The following information came from Margulis and Schwartz (1998), Buchsbaum (1938), Barnes (1980), Barnes (1984), Ruppert and Barnes (1991), Brusca and Brusca (2003), Hickman (1973), Storer and Usinger (1965), and Tudge (2000).  
I. SYNONYMS: insects, bugs, arthropods.  
II. NUMBER: >1,000,000 species known.  


A. Structure

Symmetry: Bilateral with head (5 fused segments with acron and oral region with mouthparts formed by modifies legs), thorax (3 segments, each with a walking leg; most adult insects with 2 pairs of wings), and abdomen (11 segments with considerable fusion in some groups) without walking legs.

Body Cavity: True coelom reduced and absent in adults. Haemocoel the only body cavity.

Body Covering: Covered by chitinous exoskeleton.

Support: Hardened exoskeleton.

Digestive System: Food tube simple. Mouth at anterioventral end bordered by upper lip (of epistome and labrum) and lower lip (of maxillae or labium), and has mandibles. Hypopharynx. Food tube of three regions: fore-, mid-, and hindgut. Anus terminal.

Circulatory System: Open. Haemocoel of blood sinuses with a dorsal heart. No respiratory pigments in the plasma .

Locomotion: Each segment bears one pair of unbranched, jointed legs. Lost or highly modified in some.  When present, the first leg segment is the coxa (coxae)

Excretory System: Malpighian tubules (when present) discharge into gut at junction of mid and hind- gut.

Nervous System: Circumesophagial brain and a double, ventral nerve cord with segmental ganglia. The head capsule (fused segments at the anterior end) have simple and compound eyes as well as antennae, sensory hairs.

Endocrine System: Complex, with hormonal control of sexual reproduction, development, growth, and communication.

Respiratory System: Tracheal system, usually paired spiracles at each segment.

B. Reproduction:

Reproductive System: Dioecious. Gonads one to many. Fertilization internal (by spermatophores). Oviparous.

Development: Development may include larval form that is markedly different from adult.

C. Ecology: Mainly terrestrial, a few have returned to water.



Averof, M. and M. Akam. 1995. Insect-crustacean relationships: insights from comparative developmental and molecular studies. Phil. Trans. R. Soc. London. B. 347: 293-303.

Ax, P. 2000. Multicellular Animals II. Springer Verlag. Berlin.

Brusca, R. C. and G. J. Brusca. 2003. Invertebrates. Sinauer Associates, Inc. Sunderland, Mass.

Buchsbaum, R. 1938. Animals Without Backbones, An Introduction to the Invertebrates. The University of Chicago Press. Chicago. 

Budd, G. E. 1998. Arthropod body plan evolution in the Cambrian with an example from anomalocaridid muscle. Lethaia. 31: 197-210.

Budd, G. E. 2001. Tardigrades as 'Stem-Group Arthropods': The evidence from the Cambrian fauna. Zool. Anz. 240: 265-279.

Conway Morris, S. (1998). The crucible of creation: the Burgess Shale and the rise of animals. Oxford [Oxfordshire]: Oxford University Press. pp. 56–9.

Dunn, C.W., A. Hejnol, D.Q. Matus, K. Pang, W.E. Browne, S.A. Smith, E. Seaver, G.W. Rouse, M. Obst, G.D. Edgecombe, M.V. Sørensen, S.H.D. Haddock, A. Schmidt-Rhaesa, A. Okusu, R.M. Kristensen, W.C. Wheeler, M.Q. Martindale, and G. Giribet. 2008. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 452: 745-749.

Garey, J. R. 2001. Ecdysozoa: The relationship between Cycloneuralia and Panarthropoda. Zoologischer Anzeiger 240: 321-330.

Giribet, G., G. D. Edgecombe, J. M. Carpenter, C. A. D'Haese, and W. C. Wheeler. 2004. Is Ellipura monophyletic? A combined analysis of basal hexapod relationships with emphasis on the origin of insects. Organisms, Diversity and Evolution. 4: 319-340.

