398 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 37 



the plicae orales, but also by "ingrowth" of the mandibles and maxillae 

 into the head. It is true that Philiptschenko (1912, p. 578) expressly 

 states, in accordance with Folsom, that "die Mundorgane werden 

 durchaus nicht in das Innere des Kopfes verlagert," but I think that 

 the final position of mandibles and maxillae in Diplura and Protura 

 proves this ingrowth, also indicated in Collembola by the nearness 

 of the mandibles to the roof of the head ; and furthermore the position 

 of cardo dorsal to fulcrum — if it is in fact the cardo, as I shall 

 discuss later — in my opinion supports this view. In this way, out of 

 the hypognathous head is shaped a prognathous one in a manner 

 entirely different from that by which prognathous heads are formed 

 in the other insect groups. The formation of the fulcrum and the 

 "dissolution," so to speak, of the mandibles and maxillae, are direct 

 consequences of these changes. 



3. THE HYPOPHARYNX AND FULCRUM 



The hypopharynx and the fulcrum are two independent structures, 

 though it might seem as though the fulcrum is only a skeleton sup- 

 porting the hypopharynx. This last-named structure is present only 

 in Diplura and Collembola ; in Protura I have found no trace of it, 

 though Prell (1913) depicts "ein kielformiges Mittelstiick" cor- 

 responding to the lingua and even "eine Reihe feiner Chitinstabchen" 

 corresponding to the superlinguae. The hypopharynx in the other two 

 groups is very much alike, consisting of a membranous lingua ven- 

 trally and two superlinguae more or less toothed on the inner edge. 

 The superlinguae are supported by branches from the fulcrum. 



The fulcrum itself consists of two retrograde diverging stalks 

 proximally biramously divided into a short median branch and a 

 longer lateral one reaching the lateral walls of the head. To this 

 lateral branch a rod is attached by a ligament. This rod is commonly 

 regarded as the cardo of the maxilla, but it should be noted that it is 

 placed dorsally to the lateral branch of the fulcrum, so if it really 

 is the cardo, this would indicate that the maxilla during development 

 must have grown backward "into the head." It is very much alike in 

 all groups and gives attachment in its distal part to muscles, which 

 fact might support the conception of it as a cardo. 



The shape of the fulcrum is very much alike in Diplura and Col- 

 lembola, whereas the two branches in Protura coalesce in the middle 

 line. Branches are given off anteriorly, supporting the superlinguae in 

 Diplura and Collembola (figs. 6 and 10), and in Protura to the sides 

 of the "clypeus" ; in Diplura the clypeus has a supporting sclerotiza- 



