INTRODUCTION
Many insects that usually are seen flying have long been called "flies" of various sorts - dragonflies, mayflies, fireflies, caddisflies, butterflies and so on. But these all belong to different orders of insects, only distantly related to each other. In entomology, none of these are flies proper, insects of the Order Diptera, which are often referred to as "true flies" or "two-winged flies" because they never have more than one pair of wings, those on the middle segment of the thorax. Some insects in other orders have lost a pair of wings, sometimes the front pair, sometimes the back one -- but none of these, except some male scale insects, have the hind wings reduced to halter-like structures. We follow convention in writing the English names of Diptera as two words (crane fly, house fly) and those called flies in other orders as a single word (dragonfly, stonefly).

Structure of Diptera
· When describing fly structure, we use the terminology of McAlpine (1981).
· Terms and diagrams.

Classification
The higher classification of the Diptera is in flux. That used here is from Wood and Borkent (1989) for the suborder Nematocera (lower Diptera) and from Yeates and Weigmann (2005) for the suborder Brachycera (higher Diptera). While Yeates and Weigmann (2005) indicate that the suborder Nematocera is a paraphyletic grouping, the exact relationship of the lower fly families is not certain. Thus we have deferred to the older higher classification of Wood and Borkent (1989) for this group. This amalgam of higher classification schemes is also being followed for the Manual of Central American Diptera, which is being developed (B.V. Brown, pers. com.). There is also some controversy as to whether some of the families are monophyletic. In such cases we have followed, in large part, the family classification presented in the Manual of Nearctic Diptera to allow for the use of its generic level keys, but noted the controversy in the discussion of the respective families. As well, we have used some traditional artificial higher groupings, such as Aschiza, in the development of the family keys to make the keys more user-friendly. This is because many of the characters that indicate the true evolutionary relationships of some fly families are genitalic and/or molecular, both of which would make a key unnecessarily awkward to use.

Table and Appendices
Table 1 lists figure details. All except two illustrations are reproduced with the permission of the Minister of Public Works and Government Services Canada, 2006, and taken from the Manual of Nearctic Diptera, Volume 1, Research Branch, Agriculture & Agri-food Canada, Monograph No. 27, 1981, and Manual of Nearctic Diptera, Volume 2, Research Branch, Agriculture & Agri-food Canada, Monograph No. 28, 1987.

The exceptions to the above are families Oreoleptidae and Asilidae. Family Oreoleptidae is reproduced with the permission of J. Zloty, and originates from Zloty, J., Sinclair, B., & Pritchard, G. 2005. Discovered in our backyard: a new genus and species of a new family from the Rocky Mountains of North America (Diptera: Tabanomorpha). Systematic Entomology 30(2), 248-266.

Family Asilidae is reproduced with permission by R.A. Cannings, and was originally published in Insects of the Yukon. 1997. (Eds. H.V. Danks & J.A. Downes).Biological Survey of Canada (Terrestrial Arthropods), Ottawa, ON. Robber Flies (Diptera: Asilidae) of the Yukon by R.A. Cannings.
Appendix I is a Checklist for the Order Diptera in British Columbia, and Appendix II contains illustrations of Diptera families (Figures 1-94).

References
McAlpine, J.F. 1981. Morphology and terminology - adults. pp. 9-63 In McAlpine, J.F., Peterson, B.V., Shewell, G.E., Teskey, H.J., Vockeroth, J.R., and Wood, D.M. (Eds): Manual of Nearctic Diptera. Volume 1. Research Branch Agriculture Canada Monograph 27: 1-674.

Wood, D.M., and Borkent, A. 1989. Phylogeny and classification of the Nematocera. pp. 1333-1370 In McAlpine, J.F., and Wood, D.M. (Eds): Manual of Nearctic Diptera. Volume 3. Research Branch Agriculture Canada Monograph 32: 1333-1581.

Yeates, D.K., and Wiegmann, B.M. 2005. Phylogeny and evolution of Diptera: recent insights and new perspectives. pp. 14-44 In Yeates, D.K. and Wiegmann, B.M. (Eds.): The Evolutionary Biology of Flies. Columbia University Press, New York. 1-430.