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| Beginning in the Cambrian Period (and possibly
earlier), a large number of successful groups of arthropods evolved, including
trilobites and sea scorpions. Not
only were arthropods successful in marine environments, a number of later
groups adapted to life on land. Modern
terrestrial arthropods include millipedes, centipedes, spiders, mites, ticks,
and insects. The insects have obviously
been very successful; there are more species of insects on earth today than
all other species of animal combined.
Millipedes represent the first known land invertebrates from the Siluran and appeared alongside the early vascular plant Cooksonia (Park, 2007).
Millipedes are first known from the Siluran and represent the earliest-known air breathing animals. Arthropleura was a giant millipede which could reach more than 2 m in length. The oldest centipedes are known from the Siluran and Devonian (Shear, 2010).
| Millipedes possess feed on decomposing
vegetation and many coil their bodies to protect themselves from danger. They typically possess two legs per body segment
and may possess 100 to 200 legs in total. Possible millipede burrows are known from the
Ordovician and they may have been the first land animals. Some fossil millipedes
reached 10 cm (8 inches) and another extinct group (arthroplerodira) could
reach 1 meter in length.
Some myriapods reached lengths of more than 6 feet and represent the largest known land arthropods (Thomson, 1998).
Centipedes have up to a few dozen pairs of legs. Arthropod remains from the Siluran include centipedes and arachnids. Given that these animals are predators, the existence of terrestrial ecosystems probably dates at least to the Siluran (Jeram, 1990).
A diversity of fossil myriapods are pictured below.
Marine arachnids are ancient, known possibly from the Cambrian. Scorpions are known from the Siluran (the earliest scorpions seem to have retained their gills and may have been aquatic). The earliest arachnids are known from the Siluran. A number of orders are known only from the Paleozoic, becoming extinct in the Permian (Dunlop, 2010).
The Order Uraraneida consisted of spider-like animals which first appeared in the Devonian, predating the Order Araneae which includes most spiders. These arthropods possessed silk glands although they were not yet associated with spinnerets as in spiders (Dunlop, 2010).
The Order Araneae, which includes the vast majority of fossil and living spiders, is first known from the Carboniferous (Dunlop, 2010).
Homologies of the book lungs of arachnids suggest that adaptation to land occurred once in basal arachnids (as opposed to separate adaptations for spiders and scorpions) (Scholtz, 2006; Jeram, 1990).Spiders, mites, and ticks are all known from the Devonian. Some modern arachnid embryos still retain vestiges of antennae which no modern species possesses as an adult (Sharov, 172).
The earliest spiders still retained the primitive feature of a segmented abdomen.
Insects are the most diverse group of animals today with hundreds of thousands of modern genera. About 6,000 are known from fossil record representing more than 1,260 insect families. The types of insects alive in the Paleozoic were very different from those which exist today. Of the 11 insect orders known from the Carboniferous, only cockroaches have survived to modern times (although basal members of the orders containing mayflies, dragonflies, and the true bugs of Hemiptera may also have been present; Callahan, 1972). Of the 27 orders of insects known from the Paleozoic Era, 8 do not survive the end of the Permian and 3 more do not survive the end of the Triassic. Of those Paleozoic orders that survived in the Mesozoic Era, about half underwent significant changes. Up until the Jurassic, virtually all of the insect fossils belong to groups of insects which are extinct today. By the Cretaceous, about 16% of insect fossils belong to modern families. Three genera from the Mesozoic have survived to modern times (the crane fly, a leaf-mining moth, and the beetle Tetraphaleus). By the Tertiary Period in the Cenozoic Era, almost all insect fossils can be classified in modern families (Labandeira, 1993).
