CK12 Life Science

Table 12.5: shows the structures present in each body segment.

Head Thorax Abdomen

A pair of sensory antennae,

apairofcompoundeyes, one

to three simple eyes, and

three sets of variously mod-

ified appendages that form

the mouthparts

Six segmented legs and two

or four wings

Has most of the digestive,

respiratory, excretory, and

reproductive structures

The nervous system is divided into a brain and a ventral nerve cord. Air is taken in through
the spiracles, openings on the sides of the abdomen. Insect respiration occurs without
lungs, with a system of internal tubes and sacs through which oxygen is delivered directly
to the adjoining body tissues. Since oxygen is delivered directly, the circulatory system is
therefore greatly reduced and consists of only a single dorsal tube with openings. The tube
pulses and circulates blood-like fluids inside the body cavity.

Insect locomotion includes flight, walking, and swimming. Insects are the only invertebrates
to have developed flight and this has played an important role in their success. Insect flight
is not very well understood. Primitive insect groups use muscles that act directly on the
wing structure. More advanced groups have foldable wings and their muscles act on the wall
of the thorax and give power to the wings indirectly. These muscles are able to contract
multiple times for each single nerve impulse, allowing the wings to beat faster than would
ordinarily happen.

Many adult insects use six legs for walking and have adopted a gait that uses the legs in
alternate triangles touching the ground. This gait allows for rapid walking at the same time
as having a stable stance. A few insects have evolved to walk on the surface of the water,
especially the water striders (Figure12.31).

A large number of insects live either parts of or their whole lives underwater. Water beetles
andwaterbugshavelegsadaptedtopaddleinthewater. Dragonflyyoungusejetpropulsion,
forcibly expelling water out of the rectal chamber.

Insects use a wide variety of senses for both communicating and receiving information. Many
insects have very sensitive and/or specialized sensory organs. Table(12.6) summarizes five
typesofcommunicationthatareusedbyvariousinsectsandsometimesfordifferentpurposes.