their numbers, large genetic pool resulting in ability to adapt, relative simple body systems. small size


see

http://www.shvoong.com/exact-sciences/zo...

1. Small size – due to which they can find food and shelter everywhere.
2. Adaptability – there are thousands of varieties of insects; each adapted to its ecological niche.
3. Mobility – due to their power of flight.
4. Hard Skeleton – this is lighter than bone and very resistant.
5. Fertility