412 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I37 



formed, the fulcrum, which develops independently of the hypo- 

 pharynx and takes on a curiously similar shape in all three groups, a 

 V or Y basally biramously branched, and carrying some of the muscles 

 of the maxillae. 



The mandibles and maxillae are hollow at their inner sides ; and 

 into the cavities are inserted some of the adductor muscles that meet 

 the corresponding ones from the opposite side or run to the fulcrum. 

 In Protura the mandibles are for piercing, not biting, and having no 

 use for adductor muscles, change them into protractor muscles and 

 are solid, not hollow, inside. This is most probably a secondary 

 phenomenon. Collemboles with piercing mandibles do not show these 

 changes. A cardo is present and similar in all three groups, connect- 

 ing the stipes with the fulcrum and running more or less parallel to, 

 and dorsal of, the lateral branch of this skeleton. Snodgrass (1951, 

 p. 84) expressly remarks that "the articulation of the cardo on the 

 sternal brachium is in striking contrast to the usual suspension of the 

 insect maxilla from the edge of the cranium." I have for practical 

 reasons followed the common use in calling this rod the cardo, but in 

 fact I suspect it to be actually a part of the fulcrum (to which it is 

 attached by a ligament) ; in this case the so-called stipes would arise 

 from a coalescence of stipes and cardo, and the maxilla would be 

 located in the head in the same way as the mandible. It is also to be 

 noted that at least in Collembola (fig. 11) a ligament connects the 

 base of the stipes to the head wall in the same way as the mandible, 

 and that the gnathal pouch does not seem to include the so-called cardo. 



Now, if we want to test these characters, common to the three 

 groups, according to their degree of relationship to other groups, as 

 Hennig (1953) has schematized it (synapomorph characters being 

 derived characters common to two or more groups, and symplesio- 

 morph characters being only common inheritance) we find that the 

 entognathy in itself is a synapomorphic character, being found in no 

 other arthropod group. Also the shape of the fulcrum is a synapo- 

 morph character, being not comparable to the ventral skeleton of the 

 gnathal segments of malacostracan Crustacea, though it may bear 

 some likeness to this. That the mouth parts are hollow at the median 

 side is a feature they have in common with many myriapods, with 

 which there is also a likeness in the shape of the mandible. On the 

 other hand the shape of the maxillae is a true insectan character. The 

 musculature of the mandibles suggests that of myriapodan groups, 

 though no intrinsic muscles are found ; an intermandibular muscle 

 is found in Machilidae, but in no insect with doubly articulated mandi- 



