XVI INTRODUCTION. 



cavity. The pair of organs at the extremity, the lips or la- 

 bella, are very variable 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 into small, semi-translu- 

 cent 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 pseudotracheae, as they are called, 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 standpoint, are the maxillary palpi. They are 

 always inserted at the inferior basal part of the proboscis, on 

 a thin plate which bears the maxillse, and are always extri- 

 cated. Their study has been much neglected, and doubtless 

 thorough comparative researches will reveal not a few charac- 

 ters of value in classification. They are variously described as 

 being composed of from one to five joints. Probably there 

 is never more than four articulated joints, the basal joint 

 being merely a process of the plate bearing the maxillse. 

 The tendency in diptera is toward their entire loss, and in the 

 most highly specialized families there is never more than one 

 articulated joint. They may be reduced to the merest rudi- 

 ments, even in flies which are more or less predaceous in hab- 

 its and which have the mouth-parts with the exception of the 

 mandibles, otherwise well-developed. They are never greatly 

 elongated, save among some of the Nematocera. 



Without going into Prof. Smith's arguments, he shows with 

 what seems to be much force, that the real homologies of the 

 dipterous mouth-parts are as follows : 



