1. Insects

2. Class Insecta
Most numerous and diverse of all the groups of arthropods.
(There are more species of insects than species in all the other classes of animals combined!!)
Have three pairs of legs
Usually have two pairs of wings on the thoracic region of the body, (although some have one pair of wings, or none)  
Body organization: head, thorax, and abdomen.
The head usually bears a pair of large compound eyes, a pair of antennae, and usually three ocelli (simple eyes).  

3. General Characteristics
Segmented bodies
Jointed appendages
Specialized for eating, sensing, reproduction, defense and movement
Exoskeleton
Hard covering on the outside of the animal
Limits the growth of the organism (must shed)
Muscles are attached to this layer
Contains protein, lipid, chitin, and often calcium carbonate
secreted by underlying epidermis and shed (molted) at intervals

4. Muscular system complex (contains both striated and smooth muscles)
Bilateral symmetry
Muscular system complex (contains both striated and smooth muscles)
Body cavity
Coelom is reduced;  
Most of body cavity consisting of hemocoel (sinuses, or spaces, in the tissues) filled with blood;
Digestion
Complete digestive system 
Esophagus, crop, gizzard, midgut, hindgut and anus
mouthparts modified from appendages and adapted for different methods of feeding;

5. Water retention—3 structures that aid
Circulatory System
Open system
dorsal contractile heart, arteries
Respiration
Obtain oxygen through body surface (diffusion), gills, tracheal (air tubes), or book lungs;
Tracheal tubes—carry oxygen to the muscles
Spiracles—small openings in the exoskeleton through which air opens
Water retention—3 structures that aid
Malpighian tubules—excretory structures that remove metabolic wastes from blood and return water to the cells
Exoskeleton—prevents water evaporation
Book Lungs—gas exchange without water loss (also used in respiration)

6. Excretory system Nervous system Reproduction
Paired excretory glands in some
homologous to nephridial system of annelids;  
some with other excretory organs, called Malpighian tubules;
Nervous system
Contains same system of annelid (with dorsal brain connected by a double nerve chain of ventral ganglia);  
fusion of ganglia in some species;  
well-developed sensory organs;
Compounds eyes with many lenses (can see motion and color)
Reproduction
Sexes usually separate,
paired reproductive organs and ducts;  
usually internal fertilization; often with metamorphosis;