Many insect fossils were formed after the insects were trapped in tree sap and this tree sap was converted into amber. Most amber dates from the Tertiary.
|Other insect fossils formed through replacement.|
|Devonohexapodus bocksbergensis is an early Devonian arthropod which possesses some, but not all, of the characteristics of insects. It is classified as a hexapod (the group to which insects belong) because of its three thoracic segments, three pairs of long thoracic legs, dorsal antennae, and large eyes. Early insects decreased the number of abdominal segments and reduced the size of abdominal legs. It is evident that many of the features which characterize terrestrial insects first evolved in their marine relatives (Haas, 2003).|
Insects are arthropods that have 3 body regions (head, thorax, and abdomen) and 3 pairs of legs originating on the thorax. Although most modern insects possess wings, the first known fossil insects (from the Middle Devonian) and a number of modern insects lack them. Below are photos of modern springtails and a fossil thysanuran (the group which includes modern silverfish).
|Insects can be classified into two groups depending on whether their mouthparts are exposed (Ectognatha) or not (Entognatha). All of the Entognatha are wingless. Fossil insects have shown that one of the groups of Entognatha, Order Diplura, evolved from ancestors with the same body plan as that of winged insects of the group Ectognatha. The other two orders of wingless insects, Collembola and Protura, differ from this body plan and many consider them to be sister groups to Insecta rather than true insects. Fossil Collembolans are known from the Siluran-Devonian boundary. All three of the fossil sites where the earliest known Devonian insects have been found (two sites in northeastern North America and one in Scotland) would have been situated near the equator during the Devonian Period. Thus, the first hexapod assemblages known include forms with different mouthparts for piercing and chewing, suggesting that the hexapods had begun their diversification before the Devonian, alongside the diversification of the primitive plants of the Siluran (Labandeira, 1988; Callahan, 1972).|
|One of the extinct orders of wingless insects, Order Monura, resemble crustaceans in some ways but possess the insect characteristics of 1 pair of antennae and 3 pairs of walking legs on three thoracic segments. They retained smaller legs on other segments as well, which is considered to be an ancestral condition (Sharov, 1966).|
|Vestigial abdominal legs are known in fossils insects of the groups Monura and Thysanura from the Carboniferous, fossil larvae of Paleozoic group Paleoptora, and from both modern and fossil insects of the group Diplura (Kukalova-Peck, 1987).|
|In the following images of thysanurans, two primitive conditions are evident: these insects lack wings, and they still possess additional abdominal legs, reminiscent of the condition of other arthropods.|
The evolution of
wings occurred early in the history of insects; one of the earliest insect
fossil is a winged insect of the extinct order Meganisoptera. It seems that insect wings evolved from thoracic
lobes which are often referred to as proto-wings. If this is true, then some “wingless” insects,
both fossil and modern, actually possess the proto-wings from which more
advanced wings were derived. In
the “wingless” orders Thysanura, Archeognatha, and Monura (all Ectognatha),
the proto-wings fuse to the thorax (although among the Monurans, this
only occurred in extinct species). Modern
“wingless” insects of these groups possess a pattern of blood lacunae
and tracheae in these structures similar to the venation of insect wings. In fossil species of Monura and Thysanura from
the Carboniferous, the proto-wings are not fused to the thorax and resemble
the winglets of immature winged insects from the Paleozoic (Kukalova-Peck,
Many early winged insects retained proto-wings on their first thoracic segment. One of the early orders of insects, the Paleodictyoptera, resemble mayflies and have, in addition to 2 pairs of fully developed wings, a pair of tergal lobes on the prothorax. It is possible that the ancestor of winged insects had three pairs of tergal lobes, of which the caudal two pairs developed into wings (Sharov, 1966; Callahan, 1972). Prothoracic lobes were present on some of the Protorthoptera (Carpenter, 1941)
