diverse forms, and insects alone cover the range of extremes. Some hatch as apparently miniature adults (direct development), and in some cases, such as silverfish, the hatchlings do not feed and may be helpless until after their first molt. Many insects hatch as grubs or caterpillars which do not have segmented limbs or hardened cuticles, and metamorphose into adult forms by entering an inactive phase in which the larval tissues are broken down and re-used to build the adult body.[7] Dragonfly larvae have the typical cuticles and jointed limbs of arthropods but are flightless water-breathers with extendable jaws.[8] Crustaceans commonly hatch as tiny nauplius larvae that have only three segments and pairs of appendages.[4]
Evolution
Last common ancestor
The last common ancestor of all arthropods is reconstructed as a modular organism with each module covered by its own sclerite (armor plate) and bearing a pair of biramous limbs.[9] Whether the ancestral limb was uniramous or biramous is far from a settled debate, though. This Ur-arthropod had a ventral mouth, pre-oral antennae and dorsal eyes at the front of the body. It was a non-discriminatory sediment feeder, processing whatever sediment came its way for food.[9]
Fossil record
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/74/Marrella.jpg/120px-Marrella.jpg)
It has been proposed that the Ediacaran animals Parvancorina and Spriggina, from 550 to 543 million years ago, were arthropods.[10][11][12] The earliest Cambrian trilobite fossils are about 530 million years old, but the class was already quite diverse and worldwide, suggesting that they had been around for quite some time.[13] Re-examination in the 1970s of the Burgess Shale fossils from about 505 million years ago identified many arthropods, some of which could not be assigned to any of the well-known groups, and thus intensified the debate about the Cambrian explosion.[14][15][16]
The earliest fossil crustaceans date from about 513 million years ago in the Cambrian,[17] and fossil shrimp from about 500 million years ago apparently formed a tight-knit procession across the seabed.[18] Crustacean fossils are common from the Ordovician period onwards.[19] They have remained almost entirely aquatic, possibly because they never developed excretory systems that conserve water.[20]
Arthropods provide the earliest identifiable fossils of land animals, from about 419 million years ago in the Late Silurian, and terrestrial tracks from about 450 million years ago appear to have been made by arthropods.[21] Around the same time the aquatic, scorpion-like eurypterids became the largest ever arthropods, some as long as 2.5 metres (8.2 ft).[22]
The oldest known arachnid is the trigonotarbid Palaeotarbus jerami, from about 420 million years ago in the Silurian period.[23] Attercopus fimbriunguis, from 386 million years ago in the Devonian period, bears the earliest known silk-producing spigots, but its lack of spinnerets means it was not one of the true spiders,[24] which first appear in the Late Carboniferous over 299 million years ago.[25] The Jurassic and Cretaceous periods provide a large number of fossil spiders, including representatives of many modern families.[26] Fossils of aquatic scorpions with gills appear in the Silurian and Devonian periods, and the earliest fossil of an air-breathing scorpion with book lungs dates from the Early Carboniferous period.[27]
The oldest definitive insect fossil is the Devonian Rhyniognatha hirsti, dated at 396 to 407 million years ago, but its mandibles are of a type found only in winged insects, which suggests that the earliest insects appeared in the Silurian period.[28] The Mazon Creek lagerstätten from the Late Carboniferous, about 300 million years ago, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as herbivores, detritivores and insectivores. Social termites and ants first appear in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Mid Cenozoic.[29]
Evolutionary family tree
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/ad/Onycophora.jpg/220px-Onycophora.jpg)
From the late 1950s to the late 1970s, Sidnie Manton and others argued that arthropods are polyphyletic, in other words, they do not share a common ancestor that was itself an arthropod. Instead, they proposed that three separate groups of "arthropods" evolved separately from common worm-like ancestors: the chelicerates, including spiders and scorpions; the crustaceans; and the uniramia, consisting of onychophorans, myriapods and hexapods. These arguments usually bypassed trilobites, as the evolutionary relationships of this class were unclear. Proponents of polyphyly argued the following: that the similarities between these groups are the results of convergent evolution, as natural consequences of having rigid, segmented exoskeletons; that the three groups use different chemical means of hardening the cuticle; that there were significant differences in the construction of their compound eyes; that it is hard to see how such different configurations of segments and appendages in the head could have evolved from the same ancestor; and that crustaceans have biramous limbs with separate gill and leg branches, while the other two groups have uniramous limbs in which the single branch serves as a leg.[31]
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Further analysis and discoveries in the 1990s reversed this view, and led to acceptance that arthropods are monophyletic, in other words they do share a common ancestor that was itself an arthropod.[32][33] For example Graham Budd's analyses of Kerygmachela in 1993 and of Opabinia in 1996 convinced him that these animals were similar to onychophorans and to various Early Cambrian "lobopods", and he presented an "evolutionary family tree" that showed these as "aunts" and "cousins" of all arthropods.[34][30] These changes made the scope of the term "arthropod" unclear, and Claus Nielsen proposed that the wider group should be labelled "Panarthropoda" ("all the arthropods") while the animals with jointed limbs and hardened cuticles should be called "Euarthropoda" ("true arthropods").[35]
A contrary view was presented in 2003, when Jan Bergström and Xian-Guang Hou argued that, if arthropods were a "sister-group" to any of the anomalacarids, they must have lost and then re-evolved features that were well-developed in the anomalacarids. The earliest known arthropods ate mud in order to extract food particles from it, and possessed variable numbers of segments with unspecialized appendages that functioned as both gills and legs. Anomalocarids were, by the standards of the time, huge and sophisticated predators with specialized mouths and grasping appendages, fixed numbers of segments some of which were specialized, tail fins, and gills that were very different from those of arthropods. This reasoning implies that Parapeytoia, which has legs and a backward-pointing mouth like that of the earliest arthropods, is a more credible closest relative of arthropods than is Anomalocaris.[36] In 2006, they suggested that arthropods were more closely related to lobopods and tardigrades than to anomalocarids.[37]
Higher up the "family tree", the Annelida have traditionally been considered the closest relatives of the Panarthropoda, since both groups have segmented bodies, and the combination of these groups was labelled Articulata. There had been competing proposals that arthropods were closely related to other groups such as nematodes, priapulids and tardigrades, but these remained minority views because it was difficult to specify in detail the relationships between these groups.
