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18. Hexapoda Insects are important - taking number of species of organisms, 250,000 species plants, 69,000 fungi, chordates 43,000 - most animals are arthropods, of which 103,000 are not insects and 925,000 species are insects. Either this is to do with sampling bias and simply seeing insects - terrestrial and easy to find, OR something special about insects that means they have incredible radiation - speciose. (Think about size affecting abundance). In terms of biomass, however, bacteria have greater weight. They have huge inputs on the earth's eco-system (ants move more earth than worms). Hexapods that aren't insects Insect characteristic are head, thorax and abdomen - composite segments that have fused over evolutionary time. The segments are still visible - somites have fused. Protura has no eyes or antennae, and its abdomen can have separate appendages. Hexapods that are no insects are stem groups - representing the earliest kind of insect forms and most have evolved into proper insects. Another example is Diplura that has unusual head anatomy, again can see that thorax is still externally segmented - hasn't fused into one tagmata. Insects do not have externally segmented thorax. Collombola (spring tails) have ocelli not compound eyes, and springing organs under the abdomen for rapid predator evasion. Evolution Just looking at insects, there are true insects, related species like silverfish (diplura) and spring tails which are all separated by the insects, according to DNA data. DNA data corresponds with the morphological data in that hexapods are "similar but different". They are similar to ancestral insects. Insects Successful class of organisms - found virtually in every ecosystem everywhere on the planet. Only one lineage has gone extinct - paleodictyoptera,
but most have survived over the last 350 Million Years. They have radiated enormously but have survived. There are no insects that have their complete life cycle in the sea. This is because it is full of crustaceans. If they did manage to establish itself, if they survived predation then they would face competitive exclusion as niches in the sea have already been occupied. People have argued that it is because the sea is salty - there are freshwater insects living in ponds (water boatman). Aquatic insects but not marine. Insects are also the first animals to have powered flight - not simply gliding. The only one true synapamorphy - this relates to the antennae and how they hear. All insects have a stretch receptor (chordotonal) in their antennae which moves at the antennae wave about, being batted by sound waves. This is called the JOHNSTON'S ORGAN. Division of insect lineages
Apterygota refer to the aforementioned non-insect hexapods (collembola, protura and diplura). Orthopterans are crickets, hemipterans are bugs and cicadas. Coleopterans are beetles and flies are diptera. There is one major division between the insects - above the line are the hemimetabolous insects and below the line are holometabolous. This is a major division and refers to the difference between exhibiting full metamorphosis (holometabolous) like beetle grubs. The hemimetabolous are the ancestral forms - metamorphosis is a recent evolutionary development.
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