|There is evidence to suggest that insect proto-wings were derived from respiratory structures on the legs of ancestral arthropods known as epipodites. This conclusion is supported by embryology (wings are similar to epipodites and develop from the same larval disc as legs), genetics (given expression patterns of the genes apterous, pdm, and engrailed and mutations that can cause the formation serial wings on the segments of the abdomen), anatomy (similarities in the musculature and location of wings and epipodites), neurophysiology (abdominal neurons suggest that wings were derived from serial structures which continued into the abdomen, as observed in the mobile gills of fossil mayfly larvae), and paleontology (the wing cases of fossil larvae weren’t fixed as in modern forms, but were mobile). Thus it appears that the early stages of insect wing evolution occurred in leg appendages present in the arthropodan ancestors of insects and that the early “wingless” insects possessed the protowings from which the more advanced wings were derived. (Kukalova-Peck, 1987; Damen, 2002). Below is a crustacean limb with epipodites.|
|The most primitive and the most ancient winged insect is known from the Devonian called Eopterum devonicum (which should probably be classified in a new infraclass Archoptera which has not survived to the present day). Its triangular wings were extended tergal lobes with very primitive venation. They could not have been used for flight, only gliding. The wings have some degree of protection which would have been important for an insect which would have collided with objects because of its inability to navigate its flight. It may not be a coincidence that insects capable of gliding evolved during the same period as the first tall plants (Sharov, 1966).|
|Trachopteryx, whose relationship to other insects is not clear, has a very simple wing venation pattern without cross veins and with all veins branching from a common source (Carpenter, 1975).|
Cockroaches are one of the most primitive extant lineages of winged insects. An inherited mutation in German cockroaches (Blattella germanica) named Pro-wings produces winglike extensions on the thoracic pronotal segment. These wings can reach one third the size of the forewing, possess three major longitudinal veins which subsequently branch and are linked by cross veins, and possess tracheae which correspond to the positions of tracheae in winged thoracic segments (Ross, 1964).
The most primitive groups of winged insects (including dragonflies, damselflies, mayflies) alive today can’t fold their wings over their bodies. The extinct order of insects Protodonata (some of whose members resemble dragonflies) included the largest insects in history, such as those which had a wingspan of 70 cm, about 2 ½ feet (Callahan, 1972; Carpenter, 1953). The first mayflies are known from the Carboniferous, (family Trilosobidae) and are so different that they have been classified in their own suborder while all other mayflies, modern and those of the 8 extinct families, have been grouped in a separate suborder (Tsernova, ).
|Most modern insects can fold their wings over their bodies are classified as neopterous. The earliest known neopterous insects matured through incomplete metamorphosis and were relatives of stoneflies and locusts. Many Late Carboniferous and Permian insects have been classified in the order “Protorthoptera”. This “order” united unrelated insects which could not be assigned to any modern group. Further analysis showed that many of these insects, such as those of the family Geraridae, are members of the lineage which would lead to modern Hemipteroids. Some were large, having a wingspan of 12 cm with a clypeal bump on their heads. Others possessed a broad prothorax. In many Paleozoic insects, the head was composed of separate segments while in modern insects, all the parts of the head are seamlessly fused. One extinct order, the Protohemiptera, possessed a long beak resembling that of some modern flies. While hemipterans (which use sucking mouthparts for feeding) make up less than 10% of modern insecta fauna, the sucking insects of both neoptera (protorthoptera) and paleoptera (paleodictyoptera) composed 75-90% of the insect fauna. (Kukalova-Peck, 1992; Carpenter, 1966; Carpenter, 1953).|
The most significant event in insect history was the End-Permian extinction in which the monurans, paleodictyopteroids, protodonates, protlytropterans, caloneurodeans, and hypoperlids became extinct. In the Paleozoic, there was a bout a 50:50 ratio of insects belonging to the Paleoptera and Neoptera. Ever since the end-Permian extinction, especially with the loss of the Paleopteran group Paleodictyoptera, Neopterans have been the predominant group of insects. which was survived by the ancestors of the modern groups of the Mesozoic and Paleozoic. The Permian was the geological period with the greatest insect diversity. Most of the orders of insects which are extinct today (except the Protodonata) became extinct in the Permian, resulting in a very low diversity of insects in the Triassic. (Protodonata become extinct in the Jurassic, perhaps because of predation by pterosaurs) (Carpenter, 1953; Kukalova-Peck, 1992; Labandeira, from Anderson ed.).
