NO. 7 THE INSECT CRANIUM SNODGRASS 33 



the frontal and clypeal areas embraced by the cleavage lines. In two 

 species examined by the writer, however, the anterior tentorial pits 

 lie mesad of the arms of the cleavage line (fig. ii D), though in a 

 species figured by Cook (1944) the pits are seen to lie in mesal angu- 

 lations of the arms. Inasmuch as it is shown by Cook that the labral 

 muscles and the hypopharyngeal muscles (mouth-angle retractors) 

 arise above the level of the tentorial pits, while the cibarial muscles 

 are on the widened area below the pits, it is evident that the region of 

 the apotome is frontoclypeal, though Cook regards the whole triangle 

 as the "clypeus." 



Lepidopiera. — The head of a caterpillar is characterized by an 

 upward extension of the clypeus in the form of a triangle on the 

 lower part of the face (fig. ii E, Clp), and by a complete invagination 

 (frinv) of the upper part of the frontal area embraced by the arms 

 of the cleavage line (CL). Below^ the invaginated part of the frons, 

 the arms of the cleavage line diverge and run down along the sides 

 of the clypeus to the mandibular articulations, thus defining a V-shaped 

 frontal area (fr) inverted over the apex of the clypeus. The two 

 arms of the frontal V are commonly termed the "adfrontals" by 

 students of lepidopterous larvae, who have long regarded the tri- 

 angular plate of the face as the "frons." A new interpretation by 

 DuPorte (1946) relegates the clypeus to the conjunctiva which 

 attaches the labrum to the cranium, and defines the facial triangle as 

 an "antefrons," the groove bounding the apex of the triangle above 

 the anterior tentorial pits being regarded as a "transfrontal suture," 

 and the parts below the pits as "frontoclypeal sutures." 



The several conflicting opinions that have been held concerning 

 the homologies of the external lines and enclosed areas of the cater- 

 pillar head well illustrate the diversity of interpretation that may 

 arise from a study of external features alone. Plausible evidence 

 may be deduced from superficial characters for at least three inter- 

 pretations, but, for this very reason, none of them is convincing. 



The true structure of the median facial parts of the caterpillar 

 cranium is revealed at the final ecdysis. On the head exuviae (fig. 

 II F) the frontal clefts follow the arms of the cleavage line (E, CL) 

 from the top of the head to the base of the clypeal triangle. Between 

 the separated lateral hemispheres of the cranium there is now exposed 

 the long apotome (F, frcapt), which is seen to include the triangular 

 clypeus (Clp) and both the exposed and the invaginated parts of the 

 frontal component (fr). The same structure can be demonstrated 

 by treating the intact head with caustic. The median groove of the 

 caterpillar's cranium (E, frinv), therefore, is not the coronal stem 



