THE CLASS INSECTA, AND THE ORDER DIPTERA 31 



(c) Orders which include species, outside the orders 

 already mentioned, that concern the medical officer in some 

 more or less indirect way (e.g., preying upon flies and their 

 larvae ; resemblance to " biting " insects, etc.) : — 



12. F/ecopiem (Stone-Hies) x6. Neuroptera (Ant- 



13. Ephemerida (May-flies) lions, etc.) 



14. Mallophaga (Bird-lice) 17. Mecaptera (Scorpion- 



15. Odonata (Dragon-flies) flies) 



18. Trichoptera (Caddis-flies) 



(d) Orders of no particular concern to the medical 

 officer : — 



19. Ci3//£»«^(7/« (Spring-tails) 21. Thysanoptera 



20. Embiida 22. Strepsiptera 



The orders will be dealt with here in the order of their 

 importance from the medical point of view, beginning with 

 the Diptera. 



Order DiPTERA (Gr. SiTrrepos = with two wings). 



Diptera, or Flies, are insects that possess only the 

 anterior pair of wings, which are membranous, and that have 

 the mouth-parts adapted for sucking, and sometimes also for 

 piercing. The hind wings are represented by a pair of 

 club-shaped structures known as halteres or balancers. The 

 metamorphosis is complete, and the transformation under- 

 gone in the pupal stage sometimes amounts to a complete 

 reconstruction of the body. The head is usually joined to 

 the thorax by a flexible neck that confers very great freedom 

 of movement ; the segments of the thorax are combined to 

 form a single mass ; and the number of visible abdominal 

 segments is variable, between 4 and 9. 



To the medical and sanitary officer Diptera are a group 

 of the very foremost interest, since in addition to being in 

 various ways inimical to health and comfort, certain notorious 

 species are the harbourers, or the intermediate hosts^ of 

 microscopic parasites that cause some of the most formidable 

 diseases of tropical countries. Blood-sucking flies, and the 

 profoundly grave part that some of them play in spreading 

 certain specific diseases of man, form an independent subject; 



