24 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



sclerite on which two setae are situated near the midUne. The second 

 prementum (IlPrmt) makes up the major portion of the labium. 

 Covering much of the median part of this division is a sclerite, near 

 the anterior margin of which are located two setae. The postnientum 

 has become reduced, even more so than in Cryptocephalinae (B), and 

 does not show in the figure. The labium in this group is very promi- 

 nent and protrudes from the ventral level of the head as a large flap. 

 The postmentum has become adapted for connecting the posterior 

 margin of the second prementum with the neck membrane and extends 

 more or less vertically. 



The ventral muscles consist of two pairs, the ventral adductors of 

 the labium (D, 2adlh) and the retractors of the prementum {rst). 

 The former originate from the tentorium {Tent) and insert on the 

 sclerite at the base of the first prementum. The retractors also arise 

 from the tentorium, medially to the origin of the ventral adductors, 

 and insert in about the center of the sclerite of the second prementum. 



III. THE TENTORIUM OF COLEOPTEROUS LARVAE 



The tentorium of insects in general is of considerable interest, and 

 a few examples of the structure as found in coleopterous larvae are 

 included here. The probable evolutionary development of this internal 

 "skeleton" has been given by Snodgrass (1935) and no repetition 

 of the present-day views will be included in this paper. 



The tentorium of apterygote insects at least is composed of two 

 pairs of braces or arms. One pair consists of invaginations from 

 the anterior tentorial pits. The second pair of arms results from 

 invaginations from the posterior tentorial pits. These component 

 parts may be united in various ways, as Snodgrass has shown (1935, 

 fig. 62), or the four parts may be entirely separate and greatly reduced. 

 Oftentimes there is, in addition to the two pairs of arms above men- 

 tioned, a third pair of dorsal arms. These, however, are considered 

 to be secondary outgrowths of the anterior arms and not invaginations 

 from the dorsal wall of the head, since their connection with the head 

 is usually membranous or at times entirely lacking. 



In addition to its function of bracing the walls of the head, the 

 tentorium serves as a very important region for muscle attachments. 

 From it usually originate the adductor muscles of the maxillae and 

 the labium, the retractors of the hypopharynx and the ventral dilators 

 of the stomodaeum. The antennal muscles are also attached to the 

 tentorium, usually to the dorsal arms when present. 



Of the tentoria of coleopterous larvae which were studied, that of 

 Silpha is the most nearly similar to the generalized type. The pos- 



