1904] McCLENAHAN: — RHYNCOPHOROUS COLEOPTERA. 95 



Alimentary Canal, Salivary Glands, etc. 



These have considerably altered the proportions they have in the larva. The 

 salivary glands are not the same, but a new development ; new armature has been 

 differentiated in the walls of the proventriculus, and new tracheal branches have 

 penetrated the rostrum. 



The pharynx passes over into the esophagus (Plate IX, fig. 14, oe) which 

 extends as a slender tube back into the prothorax and there abruptly widens into 

 a barrel shape proventriculus {pv) . This in Mononychus is lined with eight double 

 rows of transversely placed sharp edged plates of chitin ; each row containing 

 about sixty plates. The ventral wall of the pharynx (Plate IX, fig. 17) is made 

 of thick chitin and is incapable of contracting or expanding. That is the function 

 of the dorsal wall which is lined with but a thin coat and covered with prickles. 

 The short elevator muscles (Plate IX, fig. 14 w, and figs. 16 and 17) are attached 

 in two longitudinal rows to this wall and aid in swallowing by drawing out the 

 folds of the dorsal wall and enlarging the cavity. Around this side are several 

 rows of semi-circular muscle bands which contract it again into its position of 

 rest. The esophagus is wholly surrounded by circular muscles. 



Close beside the mandibular abductor tendons lie a pair of simple tubular 

 salivary glands which extend directly through the rostrum to the dorsal part of 

 prothorax where their ends are twisted with many convolutions (Plate IX, fig. 

 14; s.gL). 



A single pair of tracheae distribute their branches through the beak. 



IV. Metamorphosis. 



External Features of the Metamorphosis of the Head. 



A comparison of larva and imago shows great changes in both internal and 

 external anatomy. The larva merely exists to eat and grow and accumulate fat for 

 its metamorphosis. It lives a sequestered, inactive life and only the organs neces- 

 sary for such a life are developed. The adult, on the contrary, lives the active life 

 of ordinary beetles. It possesses wings, legs, eyes and antennae in addition to 

 the larval structures. 



The transformation from the degenerate larva to the highly organized adult 

 beetle, whether it occurs immediately at the end of growth, as in Mtmonychus, or 

 after a long period of quiescence with suspension of all vital activity, as in Bala- 

 ninus, when it does begin takes place rapidly. 