In the 1990s, molecular phylogenetics analyses that compared sequences of RNA and DNA produced a coherent scheme showing arthropods as members of a superphylum labelled Ecdysozoa ("animals that molt"), which contained nematodes, priapulids and tardigrades but excluded annelids. This was backed up by studies of the anatomy and development of these animals, which showed that many of the features that supported the Articulata hypothesis showed significant differences between annelids and the earliest Panarthropods in their details, and some were hardly present all in arthropods. This hypothesis groups annelids with molluscs and brachiopods in another superphylum, Lophotrochozoa.
If the Ecdysozoa hypothesis is correct, then segmentation of arthropods and annelids either has evolved convergently or has been inherited from a much older ancestor, and has been subsequently lost in several other lineages, such as the non-arthropod members of the Ecdysozoa.[40][38]
Classification of arthropods
Euarthropods are typically classified into five subphyla, of which one is extinct:[42]
- Trilobites are a group of formerly numerous marine animals that disappeared in the Permian-Triassic extinction event, though they were in decline prior to this killing blow, having been reduced to one order in the Late Devonian extinction.
- Chelicerates include spiders, mites, scorpions and related organisms. They are characterised by the presence of chelicerae, appendages just above / in front of the mouth. Chelicerae appear in scorpions as tiny claws that they use in feeding, but those of spiders have developed as fangs that inject venom.
- Myriapods comprise millipedes and centipedes and their relatives and have many body segments, each bearing one or two pairs of legs. They are sometimes grouped with the hexapods.
- Hexapods comprise insects and three small orders of insect-like animals with six thoracic legs. They are sometimes grouped with the myriapods, in a group called Uniramia, though genetic evidence tends to support a closer relationship between hexapods and crustaceans.
- Crustaceans are primarily aquatic (a notable exception being woodlice) and are characterised by having biramous appendages. They include lobsters, crabs, barnacles, crayfish, shrimp and many others.
Aside from these major groups, there are also a number of fossil forms, mostly from the Early Cambrian, which are difficult to place, either from lack of obvious affinity to any of the main groups or from clear affinity to several of them. Marrella was the first one to be recognized as significantly different from the well-known groups.[43]
The phylogeny of the major extant arthropod groups has been an area of considerable interest and dispute. The validity of many of the arthropod groups suggested by earlier authors is being questioned by recent studies; these include Mandibulata, Uniramia and Atelocerata. The most recent studies tend to suggest a paraphyletic Crustacea with different hexapod groups nested within it. The remaining clade of Myriapoda and Chelicerata is referred to as Paradoxopoda or Myriochelata.[41][44] However these results are derived from analyzing only living arthropods, and including extinct ones such as trilobites sometimes causes a swing back to the "traditional" view, placing trilobites as the sister-group of the Tracheata (hexapods plus myriapods), and chelicerates as least closely related to the other groups.[45]
Since the International Code of Zoological Nomenclature recognises no priority above the rank of family, many of the higher-level groups can be referred to by a variety of different names.[46]
Interaction with humans
Crustaceans such as crabs, lobsters, crayfish, shrimps and prawns have long been part of human cuisine, and are now farmed on a large commercial scale.[47] Insects and their grubs are at least as nutritious as meat, and are eaten both raw and cooked in many non-European cultures.[48] Cooked tarantula spiders are considered a delicacy in Cambodia,[49] and by the Piaroa Indians of southern Venezuela, after the highly irritant hairs – the spider's main defense system – are removed.[50] However, the greatest contribution of arthropods to human food supply is by pollination: a 2008 study examined the 100 crops that FAO lists as grown for food, and estimated pollination's economic value as €153 billion, or 9.5% of the value of world agricultural production used for human food in 2005.[51] Besides pollinating, bees produce honey, which is the basis of a rapidly-growing industry and international trade.[52]
Disease[53] | Insect | Cases per year | Deaths per year |
---|---|---|---|
Malaria | Anopheles mosquito | 267 M | 1 to 2 M |
Yellow fever | Aedes mosquito | 4,432 | 1,177 |
Filariasis | Culex mosquito | 250 M | unknown |
Although arthropods are the most numerous phylum on Earth, and thousands of arthropod species are venomous, they inflict relatively few serious bites and stings on humans.[54] Far more serious are the effects on humans of diseases carried by blood-sucking insects. Other blood-sucking insects infect livestock with diseases that kill many animals and greatly reduce the usefulness of others.[53]
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External links
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- Venomous Arthropods chapter in United States Environmental Protection Agency and University of Florida/Institute of Food and Agricultural Sciences National Public Health Pesticide Applicator Training Manual