Biodiversity – Oct. 22/12
- These are six-footed animals with three pairs of walking legs.
- Hexapod/insect is a monophyletic group within the arthropoda. A hexapod (insect) is an arthropod
with 3 body regions – head, thorax, and abdomen (synapomorphy). The head is sensory and it has
compound eyes, one pair of antennae, and mouthparts. The thorax has three segments and is used for
locomotion. There are three pairs of legs (synapomorphy), with one pair attached to each segment of
the thorax. The thorax also has two pairs of wings (this is a synapomorphy for derived insects). The
first pair of wings is attached to the second segment of the thorax and the second pair of wings is
attached to the third segment of the thorax. The abdomen contains the major internal organs and is
involved in reproduction.
- Animalia (metazoan) evolution major events: Eukaryotic cells were present 2BYA (Proterozoic).
Animal fossils were present 630MYA and they are relatives of today’s jellyfish and sea anemone.
Green algae and probably fungi were present (late Proterozoic). During the Cambrian period (began
550MYA), the Cambrian explosion saw a relatively rapid appearance of most major phyla (fossil
record) in oceans (early Paleozoic). Arthropod fossils from Trilobites are present from this time.
Ordovician period (began 480MYA) saw animal diversification with vertebrates in oceans and
invertebrate animals (ex. arthropods) colonizing land (along with plants and fungi). Silurian period
(began 440MYA) – On land, invertebrate animals diversified (along with plants and fungi). Insects
were undoubtedly present on land (indirect evidence, not fossil evidence). Devonian period (began
420MYA) – There was continued diversification on land, fins evolved into four limbs, amphibians
were the first land vertebrates but they were dependent on water for reproduction. The oldest
definitive insect fossils are present from this time. Carboniferous period (began 360MYA) – The
amniotic egg and amniota developed, allowing land vertebrates to become totally adapted to life on
dry land. Permian period (began 300MYA) – The End-Permian mass extinction resulted in 95% of life
going extinct. This was mostly marine invertebrates and insects, and species and higher taxa, but not
phyla (ex. arthropoda). Trilobites went extinct. Mesozoic period (began 250MYA) – There was animal
diversification in oceans and on land, and coevolution with angiosperms (flowering plants). Insects
were especially involved in coevolution with angiosperms.
- Insects today: Today insects are not only the dominant arthropods on land, they are the dominant
group of organisms on land in terms of numbers of species and biomass. Biomass is the mass of living
biological organisms per area, There are nearly 1 million described species of insects (hyperdiverse).
Insects are found in all terrestrial habitats, they are freshwater aquatic, and very few are marine.
There is incredible diversity of morphological form in insects – body parts come in a variety of forms.
- Basal/primitive insects are wingless. Silverfish are 1 inch long. Collembola are 1-2mm in length.
- Derived/advanced insects are adapted to life in the air. All derived insects (which are the majority of
insects) have wings and can fly in the adult stage. Some higher taxa have secondarily lost their wings
(ex. lice, fleas) and some species within higher taxa have secondarily lost wings (ex. some
cockroaches). Most insects are nocturnal and fly only at night. True powered flight has convergently
evolved in only three extant groups of organisms – bats, birds, and insects.
- Insect characteristics – compound eyes: Insects have two compound eyes. The eyes of dragonflies
meet at the top of their head. The surface of the eye is comprised of many cells and is a light-
gathering surface. Photoreceptors are located here. Inside the eye are neurons that transmit
information to the brain. The bigger the eye, the better the insect can see. The eyes are used in
hunting and escape from predators. The eye is also part of the exoskeleton.
- Insect characteristics – chemoreception: Sensing chemicals are involved in olfaction/smelling.
Chemoreceptors, though concentrated in the antennae, are found on other body parts as well. Antennae are segmented. Some types of antennae are related to phylogeny. Some of the com