Arthropods

Figure 8‑28. "Arthropoda collage. From left to right and from top to bottom: Kolihapeltis, Stylonurus, scorpion, crab, centipede, butterfly. Credit: Kolihapeltis 01 Pengo.jpg: Peter Halasz Stylonurus BW.jpg: Nobu Tamura SCORPIO MAURUS PALMATUS.jpg: Guy Haimovitch Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg: Wpopp Female centipede with eggs.jpg: Marshal Hedin John Kratz - Swallowtail (by-sa).jpg: John Kratz derivative work: Xvazquez, Amada44 - Kolihapeltis 01 Pengo.jpg Stylonurus BW.jpg SCORPIO MAURUS PALMATUS.jpg Blue crab on market in Piraeus - Callinectes sapidus Rathbun 20020819-317.jpg Female centipede with eggs.jpg John Kratz - Swallowtail (by-sa).jpg” Wikipedia. Used here per CC BY-SA 3.0.

The arthopod phylum has the largest number of species and the largest population of animals of all the animal phyla (Figure 8‑28). Some of the major groups of arthropods are crustaceans, spiders, and insects.

There are several reasons for arthropod success in the sea, land, and air. One of the most important is their jointed appendages. They are extremely mobile and can move quickly away from threats or toward food. They have highly developed sensory and reproduction systems. Their circulatory systems are incomplete (no blood vessels); however, they are small, so they do not need to circulate nutrients across large distances. Their exoskeletons provide three advantages: sites for muscle attachment, prevent water loss to the environment, and provide protection. The disadvantages are that the exoskeleton is heavy and that that they must shed the exoskeleton in order to grow. Because they have heavy exoskeletons and incomplete circulatory systems, they will never become extremely large on land, despite what you might have seen in horror movies. The ability to fly is an enormous advantage for many arthropods. This was particularly true prior to the evolution of birds in the early Cretaceous (150 Ma). Insects ruled the skies until that time. Pterosaurs were irrelevant to insects since they were so large.

The most numerous, diverse (17,000 species), and widely distributed invertebrate in the early Phanerozoic oceans was the trilobite, which was a crustacean, an arthropod subphylum. Because different species of trilobites evolved in different periods, they are indicators for biostratigraphy, the science of dating geologic formations based on their fossils. For example, Kolihapeltis (upper left, Figure 8-28) was a trilobite that lived in the early Devonian period; thus, geologists know that if a formation has this trilobite, then it is a Devonian formation.

Figure 8‑29. View to the south from Mt. Burgess. 1911. A Geologists’s Paradise by Charles Walcott. National Geographic Magazine. Vol 22:6. p. 512

Charles Walcott was a geologist, paleontologist, and government administrator of the early 20^th century. He discovered (or possibly rediscovered or discovered fully) the Burgess Shale (505 Ma) in 1909 on the north side of Mt. Burgess in the Canadian Rockies. Walcott took a picture of this rugged region for National Geographic Magazine in 1911 (Figure 8‑29). Even a century ago, he described how geologic pressure from the west lifted the underwater strata 16,000 ft above sea level. Subsequently, this high region was worn down by erosion and formed the Canadian Rockies. The millions of fossils in the Burgess Shale revealed the amazing diversity of the Cambrian ocean ecosystem and the early dominance of the arthropods (Figure 8‑30), particularly the trilobites. The Burgess Shale may have been the greatest discovery of fossils in history. Walcott also discovered Cambrian trilobites in China. Prior to his discoveries, Cambrian fossils had been discovered here and there, but people did not realize the amazing diversity of animal life in the Cambrian.

Figure 8‑30. Trilobites and other middle Cambrian crustaceans. Smithsonian Miscellaneous Collections. Cambrian Geology and Paleontology, Vol II. By Charles Walcott. 1914 . p. 208.

The millions of fossils in the Burgess Shale revealed the amazing diversity of the Cambrian ocean ecosystem and the early dominance of the arthropods (Figure 8‑29), particularly the trilobites. The Burgess Shale may have been the greatest discovery of fossils in history. Walcott also discovered Cambrian trilobites in China. Prior to his discoveries, Cambrian fossils had been discovered here and there, but people did not realize the amazing diversity of animal life in the Cambrian.

Figure 8‑31. Modern arthropod lobster. Credit: Bart Braun. Public domain.

The arthropods are divided into the Chelicerata and Mandibulata. The Chelicerata have jaws that work from the side while the Mandibulata have mouth parts that work from the top and bottom. The Mandibulata includes crustaceans, insects, centipedes, and millipedes. Early trilobites and other crustaceans that dominated the Cambrian were Mandibulata. The Crustacea have highly developed sensory systems. They have a brain and nerve cord running down the ventral (front side). Crustaceans are scavengers and their mouth is retracted and under their body. Cambrian arthropod crustaceans such as trilobites have gone extinct, but other arthropod crustaceans such as crabs and lobsters replaced them in the sea (Figure 8‑31). Crustaceans, as fast-moving animals have highly developed nervous systems. Another descendent of the Cambrian arthropods, insects, became the most diverse and populous land animals on earth.

