Structure & Function

Definitions
Morphology - the study of external form and structure, e.g.., the legs, wings, head, etc.

Anatomy - the science of structure, usually internal.

Anatomical positioning terms

Anterior = toward the head or front end
Posterior = toward the hind end or tail
Dorsal = toward the upper surface or back
Ventral = toward the lower surface or belly
Median or mesal = toward the middle or inner part
Longitudinal = oriented along the long axis of the body or appendage
Transverse = oriented at right angles to longitudinal

Somite - an individual segment of the insect body. A segment consists of a tube-like structure with a pair of appendages per segment.

Much like a train is assembled from a series of rail cars, the insect body is built from a series of connected segments. In a train not all the cars are exactly alike, some have tanks, some are open bins, others are boxes. The basic rail car design has been modified for different functions, to carry liquids, solids, and containers. The insect body has a similar design, the basic somite has been modified by evolution for different functions. It may be enlarged, shrunken, lost and it may or may not have appendages. And the appendages themselves may be greatly modified. They can be used for walking, eating, grasping, or touching.

Proto-insect or ancestral body plan. Note the numerous somites that make up the body plan. All the somites are similar although they vary in size, shape, and some parts may be absent.

Generalized Insect Body Plan

The insect body plan, as seen in this grasshopper, is divided into 3 areas, the head, thorax, and abdomen. Each area is made of of a number of somites.

Head

The anterior region of the insect body or the head is composed of several highly modified and fused segments. The head functions to:

ingest food
major sensory perception center,
coordinate body activities (brain)
protect the coordinating center

Morphological traits

compound eyes
one to 3 simple eyes (ocellus, plural ocelli)
antennae
mouthparts = labrum and 3 sets of paired and modified appendages
labrum - plate (sclerite) that serves as upper lip in insects with chewing mouthparts, analogous to upper lip
mandible - appendage of the 4th body segment that becomes the 1st pair of mouthparts, analogous to jaw
maxilla - 2nd pair of feeding appendages, used for food handling and sensing
labium - fused, 3rd pair of feeding appendages, analogous to lower lip
encloses the brain

Generalized Insect Head

Thorax

Middle section of the body with 3 segments. Walking and flying functions.

Morphological traits

three segments (PROthorax, MESOthorax, and METAthorax)
one pair of walking legs per segment (appendages)
wings, if present, on 2nd and 3rd segments
dorsal (upper) surface = notum
ventral (lower) surface = sternum

Abdomen

11 segments, not all segments may be easily visible

Morphological traits

more flexible than other regions to allow breathing
usually 1 pair of spiracles (breathing holes) per segment
dorsal (upper) surface = tergum
ventral (lower) surface = sternum

Link to: Introduction to Insect Anatomy

Go to: top of page

home page

topics list

Last updated Dec. 30, 2000
Gary Brewer