CHAPTER IV 



MOSQUITOES AND GNATS 



Order I. Diptera. — Insecta (p. 15). The dipterous insects have onty 

 the anterior pair of wings developed, the posterior pair being repre- 

 sented by rudimentary structures called halteres, or balancers, which 

 are supposed to function as organs of balance. In some parasitic forms 

 (sheep "tick," bat fly) wings are entirely wanting. 



The head, thorax, and abdomen are sharply defined. The mouth 

 parts are adapted for sucking, the haustellum, or sucking tube, being 

 formed by the labium and labrum, within which lie the mandibles and 

 maxillae, which may be modified into blade-like structures for piercing. 

 With this structure the insect sucks the juices of plants or penetrates the 

 skin of animals and feeds upon their blood. In the flies the antennae are 

 short, consisting of but three well-developed joints. The three thoracic 

 segments are frequently fused, and the tarsi have five segments. 



Metamorphosis is complete. The larvae are apodal grubs, maggots, 

 or wrigglers, the latter aquatic (mosquitoes). 



Parasitism. — The dipterous group of insects includes a number of 

 species varying in their grade of parasitism from optional occasional to 

 obligate occasional and permanent. They are chiefly of importance 

 from the medical viewpoint as carriers of bacterial and animal parasitic 

 infection, investigations within recent years well establishing the fact 

 that certgiin serious and often fatal diseases of man and domestic animals 

 are spread by these insects either as essential hosts or as direct carriers 

 of the infecting organism. As essential hosts a part of the development 

 of the pathogenic organism must essentially be undergone in the insect. 

 As direct carriers they may inoculate directly into the blood with con- 

 taminated piercing or biting mouth parts, or they may simply trans- 

 port disease germs upon their bodies and appendages, contaminating 

 wounds, food, or any object upon which they may alight. 



As blood-sucking pests and sources of torment in the habitations of 

 man and in the fields and stables of his live stock, many of these two- 

 winged insects are of very considerable economic as well as pathologic 

 importance. In view of all that at the present time can be charged 

 np against them, they are well worthy of the increasing attention they 

 are receiving with a view to their more effectual control. 



Of the families of the order Diptera containing parasitic species, six 

 are here considered, as follows: 



