192 AN ESSAY ON THE DEVELOPMENT 
of the Tabanide. Now it will be remembered 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 Pl. 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 Hristalis tenax (Pl. 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 maxillary. 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 structures. 
I have now traced out what seems to me a continuous development of the modifi- 
cations of the subgalea and galea, and have shown, I think, that from Pteromalus in 
the Hymenoptera to Hristalis in the Diptera, a continuous chain may be constructed, 
requiring nowhere any change of character, function or location. No disassociation 
from other maxillary structures and no connection with labial structures. 
In taking up the modifications of the palpifer I am confined almost entirely to 
the Diptera, in which this sclerite is best developed. In Bettacus 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 Pronuba 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 
others. It undergoes a curious and interesting change in function as the Dipterous 
mouth changes from the piercing to the scraping or lapping type, and as it becomes 
flexed. 
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 tip, at which latter pomt it is also toothed. The 
connection of the palpus with the subgalea was already shown on PI. I, Fig. 1%, and 
this shows how the chitinous palpifer 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 piercing structures inside of that organ. How it gets there is illustrated in 
the Anglesea Simuliid (Pl. I, Fig. 2"), where all the maxillary parts are shown in 
proper connection, and it is seen that the palpifer enters the galear envelope in the 
