NO. 13 LABIUM OF COLEOPTEROUS LARVAE ANDERSON 9 



Dytiscidae (pi. 2, E). 



The labium of Dytiscus sp., as in other representatives of the 

 Adephaga, has the prementum (Prmt) and the mental (Mt) subdi- 

 vision of the postmentum distinguishable from the head capsule. The 

 remainder of the postmentum, i. e., the submentum (Smt), has become 

 immovably united with the head. The prementum bears the palpi. 

 The mentum is short and does not show in a live specimen when 

 the prementum is retracted. Korschelt (1924) states, in regard to 

 Dytiscus marghmlis, that, " Das Mentum is rudimentar. Das Sub- 

 mentum fehlt ganz, und ebenso ist die Ligula voUstandig riickgebildet." 

 The submentum (Smt) as a separate sclerite has disappeared, but it 

 is interpreted as lying anterior to the tentorial pits (pt). 



The muscles inserting on the prementum consist of two pairs of 

 adductor muscles. The ventral adductors (2adlb) originate on the 

 tentorium, at or close to the point of invagination of the latter, and 

 insert on the base of the prementum. Their points of insertion are 

 closer to the midline than is the case in many larvae. The dorsal 

 adductors of the labium (ladlb) originate also on the tentorium but, 

 contrary to their custom, they insert ventrally on the base of the 

 prementum. Proof that these are really the migrated dorsal adductors 

 lies in the fact that their point of origin is, as usual, ventral to that 

 of the ventral adductors. Furthermore, the point of insertion of the 

 ventral adductors has migrated perceptibly toward the midline of the 

 base of the prementum, allowing room for the dorsal adductors. 



Gyrinidae (pi. 2, F). 



In Dineutes the two parts of the labium that are present and sepa- 

 rate from the head capsule are the mentum (Mt) and a split pre- 

 mentum (Prmt). The submentum (Smt) has become completely 

 fused with the skeleton of the head and is not distinguished from the 

 latter by any indication of a suture. The prementum in Dineutes is 

 unique among the larvae studied in that it is very deeply furcate in 

 the midline. The two parts have the appearance of basal segments 

 of a three-segmented palpus, but their musculature proves them to 

 be halves of a cleft prementum. The mentum (Mt) is better developed 

 than it was in the larvae previously discussed. 



The muscles of the labium are the two pairs of adductor muscles 

 which usually insert on the prementum. The ventral adductors (2adlb) 

 originate on the submentum, apparently having migrated there from 

 their more primitive position on the tentorial arms. They insert on 

 the bases of the halves of the prementum, and to judge from their 

 position, no doubt act as depressors of the palpuslike structure, which 

 is made up of a palpus (Pip) and one-half the prementum (Prmt). 



