32 ILLINOIS BIOLOGICAL MONOGRAPHS [202 



MOVABLE PARTS OF THE HEAD 



In arrangement and structure the movable parts of the head of 

 the generalized Diptera are homologous with the movable appendages 

 of other generalized insects. In the Cyclorrhapha the parts retain their 

 relative position, but structurally they undergo striking modifications 

 and in some cases almost complete reduction. 



To make clear the use of a number of terms found in the following 

 discussions, the mouth-parts as a whole will be considered at this point. 

 The appendages of the mouth of the generalized Diptera are free, inde- 

 pendent structures, with their proximal ends adjacent to the head-cap- 

 sule. The cardines and stipites of the maxillae are exceptions to the 

 above statement, in that they are embedded in the mesal membranous 

 area of the caudal aspect of the head. The mouth-parts, the labrum- 

 epipharynx, and the hypopharynx constitute in the Calyptratae a single 

 complex mouth-appendage designated as the proboscis. The chitinized 

 parts of the proboscis are far removed from the head-capsule, but in 

 this projection of the parts, the proximal ends of the chitinized ap- 

 pendages are joined together and have the same relationship with each 

 other as in generalized insects. 



The term proboscis is most applicable among the Cyclorrhapha to 

 those whose mouth-parts resemble those of Musca. The proboscis is 

 naturally divided into three areas by the two bends which it makes as 

 it is withdrawn into the oral cavity. The parts of the proboscis have 

 been given varied and confusing names. Hewitt divides it into two 

 general areas — the rostrum and the proboscis proper. He says: "The 

 proboscis consists of two parts, a proximal membranous conical por- 

 tion, the rostrum, and a distal half, the proboscis proper, which bears 

 the oral lobes. The term haustellum is also used for this distal half 

 (minus the oral lobes) and as a name it is probably more convenient, 

 as the term proboscis is used for the whole structure, — rostrum, haustel- 

 lum and oral lobes". 



The terms rostrum and haustellum have been used in various ways 

 by numerous workers in different orders; consequently the parts which 

 they designate are by no means homologous. A more comprehensive 

 set of terms based upon the word proboscis has been used by a few 

 workers, who divide the proboscis into basiproboscis, mediproboscis, and 



