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-tracheae 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-tracheae 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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