No. 429.] THE MOUTH PARTS OF INSECTS. 699 
those families by common agreement held to constitute the 
more generalized portion of the order, —I came to the conclu- 
sion that the old and most widely, if perhaps uncritically, 
accepted interpretation of the homologies of the dipterous one 
is the true one. This interpretation homologizes the labella- 
bearing proboscis common to all the more specialized flies with 
the labium of other insects, finds the maxillae represented in 
these specialized forms chiefly or only by a pair of palpi, and 
finds the mandibles wholly wanting in all but the females of a 
few families. In the case of most of the nematocerous families 
the labium retains a truly labiumlike character and has not 
developed the pseudotrachez-bearing labella, while the max- 
illa are represented by a well-developed bladelike terminal 
lobe as well as by the palpi. The mandibles when present are 
of the character of elongate blades or stylets, never of the 
Character of true crushing or biting jaws. The structural 
character of the mouth in each of the nematocerous families 
is described and illustrated in my series of papers (1899) in 
Psyche, previously referred to. 
But several interpretations of the homologies of the mouth 
` parts widely at variance with the above have been offered. In 
these various interpretations the possession of mandibles by 
any flies at all is denied; the so-called labium is considered to 
be composed of modified parts of the maxilla, and the so-called 
maxillae are believed to be parts of the labium ; in fact, most of 
the possible changes which an active speculation could invent 
have been rung on the theme. Nor are these interpretations 
based on mere speculation ; they are the results, in several cases, 
of prolonged and disinterested examination of considerable 
series of specimens. 
In the face of such differences of opinion, and with the 
apparent limits of the method of the comparative study of the 
fully developed mouth parts of various members of the order 
reached, it becomes imperative to seek the clue to these lost 
homologies in the facts of development. And this is really the 
first object of this present study. Can the homológies of the 
dipterous mouth parts be discovered by the study of the devel. 
opment of the parts? 
