36 ILLINOIS BIOLOGICAL MONOGRAPHS [206 



embedded in the membrane. These strips extend cephalad from their 

 caudal connection to the cephalic margin of the membrane about the 

 mouth-parts. At this point, where distinct tendons are attached, they 

 turn abruptly ventrad and become free appendages. All connection 

 between the mandibles and the invaginations of the anterior arms of 

 the tentorium is lost. The relationship between the tentorium and the 

 mandibles has not been observed in Culicoides for the lack of material. 

 No other families of the Diptera outside of those to which the above- 

 named genera belong, so far as observed, possess true mandibles or 

 rudiments of the same. When mandibles are present, they are always 

 of considerable size and probably functional. 



A number of investigators have described mandibles for many 

 species not included in the above families. Langhoffer (1901) describes 

 mandibles for the Dolichopodidae which are shown in this paper to be 

 modifications of the epipharynx (Fig. 524 and 528). The apodemes 

 of the muscids (Fig. 304, 308, and others) have been called mandibular 

 tendons by MacCloskie and others. This is incorrect as shown by the 

 figures and in the discussion of the maxillae. A number of workers 

 (e.g., Wesche, 1909) believe that the mandibles have united with the 

 labium and exist as chitinized strips on the cephalic aspect of the 

 labium or as thickenings on the meson of the theca. Neither of these 

 interpretations can be accepted when one takes into consideration the 

 relative position of these so-called mandibles and the manner of devel- 

 opment of the proboscis of the Calyptratae. The chitinized thickenings 

 on the cephalic aspect of the labium are located caudad of the maxillae 

 and the hypopharynx. This does not agree with the position of the 

 mandibles of other insects. Furthermore, these thickenings are present 

 in Tabanus where true mandibles occur. The chitinized thickenings 

 on the meson of the theca in some of the Diptera can not be considered 

 as rudiments of mandibles for many reasons. The most conclusive 

 objection to this interpretation lies in the fact that these thickenings 

 are very prominent in Simulium which has distinct mandibles in both 

 sexes. 



When interpreting mouth-appendages, it is always necessary to 

 take into consideration the generalized relationship between the mouth- 

 parts and their association with the invaginations of the tentorium. 

 It is also very desirable to observe a large series of forms before attempt- 

 ing to homologize the parts. The above interpretations were apparently 

 not made from either of these vantage-points. 



Maxillae. — ^All Diptera having functional mouth-parts have max- 

 illae. They are, however, greatly reduced and modified in some genera,, 

 and at first glance bear little or no relation to the structure or location 



