192 AN ESSAY ON THE DEVELOPMENT 



of the Tdbanidcu. Now it will be remembei-ed that in this genus I showed the con- 

 nection of all the labial parts with the mentum, where they normally belong; hence 

 all the other parts must be, of necessity, maxillary. So we find also in PI. II, Fig. 14, 

 that the central labellate structure, two of the piercing structures and the maxillary 

 palpi all arise from a single united basal sclerite, the stipes. 



In Eristalis tenax (PI. I, Fig. 3) these labellate structures are shown, turned 

 aside to expose the labial structures. Here also I showed the presence of labial palpi 

 in close connection with the ligula and hypopharynx, normally attached to the men- 

 tum, and again it follows that the other structures must be maxillai-y. Again also 

 I must call attention to the fact that the palpi are mere continuations of the enveloping 

 membrane, and that this membrane continues without break to the tip of the labella. 

 Unless we are to believe that a continuous membrane may give rise to both the maxil- 

 lary and labial palpi, we cannot possibly consider the labella as labial structure'^. 



I have now ti-aced out what seems to me a continuous development of the moditi- 

 cations of the subgalea and galea, and have shown, I think, that from Pteromalus in 

 the Hymenoptera to Eristalix in the Diptera, a continuous chain may be constructed, 

 requiring nowhere any change of character, function or location. Xo disassociation 

 from other maxillary structures and no connection with labial structures. 



In taking up tlie modifications of the palpifer I am confined almost entii'ely to 

 the Diptera, in Avhich this sclerite is best developed. In Bittacus I showed its devel- 

 opment to an elongated structure of no particular type or function and of about the 

 same texture as the galea. In Proniiha I showed its development into a highly spe- 

 cialized " tentacle," tactile and sensory as well as mechanical in character. In the 

 Diptera it is quite usually present as an elongated, rigid, chitinous organ adapted for 

 piercing. It occurs in all the piercing types and is present as a rudiment in many 

 othei-s. It undergoes a curious and interesting change in function as the Dipterous 

 mouth changes from the i)iereing to the scraping or lapping type, and as it becomes 

 fiexed. 



The simplest form occurs in those piercing Diptera in which the proboscis is not 

 flexed. Thus in the Buffalo gnat (PI. II, Fig. 9) it is a stout, semicylindrical piercing 

 organ, enlarged both at base and at tij), at which latter point it is also toothed. The 

 connection of the palpus with the subgalea was already shoAvn on PI. I, Fig. 1", and 

 this shows how the chitinous paljjifer forms part of the combination. The palpifer 

 arises, normally, outside of the galea ; yet at the tip it is found in connection with all 

 the other piei'cing structures inside of that organ. IIow it gets there is illusti-ated in 

 the Anglesea Simuliid (PI. I, Fig. 2"), whci-c all the maxillary i)arts are shown in 

 proper connection, and it is seen that the pal])iler enters the galear envelope in the 



