26 NORTH AMERICAN DIPTERA. 



the piercing parts, the proximal portion is more or less 

 membranous and retractile; or, the inner organs may be 

 capable of elongation, being coiled np in some cases, as 

 in Pangonia t within the pharyngeal cavity. The pair of 

 organs at the extremity, the lips or labella, are very va- 

 riable in shape, position and function. In the mosquito, 

 for instance,, they serve merely as a pair of fingers to 

 guide the piercing parts. In many of the flower-flies with 

 long proboscis, they are small, oval, divaricable organs, 

 that seem to be chiefly sense organs, as they are usually 

 provided with hairs inserted in small, semi-translncent 

 spots on the outer sides and margins. In the greater 

 number of flies, however, the labella are of considerable 

 size, and are provided with radiating ridges on the inner, 

 opposable sides. These pseudotracheas, as they are call- 

 ed, serve as means of attrition, by which the insects rub 

 off particles of food from firm substances. Sometimes 

 the labella are long and slender and folded back under 

 the labium when at rest. In the Asilidse and some others, 

 they are rigid and horny* 



Perhaps the most important of all the mouth-parts, 

 from the systematic stand point, are the maxillary palpi. 

 They are inserted at the inferior basal part of the pro- 

 boscis, on a thin plate which bears the maxillae, and 

 are always extricated. Their study has been much neg- 

 lected, and doubtless thorough comparative researches 

 will reveal not a few characters of value in classification. 

 They are variously described as being composed of from 

 one to five joints.* There are never more than four articu- 

 lated joints, the basal joint being merely a process of the 

 plate bearing the maxillae. The tendency in diptera is 

 toward their entire loss, and in the more highly special- 

 ized families there is never more than one joint. They 



Theobald saya some Culicidae have six joints, but his statement 

 needs confirmation. 