Figure 8‑32. Drosophila melanogaster. Credit Andre Karwath. Used here per CC BY-SA 2.5.

From a research perspective, one of the most important arthropods is the fly (Figure 8-32), Drosophila melanogaster. Researchers have studied the DNA in flies and other protostomes and found an amazing number of similarities with the vertebrates, as well as annelids and other protostomes. Arthropods have highly developed nervous systems, which makes sense, since many are able to fly. Two of the insects with the largest brains are bees and cockroaches.

Flies and other arthropods have rhabdomeric compound eyes, which are composed of 700 individual ommatidia, each of which has eight photoreceptor neurons, 2 pigment cells, and 4 cone cells. They fly eye is divided into seven rhabdomeres, two of which can see color. Each of the ommatidia creates an individual picture, and the fly brain puts together the 700 images. It is difficult to admit, but flies are amazing.

W. T. Calman (1909) was the first to propose that insects evolved from crustaceans (arthropods such as trilobites, etc.),[1] which has been supported by research in the last 100 years.[2] [3] Insects adapted to life on land in the early Devonian and are found in the Rhynie chert (400 Ma), which preserved fossils in salt. The Rhynie chert had hexapods, freshwater crustaceans, centipedes, mites, and arachnids (spiders). Most insects in the Rhynie Chert did not have wings, but one insect fossil in the Rhynie chert, Rhyniognatha hirsti, may have had wings.[4] Grimaldi and Engel concluded that if insects had wings 400 Ma (the earliest known insects), then they probably evolved from aquatic arthropods approximately 420 Ma.[5]

It is unknown whether the first insects were flyers, but the fossil record of insects prior to flight is almost nonexistent.[6] Many of the older insect fossils are of isolated wings, which were fully functional; thus, the earliest known winged insects were already competent flyers (not just gliders).[7] How did the wings evolve? This has been a debate that has raged for 200 years. For example, one study proposed that insect wings arose from gills due to common genes, [8] but other studies propose different origin locations. This debate is far from over.

After the Rhynie fossils, there are many fossils of winged insects in the fossil record of the Carboniferous period (360–290 Ma). The Protodonata were very large insects at the end of the Carboniferous and at the beginning of the Permian. One family of Protodonata had a wingspan of 75 cm. The superclass hexapoda (includes insecta, collembola, diplurans, and proturans) are now the most diverse and populous animals on land, with over 750,000 species. Within the hexapoda, insects have the most species: 75% of all animal species are insects.

Insects are the largest class in the animal kingdom (Figure 8‑33). There are several reasons that insects are so successful. Insects evolved flight and are highly motile. They have highly developed social systems, such as ants and bee hives. They have a high reproductive capacity. As fliers, they have highly developed nervous systems. The gas exchange system is a set of tracheal tubes connected to openings in the outside of the body. As with vertebrates and annelids, insects have a larval stage and undergo metamorphosis.

Figure 8‑33. Collage showing the diversity of insect species. Insect species ordered from top left to bottom right: Long dance fly (Empis livida) Long Nosed Weevil (Rhinotia hemistictus) Assassin bug in the family Reduviidae sub-family Harpactocorinae Mole Cricket (Gryllotalpa brachyptera) Emperor gum moth (Opodiphthera eucalypti) European Wasp (Vespula germanica). Credit: Bugboy52.40. Derived from images uploaded by Fir0002. Used here per CC BY-SA 3.0.

References

[1] Calman, W. T. 1909. Crustacea. In E. R. Lankaster (ed.), A Treatise on Zoology. Part VII, Appendiculata. Adam and Charles Black, London, pp. 1-332.

[2] Nardi, F., G. Spinsanti, J. Boore, A. Carapelli, R. Dallai, F. Frati. 2003. Gexapod Origins: Monophyletic or Paraphyletic? Science. 299(5614): 1887 - 1889

[3] Luan Y., J. Mallatt, R. Xie, Y. Yang and W. Yin. 2005. The Phylogenetic Positions of Three Basal-Hexapod Groups (Protura, Diplura, and Collembola) Based on Ribosomal RNA Gene Sequences. Molecular Biology and Evolution. 22(7):1579-1592.

[4] Engel M. & D. Grimaldi. 2004. New light shed on the oldest insect. Nature 6975: 627–630. doi:10.1038/nature02291

[5] Grimaldi, D. and M. Engel. 2005. Evolution of the Inects. Cambridge. pp. 735.

[6] Alexander, D. 2002. Nature’s flyers: birds, insects, and the biomechanics of flight.

[7] Alexander, Flyers.

[8] Averoff, M. and S. Cohen. 1997. Evolutionary origin of insect wings from ancestral gills. Nature. 385: 627-630.

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