Their food generally consists of decayed animal and vegetable matter, and thus they are very useful as scavengers, particularly in hot climates. The _larvæ_ frequently live in the substance upon which they feed, while others reside in water, and many are parasitic. For the classification of the _Diptera_ we owe much to that eminent entomologist, Osten-Sacken, whose arrangement has been here generally adopted. TABULAR VIEW OF THE PRINCIPAL FAMILIES OF THE DIPTERA. TRIBE I.--NEMOCERA. Family 1. Cecidomyiidæ or Wheat Midges. Family 2. Mycetophilidæ or Army Worms. Family 3. Simuliidæ or Sand Flies. Family 4. Bibionidæ or Garden Flies. Family 5. Blephariceridæ or Blood Flies. Family 6. Culicidæ or Gnats. Family 7. Chironomidæ or Midges. Family 8. Orphnephilidæ or Bread Flies. Family 9. Psychodidæ or Fungus Midges. Family 10. Tipulidæ or Crane Flies. Family 11. Dixidæ or Wood Gnats. Family 12. Rhyphidæ or Dung Midges. TRIBE II.--BRACHYCERA. Family 13. Xylophagidæ or Tree Flies. Family 14. Cœnomyiidæ or Poplar Flies. Family 15. Stratiomyiidæ or Manure Flies. Family 16. Acanthomeridæ or Pomade Flies. Family 17. Tabanidæ or Breeze Flies. Family 18. Leptidæ or Fox Flies. Family 19. Asilidæ or Hawk Flies. Family 20. Midaidæ or Wolf Flies. Family 21. Nemestrinidæ or Flower Flies. Family 22. Bombyliidæ or Bee Flies. Family 23. Therevidæ or Hairy Flies. Family 24. Scenopinidæ or Window Flies. Family 25. Acroceridæ or Sloth Flies. Family 26. Empidæ or Little Hawk Flies. Family 27. Dolichopodidæ or Fairy Flies. Family 28. Lonchopteridæ or Water Flies. Family 29. Syrphidæ or Dart Flies. Family 30. Conopidæ or Wasp Flies. Family 31. Pipunculidæ or Hedge Flies. Family 32. Platypezidæ or Fungus Flies. Family 33. Œstridæ or Bat Flies. Family 34. Muscidæ or Meat Flies. _Section 1.--Calypteræ._ Sub-family 1. Tachininæ or Parasitic Flies. Sub-family 2. Descinæ or Rainbow Flies. Sub-family 3. Sarcophaginæ or Screw Worms. Sub-family 4. Muscinæ or Blow Flies. Sub-family 5. Antromyiinæ or Summer Flies. _Section 2.--Acalypteræ._ Sub-family 6. Scatophaginæ or Dung Flies. Sub-family 7. Ortalinæ or Wood Flies. Sub-family 8. Trypetinæ or Fruit Flies. Sub-family 9. Piophilinæ or Cheese Flies. Sub-family 10. Diopsinæ or Horn Flies. Sub-family 11. Chloropinæ or Corn Flies. Sub-family 12. Drosophilinæ or Mould Flies. Sub-family 13. Agromyzinæ or Holly Flies. Family 35. Phoridæ or Plant Flies. TRIBE III. HOMALOPTERA. Family 36. Hoppoboscidæ or Forest Flies. Family 37. Nycteribidæ or Bat Lice. Family 38. Braulidæ or Bee Lice. TRIBE IV. APHANIPTERA. Family 39. Pulicidæ or Fleas. The number of families comprised in the _Diptera_ is so great that space will only permit us to deal very briefly with each. [Illustration: Fig. 122.--Epidosis Leucopeza (Mag.)]