160 The Proboscis of the House-fly. [ March, 
mid segment and the tip, we have still the largest structure of all 
(the fulcrum) to explain. It seems to be general in Diptera; even 
the mosquito possesses it; in other insects it is unknown. It could 
not be what Prof. Huxley suggests, “the labrum, mandibles and 
maxillz coalescing;”’ at least its structure and forms in various 
Diptera give no evidence of such union, and how then are we to 
explain the mid segment with the great tendons? Mr. Lowne 
makes it a composite structure, the dorsal part being epistoma, 
and the ventral part pharyngeal, formed in the wall of the aliment- 
ary canal. This explanation will not satisfy, for the inner surface 
of the fulcrum has many muscles, which could not be there if it 
were only a chitinous lining of the cesophagus. One might as 
well expect to find muscles growing on the outside of a lobster as 
within its throat. 
In searching after the homology of this piece, I soon found that 
I must go outside the Diptera, nor was I long searching till the 
secret came out. Opening the head of a katydid and of a wasp, 
I found in both what I wanted; the endocranium, which runs 
from back to front of skull, strengthening it. Long ago Burmeis- 
ter informed us that the Diptera have no endocranium, but their 
skulls are as empty shells, easily fractured. But here we see that 
Burmeister was wrong; they have the endocranium in the proper 
position when the insect is being hatched and when its proboscis is 
withdrawn ; but instead of having it rigidly fixed in the skull, they 
have it free posteriorly, hinged in the front and able to swing out 
so as to form a pedestal for the mouth parts which make up the 
proboscis. 
Comparing Mr. Huxley’s excellent description of the endocran- 
ium of the cockroach} we find the relation of parts with the retract- 
ed proboscis of the house-fly to correspond exactly. The endo- 
cranium has axis and wings corresponding to the structure of 
the fulcrum (Fig. 3). Its posterior extremity close to the foramen 
magnum, and the cesophagus pierces it; so with the house-fly 
when the fulcrum is turned in. The great tendons of the mandi- 
bles are right and left of it,as we have seen them to be in the 
house-fly. What is true of the house-fly is, we believe, generally 
true of its order. 
Thus we have fallen upon a modification of structure dependent 
on metamorphosis of function, almost as striking as that which 
1 Anat. Invert. An., p. 403-404 (348 of American edition). 
