How do you tell apart arachnids, insects, and crustaceans?

Arthropods are animals which have articulated limbs.
a) Arachnids
b) Crustaceans
c) MillipedeS
d) Insects
Ex.: Spider, cancer, millipede,ant,scorpio, crawfish, lobster
 General character of arthropods:
- Segmented body covered with an exo skeleton.
- Three or four pairs of Appendix  united among themselves.
- Muscles well developed, the link with exo skeleton.
- The body is divided into two (three at insect) main regions: cefalo thorax(insect - head and thorax) and abdomen.
 -Well developed Mouth.
- Specialized senses, highly developed.
- Circulatory system, open type.
-  Articulated limbs.
- Breathing through gills, trachea and pulmo-trachea.
Insects are the arthropods  most valued and most numerous (approximately one million species).
General characters of insects:
- The two pairs of wings.
- Oral apparatus.
- Metamorphosis is complete.