CHAPTjER IX. 

 ORDER DIPTERA. 



This is a \ery large and important order and includes a 

 number of important economic species such as the house 

 fly and the mosquito. They are separated from all other 

 insects by the wing structure, the front wings only being 

 fully developed as means of flight and the second wings 

 being modified into knob-like structures which seem to have 

 some use in sustaining the insect in a certain position. 

 They vibrate but cannot serve any distinct function as a 

 means of flight. They are modified wings. This is a case of 

 specialization, reduction in size. This is coordinated with 

 the centralization of the nervous system. There is distinct 

 metamorphosis whose stages are usually sharply marked. 

 The larva? are usually called maggots, those of the mosquitoes 

 "wrigglers." The larvae occur in all manner of situations. 

 Some are aquatic, some terrestrial, some subterranean, etc. 

 The group is separated mainly by larval characters — one 

 group including mosquitoes and gnats, another the more 

 specialized flies. The larva contracts and the skin hardens 

 and forms a pupal case whose end splits oft' like a cap. 



The Orthorapha, mosquitoes and gnats, are the less 

 specialized or the more primitive group. 



Of the groups in the first series the mosquitoes (CulkidcB) 

 are the most in evidence, if not the most important, and 

 are receiving more attention lately because of their asso- 

 ciation with the transmission of various diseases. The 

 mosquito is not a single species. There are a great many 

 dift'erent species and they differ a great deal in life history 

 and habits and in their relation to disease. The most general 

 statement to be made regarding their cycle is that the eggs 

 (248) 



