SOFT PARTS AND MECHANISM OF THE PROBOSCIS. 399 
system ; and the elasticity of the compressed air, acting against 
the tension of the tendinous cords on the one side, and the 
elastic recoil of the pseudo-trachez on the other, is, I have 
no doubt, the means by which the varied movements of the lips 
are mainly produced. Kraepelin admits that muscle-fibres are 
absent in the lips, except a few transverse bands near their 
anterior border, and these I have failed to recognise, although 
fibrous, probably elastic, bands of tissue are present. 
It will be seen that the injection of air into the vessels between 
the tendinous cords and the pseudo-tracheze would render the 
disc tumid and convex ; the tension of the paraphysal muscles, 
on the other hand, acting on the tendinous cords, would draw 
the centre of the disc back and render it concave. 
The Act of Suction—The movements of the surface of the oral 
sucker during the act of suction were most graphically described 
by Reaumur. He says: ‘ There is no one who has not seen 
flies apply the extremity of the proboscis or their lips to 
syrup or sugar thousands of times, but few have seen the act 
well, yet the method of doing so is very simple. I spread a 
little transparent syrup on the inner surface of the thin glass 
cover of a small box, and enclose flies of different kinds. These 
are so greedy for the syrup that they forget their captivity, and 
there are always some which at once alight upon their favourite 
food. If one observes these, he will see that they commence by 
extending the proboscis and applying the channelled surface of 
their lips to the syrup; the glass cover and the transparent 
syrup do not prevent a careful observation with a lens of what 
occurs at the extremity of the proboscis. I invite the curious 
to give themselves to this observation, with which they will 
assuredly be satisfied, as I have been many times.’ 
‘Whilst the body of the proboscis is immobile its extremity is 
active, and one observes the most varied and rapid movements 
of the lips. The short transverse diameter of the disc is 
alternately increased and diminished. At one moment the two 
halves lie in the same plane, and at another they make an 
angle with each other which varies from moment to moment 
from an acute to an obtuse, and from an obtuse to an acute 
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