158 The Proboscis of the House-fly. [ March, 
tip, and at once the tip collapses upon the mentum.’ If you tear 
the membrane about the base of the proboscis that part collapses. 
If you press the head over much, the membrane-sheath sends out 
bulging processes which soon burst, sending bubbles of air 
through the water in which you are examining it. 
I have repeated these experiments so often as to be satisfied 
that the rich tracheal system which crowds the lower part of the 
cranial chamber is the chief agent in protruding the proboscis? 
The examination of the muscular arrangement justifies this con- 
clusion. Muscles cannot directly protrude anything, they only 
pull. In the fly they may and do aid in protruding the proboscis 
by swinging. out the fulcrum. The long muscles which retract 
the mentum aid in straightening the proboscis when it is pro- 
truded, but the mentum is not attached proximally to any hard 
structure, and its firmness and power of supporting the tip depends 
on the tense condition of the membrane in which it lies, and this 
tenseness is due to inflation 
The great tendons which run back from the opercular piece 
(Fig. 3, 2) have their tips united by muscles to the distal and the 
sub-distal processes of the fulcrum (f). Lowne understands these 
muscles to be flexors of the mid segment upon the basal segment. 
Their tendency on contracting would be rather as extensors; but 
both suppositions are wrong. When they contract, instead of 
flexing the mid segment, the great tendons themselves bend, for 
they are too weak and too slightly articulated to the operculum 
to stand much pulling. Their work is of a more delicate nature. 
By acting alternately on the tendons, ‘these muscles bend the tip 
of the fly from side to side, enabling this organ to move nimbly 
from place to place, as you may see it when foraging on your 
breakfast table. This mechanism is well developed in Stomoxys, 
where only the basal part of the probascis is protrusible. We 
have already seen that the muscles extending from the mentum to 
the divergent rods which embrace the tip, serve to expand the 
lips to their fullest width; at the same time the tips become 
tensely swollen by air. 
It occupies this position when the proboscis is withdrawn, but never so in thé 
living fly when the proboscis is protruded. Most of the Tr in books represent 
it in pasts aI state, probably drawn from dead specim 
x e this discovery before I was aware that Gleichen had fallen upon it 
so long ago. a rejection of his views may — bag so little attention has uee 
p to it by others. 
