34 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I07 



of the cleavage line, nor is it a midcranial sulcus ; its margins are the 

 approximated frontal arms of the cleavage line along the sides of the 

 narrow invaginated part of the frons (F, fr). 



Further evidence that the triangular plate of the caterpillar's head 

 is the clypeus is seen in the fact that upon it (fig. 13 B, Clp) are 

 attached the facial muscles (cbmcls) that lie anterior to the frontal 

 ganglion and its brain connectives (FrGng). These preganglionic 

 muscles are the cibarial dilators, though the cibarium of the cater- 

 pillar (Cb) appears to be continuous with the stomodaeum, and in no 

 known insect are the cibarial muscles attached elsewhere than on the 

 clypeus. On the other hand, the postganglionic muscles (phmcls), or 

 precerebral dilators of the pharyngeal region of the stomodaeum, 

 take their origins on the narrow strips (fr) between the cleavage lines 

 and the apex of the clypeal triangle, and these muscles in all insects 

 are consistently frontal muscles. Finally the labral muscles, which 

 also characteristically arise on the frons, take their origins in the 

 caterpillar far back on the median invagination of the cranium above 

 the clypeus. All these skeleto-muscular relations, therefore, demon- 

 strate that the exuvial apotome of the caterpillar head represents the 

 frontoclypeal region between the arms of the cleavage line in other 

 holometabolous larvae. 



A rupture of the head cuticle does not take place at each ecdysis 

 with all caterpillars. Many species shed the head capsule entire 

 except at the last ecdysis, when the cranial cuticle splits in the usual 

 manner. The head exuviae of the younger instars may be detached, 

 or remain connected with the body skin. According to Frost (1922), 

 in the bud moth, Tmetocera ocellana Schiff., the head capsule comes 

 off separately at each ecdysis, the cuticular rupture taking place 

 between the head and the thorax. In the Hesperidae, Henriksen 

 (1932) says, "no opening of the head capsule takes place; the head 

 of the new instar is drawn back through the occipital foramen of the 

 old head capsule and out through the rent in the thorax." Usually 

 in such cases it is to be observed that the head of the new instar 

 develops in the thorax of the old skin before ecdysis. Hess (1937) 

 describes the moulting of Henierocampa leucostigrna (A. & S.), 

 Hyphantria cunea (Drury), and Malacosoma americana (F.) in all 

 of which apparently the head capsule is shed entire at each ecdysis 

 except the last, which takes place in the cocoon. 



Diptera. — For a study of the cleavage lines on the larval head of 

 lower Diptera, the mosquito furnishes a good subject, since many 

 species have been described and figured, and it is easy to observe the 

 fact that the ecdysial splits follow the lines ordinarily called the 



