1880. ] The Proboscis of the House-fiy. 157 
functions—t. The proboscis is an organ of suction. The 
cesophagus traverses the inferior central part of the fulcrum, 
thence passes through the mid segment in the canal made by the 
operculum and the axis piece, being here joined by a pair of sali- 
vary ducts; it then opens at the mouth, communicating with the 
false trachea. It can exude a drop of clear fluid from the salivary 
ducts, and when the proboscis is distended it can act as an organ 
of suction, receiving fluids from the false trachez and conveying 
them to the digestive organs. The large supply of muscles 
within the fulcrum and in the axis piece, appears to be subservient 
to this process. 
2. Retraction. — Two long and: powerful retractile muscles 
extend from the back part of the skull (near the foramen magnum) 
to the proximal end of the mentum (Fig. 1, 4, m). By contracting, 
these draw in the mid segment, so that its proximal end is close to 
the neck of the fly. Other muscles attached to the ventral proxi- 
mal processes of the fulcrum ‘assist in drawing it in and up, thus 
turning the fulcrum upon its upper proximal processes which are 
hinged to the frontal piece of the skull. Thus the two proxi- 
mal segments of the proboscis are folded on each other, and are 
both drawn inwards and upwards into the skull, so that they are 
like the letter V lying on its side, with its acute angle backwards. 
One arm of this V is hinged to the lintel of the door-way, whilst 
the other arm bears the collapsed tip of the proboscis, which now 
Serves as a door to close the entrance. The ends of the palps 
then protrude from the upper part of the doorway on both sides 
of the proboscis tip. ; 
3. Protrusion.—The part taken by inflation in extending the 
proboscis, is so obvious that it was suggested nearly a century 
ago by Gleichen, but the suggestion was rejected, and W. T. Suf- 
folk! infers that the structure of the interior of the head was 
unknown to Gleichen, “as the extension of the organ is attributed 
to inflation, and not to muscular action.” se 
It is easy to dispose of Mr. Suffolk’s hasty criticism. Immerse 
the head of the fly in caustic potash, which destroys the muscles, 
the chitine of the membranous sheath and the tracheal tubes 
remaining intact, and you can still protrude the organ by slight © 
Pressure. Further, when the proboscis is pressed out and all its 
parts distended, pierce with a needle the swollen air sacs under the 
1 Op. cit. 
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