8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 97 



Mechanism of coiling and extension. — It should now be ixDssible to 

 understand the functions of these muscles. A study of figure 2 shows 

 that by their contraction, the anterior tentorial proboscis extensors 

 draw the tubular part of the stipes up against the recurved end of the 

 gena. (This action is shown diagrammatically in fig. 2 A.) It will be 

 noticed that there is a valve arrangement between the tubular part of 

 the stipes and the flat sclerite on which the muscles are inserted. As 

 the muscles draw the stipes upward, the valve {vlv) closes, with the 

 result that the tubular part becomes a closed cylinder. Thus pressure 

 is exerted against the blood within the stipes cylinder as it is forced 

 against the recurved flange of the gena. The stipes cylinder forms a 

 closed point at its proximal end, and therefore the blood displaced as 

 the pressure continues must move outward through the stipes, toward 

 the proboscis. The stipital ridge is enlarged at the proximal end of the 

 stipes and thus practically covers the lumen of the proboscis unit. The 

 posterior tentorial proboscis extensor is inserted on this ridge, and con- 

 traction of this muscle not only creates pressure on the blood enclosed 

 within the stipes, but also moves the base of the proboscis unit upward, 

 which effects a tight seal with the functional mouth (fig. 2 G, H ; 9 D) . 



The blood displaced from the stipes is thus forced out into the lumen 

 of each tightly coiled proboscis unit, thereby causing the proboscis to 

 unroll. The diagonal muscles within each proboscis unit, described by 

 Burgess, cause the proboscis to coil. That blood pressure might be the 

 agency for uncoiling the proboscis was first suggested to the writer by 

 R. E. Snodgrass, who, in his " Principles of Insect Morphology " 

 points out the mechanical analogy of such a mechanism with the toy 

 paper snake which a child uncoils by blowing into it. The uncoiling 

 action of one proboscis unit is shown diagrammatically in figure 4. 



The mechanism described above is the simplest which the writer has 

 seen. In many moths and butterflies the stipital cylinder is further 

 modified, but the principle is invariably the same, as may be seen in 

 figure 2. The musculature concerned in the extension of the proboscis 

 seems to be fundamentally the three pairs of muscles described, but 

 in a large number of insects one or two pairs may be absent. However, 

 functional maxillae always have at least two pairs. 



Comparative structure in lepidoptcrous families. — The maxillae of 

 a number of species representing the more important families were 

 examined, primarily to determine the fundamental musculature of the 

 lepidoptcrous maxilla. Moths having degenerate or obsolete mouth- 

 parts were also studied and, indeed, proved to be one of the most in- 

 teresting phases of this investigation. To expose the proboscis ex- 

 tensor musculature, a simple procedure is first to make a complete 