Hickman, C. P. 1973. Biology of the Invertebrates. The C. V. Mosby Company. Saint Louis. 

Ivantsov, A. Yu. 2004. New Proarticulata from the Vendian of the Arkhangel'sk Region. Paleontological Journal. 38(3): 247-253.

Lavrov, D. V., W. M. Brown, and J. L. Boore. 2004. Phylogenetic position of the Pentastomida and (pan)crustacean relationships. Proceedings of the Royal Society of London. Series B. 271: 537-544. 

Mallatt, J. M., J. R. Garey, and J. W. Shultz. 2003. Ecdysozoan phylogeny and Baysean inference: first use of nearly complete 28S and 18S rRNA gene sequences to classify the arthropods and their kin. Molecular Phylogenetics and Evolution. 31: 178-191. 

Manton, S. F. 1977. The arthropod habits, functional morphology, and evolution. Clarendon Press. Oxford.

Margulis, L. and K. Schwartz. 1998. Five kingdoms, an illustrated guide to the phyla of life on earth. 3rd Edition. W. H. Freeman and Company.  New York.

Mayer, G. 2006. Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods? Arthropod Structure and Development. 35: 231-245.

Mayer, G. and P. M. Whittington. 2009. Velvet worm development links myriapods with chelicerates

Nielsen, C. 2001. Animal Evolution: Interrelationships of the Living Phyla. 2nd Edition. Oxford University Press. Oxford.

Patel, N. H., E. Martin-Blanco, K. G. Coleman, S. J. Poole, M. C. Ellis, T. B. Kornberg, and C. S. Goodman. 1989. Expression of engrailed proteins in arthropods, annelids, and chordates. Cell. 58: 955-968. 

Pechenik, J. A. 2005. Biology of the Invertebrates. McGraw-Hill. New York.

Regier, J. C., J. W. Shultz, and R. E. Kambic. 2005. Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proceedings of the Royal Society of London. Series B. 272: 395-401.

Reiger, J. C., J. W. Schultz, A. R. D. Ganley, A. Hussey, D. Shi, B. Ball, A. Zwick, J. E. Stajich, M. P. Cummings, J. W. Martin, and C. W. Cunningham. 2008)Resolving arthropod phylogeny: exploring phylogenetic signal within 41 kb of protein-coding nuclear gene sequence. Syste. Biol 57(6): 920-938.

Ruppert, E. E. and R. D. Barnes. 1994. Invertebrate Zoology. 6th edition. Saunders. Ft Worth, TX. 

Ruppert, E. E., R. S. Fox, and R. D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole. New York. pp. 1-963.

Strausfeld, N. J., C. M. Strausfeld, R. Loesel, D. Rowell, and S. Stowe. 2006. Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage. Proc. R. Soc. London. B. 273: 1857-1866.

Telford, M. J. S. J. Bourlat, A. Economou, D. Papillion, and O. Rota-Stabelli. 2008. The evolution of Ecdysozoa. Phil. Trans. R. Soc. B. 363: 1529-1537. 

Tudge, C. 2000. The Variety of Life, A Survey and a Celebration of all the Creatures That Have Ever Lived. Oxford University Press. New York.

Waggoner, B. M. 1996. Phylogenetic hypotheses of the relationships of arthropods to Precambrian and Cambrian problematic fossil taxa. Systematic Biology 45(2): 190-222.

Whittington, H. B. and D. E. G. Briggs. 1985. The largest Cambrian animal, Anomalocaris, Burgess Shale, British Columbia. Phil. Trans. R. Soc. London. B. 309: 569-609.

Willmer, P. 1990. Invertebrate relationships, patterns in animal evolution. Cambridge University Press. Cambridge.


By Jack R. Holt and Carlos A. Iudica.  Last revised: 02/03/2013