194 AX ESSAY ON THE DEVELOPMENT 



this disappears and the chitinous rod is entirely disassociated from the palpus. Finally 

 in Stomoxys calcitrans (PI. II, Fig. 12) there remains nothing to indicate the existence 

 of any i-elation between the slender chitinous rod and the distant maxillary })alpus. 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 I'emains 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 Hynienoptei-a 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 (PI. I, Fig. 

 2") its relation to the other maxillary parts is shown. In PI. I, Fig. 1*^, is illustrated 

 the connection between the palpi fer and lacinia in the Simulkim sent me by Mr. 

 Aldrich. This connection is not fonciful but actual, and no sclerite so intimately con- 

 nected with an admitted maxillate structure can be anything but maxillary. 



Again in CJirysojJS (PI. II, Fig. 14) I have illustrated the fact that all the struc- 

 tures which I consider maxillary have a common 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 maxillar}^ sclerite, why should it be modified to 

 accommodate the maxillai-y 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 jialpus V 



One of the most serious ditiiculties 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 theoi-y 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 fiicts. Their functional character never 

 changes ; they simply dwindle from disuse and gradually disappear. So we find them 

 in the Lepidoptcra 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 Slmulium. 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 



