|
|
|
Insects have the traits already discussed for the
arthropods and the mandibulates - exoskeleton (of chitin,
a light material but very supportive if a structure is small enough
- in some ways like fiberglass), open circulatory system, segmented
worm-type nervous system, mandibles, antennae - and a few specific
traits of their own. Insects typically are divided into three
sections: a head, thorax, and abdomen. The thorax is
where all of the structures used for movement are located: six
walking legs (several types of
mouthparts and perhaps other
structures are built from what used to be legs, but only six real
legs remain), and typically four
wings. Insects, unlike other winged animals, have not
evolved
their wings from pre-existing appendages, but rather from outgrowths
of the thorax exoskeleton, possibly flaps that had been used to
cover the gills of fresh water insects, or from the gills themselves. The progression probably went from
blunt gliding structures, through steerable glider wings, to
flappable wings, to systems that can beat wings up to at least a
thousand times a second. Many types of insects use only the
back pair of wings for flying; the front pair in those insects
have generally evolved
into protective covers for the back wings.
Insects typically go through a limited number of molts
(6 is the typical number) between hatching and adulthood, and with
very few exceptions only develop fully-functional wings during the
last molt. If you are looking at an insect with full wings,
its almost certainly an adult and never going to get any
bigger. Insects also go through
two different pathways of
development. In direct
development (sometimes called simple
or incomplete metamorphosis), young
insects look like miniature, wingless versions of the adults (these
young are typically called nymphs). Crickets and
grasshoppers develop this way. In indirect
development (sometimes called complete
metamorphosis),
the larva is physically quite different from the
adult. They often occupy different niches than
adults as well, allowing more individuals to exist together in an
ecosystem. Larvae grow, molt, grow, molt, et cetera, until
they encase themselves in a pupa form and completely
reorganize themselves, a process called metamorphosis (sometimes breaking down to a "soup"
of cells that then completely build a new individual) and emerge as
adults. The terms "simple metamorphosis" and
"complete metamorphosis" mentioned above can be confusing,
and "metamorphosis" is usually just applied to animals
that go through a major change. This is what butterflies do, as you probably know, but
also beetles, bees, ants, and flies develop this way. In some
cases, the adult acts almost exclusively to reproduce and spread
offspring, not even being able to feed during their short time in
that form.
|
|
INSECT SUBGROUPS - |
|
PRIMITIVE WINGLESS INSECTS -
There are still some species of insects around
descended from ancestors who never evolved wings. Most of
these are small and, although youve been around them your whole
life, you probably have never noticed them, except for maybe
the insects commonly called silverfish, which
encounter humans usually around drains. |
|
PRIMITIVE WINGED INSECTS -
Flying insects use two different drive systems in
their wings - the more primitive system attaches directly to the
inner ends of the wings, while the more modern drive a
lever-and-pivot system built into the structure of the thorax.
The primitive winged insects include the
dragonflies,
mayflies
and
damselflies (Odonata). |
|
MODERN WINGED INSECTS -
These use the thorax drive system, which can
produce wing beats so rapid that a special type of muscle had to
evolve just to keep things moving. This group includes (these
are only the well-known groups, there are several more) -
- The
grasshoppers and their relatives,
which are obvious insects such as crickets and katydids and insects
maybe not so obviously related, such as
cockroaches,
mantises, and
walking sticks. These are direct developers. Orthoptera.
- The "true bugs," which includes
leaf hoppers and
cicadas. These insects often seem more
"boxy," with angles and straight lines. They
are direct developers. Hemiptera.
- The beetles, which have more
named species than any other insect group. These typically
have hard
shiny coverings, including
front wings that cover and
protect the back wings. They are indirect developers, often
going through a grub or wormy larval stage. Coleoptera.
- Butterflies and
moths.
Although usually moth wings
fold back over the abdomen and butterfly
wings
don't, that is not a reliable way to tell them apart - there
isn't really an easy way to do it, since there are some very mothlike butterflies and butterflylike moths. These are
indirect developers, often with
caterpillars as larvae. Lepidoptera.
- Flies. These roundish
insects have only a single pair of wings, which often beat at
extremely high speeds. The other pair of wings have evolved
into short knobby structures (halteres) that beat
opposite to the wings and counterbalance those forces. These
are indirect developers, often with soft wormy
larvae, including
maggots. Diptera.
- Termites. These
ant-like insects include the famous wood-eaters, which depend upon a
whole ecosystem of unusual
protozoans that live in their guts and
actually digest the wood, but some termite species are
mound-builders. They are indirect developers, with usually
only reproductive adults having wings, and commonly exist in social
colonies. Isoptera.
- Ants, Bees, and
Wasps.
Sometimes combined with the termites, these also are indirect
developers and live in social colonies. Ants are ground
foragers, bees are mostly plant eaters and wasps are mostly meat
eaters. Hymenoptera. |
|
MODERN WINGLESS
INSECTS.
These insects show evidence of wings, so it is
assumed that their ancestors were winged and evolution has led to
the loss of the wings. Both of the types of insects in this
group are commonly parasites for whom wings might interfere with staying on
their host.
This group includes the lice (Phthiraptera
and Psocoptera)
and the
fleas (Siphonaptera). |
|
|