Of the modern orders of winged insects, one is known from the Carboniferous (Blattaria, [roaches]), some from the Permian (Odonata [dragonflies], Ephemerida [mayflies], Perlaria [stoneflies], Corrodentia [book lice], Hemiptera [true bugs], Thysanoptera [thrips], Mecoptera [scorpionflies], Neuroptera [dobsonflies], and Coleoptera [beetles]), one from the Triassic (Orthoptera [grasshoppers]), sime from the Jurassic (Dermaptera [earwigs], Trichoptera [caddis-flies], Diptera [flies], Hymenoptera (bees)), some from the Tertiary (Isoptera [termites], Embiaria [embilds], Siphonaptera [fleas], Lepidoptera [butterflies], Strepsiptera [stylops]), and one group known from the Pleistocene (Anoplura [sucking lice]). Once members of a modern insect order had evolved, modern families appeared at varying points. (Carpenter, 1953).
Today, insects which have an anatomically and ecologically distinct larva (the holometabolous insects) make up more than 90% of insect species. The earliest example of this life style is known from insect galls on plants of the Carboniferous (Labandeira, 1996).
A number of “winged” insect groups have reduced
or lost their wings and about 5% of modern pterygote lineages are wingless.
Although the ancestral condition of stick insects (Phasmatodea)
was a wingless condition, some lineages (perhaps four) have re-evolved
partial or full wings over time (Whiting, 2003).
Four-winged flies at the base of the Dipteran evolution are known from the Upper Permian. The insect order Strepsiptera may represent the sister group of Diptera and their pseudohalteres have been compared to the halteres of Dipterans (Yeates, 1999).The major groups of flies are known from the Upper Triassic. The earliest flies were similar to midges and crane-flies. The diversity of flies increased during the Jurassic and six of the Jurassic families have become extinct (Kovalev, 1981).
|The first Hymenopterans were similar to sawflies or were parasitic; bees and wasps evolved later (Carpenter, 1953).Bees seem to be a clade which arose from within the hunting wasps (Danforth, 1999). Late Cretaceous fossils of termite workers are one of the earliest known examples of insect social organization (Marinez-Delclos, 1995). The insect order Hymenoptera includes both social and nonsocial lifestyles. Social behaviors are thought to have evolved a number of times, once in ants and the wasp family Sphecidae, twice in the wasp family Vespidae, and several times in bees (Hines, 2007). The Cretaceous family of hymenopterans, Armaniidae, presents characters which link it to the early ants, such as the Cretaceous family Sphecomyrmidae (Dlusskiy, 1983).|
|Some winged female queen ants had wingspan of 16 cm. Some weighed 10 g (about the weight of a wren; some hummingbirds only weigh about 2 g).|
|As angiosperms spread
in the Late Jurassic and Cretaceous, the diversity of hymenopterans gradually
increased (although bees are not known before the Cenozoic), ants appear,
aphids became more abundant, and leaf eating beetles became common (Ponomarenko,
1998). Most eusocial insects (ants,
bees, vespid wasps, and termites) first appear in the Creatceous. Ants are known from the Jurassic, which is earlier
than previously thought (Grimaldi, 2000; Staedter,1997)
There are about 11,000 known species of ant. They compose less than 2% of the number of insect species but their combined biomass represents about 1/3 the insect biomass on earth. In tropical regions, the total mass of ants in an area can exceed the total mass of all tetrapods combined ( Wilson, 2005).
The first ant fossils date from the Lower Cretaceous. The most primitive Mesozoic ants, subfamily Sphecomyrminae, present a combination of ant and wasp characteristics ( Wilson, 2005).