l8 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 95 



The ventral muscles consist of two pairs, the retractors of the pre- 

 mentum (rst) and the ventral adductors of the labium {2adlh). The 

 retractor muscles, as is their custom, originate from the postmental 

 sclerite and insert on the base of the second prementum. The ventral 

 adductors also arise on the postmentum, instead of on the tentorium. 

 Their point of origin is almost directly posterior to that of the retrac- 

 tor muscles. As is the usual condition, they insert on the base of the 

 first prementum. 



Byrrhidae (pi. 5, G). 



Since this family shows a type of labium which might be considered 

 generalized for coleopterous larvae, it was adopted as a basic example 

 (pi. I, D). It shows the three divisions of the labium definitely, the 

 first prementum (pi. 5, G, IPrmt), the second prementum (IlPrmt), 

 and the postmentum (Pmt). The two segmented palpi, conforming 

 to the general condition, are borne on the first prementum. The latter 

 is marked ofif from the second prementum by a membranous ring, and 

 the second prementum is separated from the postmentum in a similar 

 manner. 



Inserted on the base of the first prementum are found the ventral 

 adductors of the labium {2adlb). They originate on the cross bar 

 of the tentorium {Tent). The retractors of the prementum {rst) 

 insert on the base of the second prementum, with their origin on the 

 postmental sclerite. 



Helodidae (pi. 5, H). 



The labium of Prioncyphon consists of but two well-marked divi- 

 sions. Most of the labium is made up of a large prementum (Prmt), 

 which bears several pairs of scattered setae. From its anterior margin 

 arise the palpi, which are rather small in comparison with other larvae. 

 Between the prementum and the posteroventral margin of the head 

 capsule is the postmentum (Pmt). At its posterolateral margins are 

 the posterior tentorial pits (pt). 



There are two pairs of ventral muscles, both of which insert on 

 the prementum. The retractors of the prementum (rst) originate 

 from the tentorial arms and go diagonally to the base of the pre- 

 mentum where they insert close together in the midline. The ventral 

 adductors of the labium {2adlh) are also present. They originate, 

 however, from the postmentum. Their point of insertion is slightly 

 anterior of the center of the premental sclerite. Thus in this insect 

 the origins of the two pairs of ventral muscles are reversed from the 

 more primitive condition as found in the roach (pi. i, B). 



