256 ANATOMY. 



two first (PI. VI. f. 5. 1.) lie in the .anterior margin of this mem- 

 brane, in a transverse direction to the proboscis, but linearly with 

 respect to each other, directly behind the mentum. From the exterior 

 ends of each of these two pieces there originates a similar (2) bone, 

 which extends posteriorly upwards, the point of which touches a third 

 (3) bone, which furcates and descends from here to the posterior end 

 of the membrane. Both the prongs of the fork join at their ends a 

 fourth (4) uneven main bone, which lies transversely at the end of the 

 membrane, and opposite to the two first, which lie immediately behind 

 the mentum ; the fifth paired main bone (5) originates likewise at each 

 end of this fourth unpaired bone, and runs at the margin of the mem- 

 brane close to the horny aperture of the head. All nine thus construct one 

 valve, the anterior lobes of which are formed by the two first transverse 

 and anterior lateral bones, and the posterior lobes by the second lateral 

 bones, the fourth transverse and the two marginal bones originating 

 from its end. The articulation takes place at the point of con- 

 nexion of the two second and third bones. If the mentum (the same, a.) 

 be withdrawn, the membrane and bones lie like a valve together, but if, 

 on the contrary, the suctorial apparatus be distended, the membrane is 

 stretched out by means of the bones, and these push the chin forward be- 

 fore it. The motive apparatus of the butterflies is much more simple ; 

 in them a double band-shaped muscle runs along each half of the pro- 

 boscis, which clothes the entire cavity, leaving merely a narrow central 

 canal. Both these muscles roll up and distend the proboscis, and also 

 unite it with the head, inserting themselves partially upon the horny 

 wall, and partly upon the, indeed very small, internal frame-work of 

 the head. The smallness of their head arises from the disappearance 

 of the muscles of the mandibles. The same may be maintained of the 

 Hemiptera ; they also have but delicate muscles, which elevate and 

 withdraw the sheath, as well as still smaller ones, which rein the setae. 

 The Diptera, although they have in general a large head, derive it 

 from the preponderance of their eyes, for the muscles which pass to 

 their mouth are likewise abortive ; the fleshy proboscis alone, which we 

 consider as the labium, receives two large and tolerably broad band- 

 shaped muscles, which originate from two ridges placed internally 

 over the aperture of the mouth, and arched from the cheeks to the 

 clypeus, and which extend also to the apex of the proboscis. They 

 withdraw the proboscis within its cavity, and are therefore called the 

 extensors of the haustellum. 



