194 AN ESSAY ON THE DEVELOPMENT 
this disappears and the chitinous rod is entirely disassociated from the palpus. Finally 
in Stomoxys calcitrans (PI). Il, Fig. 12) there remains nothing to indicate the existence 
of any relation between the slender chitinous rod and the distant maxillary palpus. It 
is not in the least strange that guesses as to the character of this structure in Musca 
domestica should have been so often wide of the mark; though with a proper series as 
now shown, its origin is clear. 
There remains to be accounted for the lacinia, and this in the Diptera is the flat, 
blade-like structure generally identified as the mandible. It has been shown that 
while the lacinia is often the dominant organ in many mandibulate insects, the tendency 
is, on the whole, to a decrease in size, ending in the Hymenoptera in its entire elimina- 
tion. In the Diptera it is present in the blood-sucking species only, and it may be 
identified by its position and its relation to the other maxillary structures. It has 
been several times referred to incidentally, and in the Anglesea Simuliid (Pl. I, Fig. 
2") its relation to the other maxillary parts is shown. In PI. J, Fig. 1%, is illustrated 
the connection between the palpifer and lacinia in the Simuliwm sent me by Mr. 
Aldrich. This connection is not fanciful but actual, and no sclerite so intimately con- 
nected with an admitted maxillate structure can be anything but maxillary. 
Again in Chrysops (Pl. II, Fig. 14) I have illustrated the fact that all the struc- 
tures which I consider maxillary have acommon origin. At Fig. 14* I show the lacinia 
alone, and it is to be noted that at the base it is modified for attachment with reference 
to the palpus. Now unless this is a maxillary sclerite, why should it be modified to 
accommodate the maxillary palpus? Does it not seem rather absurd to believe that 
this can be a mandible brought to originate from one point with the palpifer and modi- 
fied to allow it to envelope at base the maxillary palpus ? 
One of the most serious difficulties in the way of the proper understanding of the 
mouth parts of haustellate insects has been the desire to provide for the mandibles on 
the theory that they are among the permanent structures. Yet I cannot understand 
why this should necessarily be the case. When functional, mandibles are essentially 
chewing or biting organs, and when the insects do not require such structures, it seems 
to me most natural that they should become obsolete: and that is exactly what has 
occurred according to my reading of the facts. Their functional character never 
changes; they simply dwindle from disuse and gradually disappear. So we find them 
in the Lepidoptera as mere rudiments, connected with a highly specialized maxilla ; 
and in the Rhynchophora they are sometimes mere remnants, occasionally reversed in 
position—exactly as I pointed them out in Simulium. I think that in view of all the 
evidence presented by me, none of the piercing organs of the Diptera can be consid- 
ered mandibles, and I cannot even yet, after carefully weighing all that Dr. Packard 
