B. T. LOWNE ON THE PROBOSCIS OF THE FLY. 131 



phagcal tube acts as a valve, closing the opening in the labium, by 

 which the labial tube communicates with the large tracheal duct, 

 before mentioned, for although this has not been demonstrated by me, 

 its form and position would enable it to act in that manner, and if such 

 be the case, the labial cavity and tube, as well as the cavity of the 

 sucker, would continue to draw in fluid as long as the valve re- 

 mained closed, a single exhaustive effort on the part of the insect 

 enabling it to maintain suctorial power for some seconds. 



A kind of rhythmic muscular action of the lips assists the ascent 

 of the fluid into the labial tube, from which it is drawn by the 

 open corneous oesophageal tube. 



Gleichen long ago noticed the copious escape of air from the 

 opening between the lips of the fly's proboscis, when the body of the 

 insect is subjected to pressure ; he supposed that its purpose was the 

 inflation of the whole organ, which he considered as an erectile one. 

 It is a curious coincidence, that I thought the same thing myself 

 when I commenced the investigation of the subject, substituting a 

 fluid for a gaseous agent, and expected for a long time that I should 

 discover a special contractile sac for effecting this. I can confi- 

 dently state now, however, that nothing of the kind happens, and 

 that the exsertion of the proboscis is entirely a muscular act. 



"Within the labial cavity, and inserted into the triangular opening 

 of the labial tube, are two rows of rods, bidentate at their ex- 

 tremities. I had never been able to make out their import, 

 until Mr. Suffolk informed me that he has examined sugar on 

 which flies had been feeding, and that it was striated with lines 

 which, when measured by a micrometer, corresponded exactly to the 

 distance between these teeth. I have never observed this myself, 

 but can account for not having done so, as I have always used 

 young flies, which are plentifully supplied with fluid in their abdo- 

 minal crops with which they melt the sugar ; in these the labial 

 teeth are comparatively soft. I can quite understand that when 

 the abdomen is heavy with the developed ovaries, or testes, that the 

 abdominal stomachs are comparatively reduced, then the labial 

 teeth, hardened, like all the chitinous structures, by age, would 

 afford powerful auxiliaries to the solvent action of the saliva. 

 They would be brought into play by the opening of the fissure 

 between the lips, for which a special muscle is found on either 

 side. 



Another set of organs is found upon the inner surface of the 



