60 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



rudiments in the embryo of a spider (fig. 22 C, Ch) bear a relation- 

 ship to the head so similar to that of the tritocerebral rudiments in 

 the insect embryo (D, Put), that the identity of the two sets of organs 

 can scarcely be questioned. Holmgren (1916), furthermore, claims 

 that the histology of the arachnid brain shows that the chelicerae are 

 innervated from the tritocerebral region of the brain. If this homology 

 is correct, there is no reason for calling the tritocerebral appendages 

 " second antennae " except in the Crustacea. The arachnid chelicera 

 is a uniramous organ, that of a scorpion (fig. 24 A) having three 

 well-developed segments. 



Fig. 24. — Postantennal appendage of adult arthropods. 



A. chelicera of a scorpion, left, ventral view, showing uniramous structure 

 and three segments. B, second antenna of a decapod crustacean (Spirontocaris 

 (jrociilaiidiciis), left, ventral view, showing biramous structure, consisting of 

 two-segmented base (Prtp) bearing an exopodite (Exp) and an endopodite 

 (Endp). 



THE GNATHAL APPENDAGES 



There can be no doubt that the gnathal organs — the mandibles, the 

 first maxillae, and the second maxillae — constitute a distinct group 

 of appendages in the eugnathate arthropods. The mandibles are the 

 most highly modified of the gnathal appendages, and, in most cases, 

 their structure has lost all resemblance to that of the more generalized 

 insect maxillae. A maxillary appendage, therefore, should be studied 

 first as affording a better example of the basic structure of the gnathal 

 organs, and, in insects, the first maxilla preserves most nearly the 

 primitive structure, since the second maxillary appendages are united 

 to form the labium. 



The first maxilla of an insect with typical biting mouth parts, of 

 which the roach oft'ers a good example (fig. 25 A), consists of a basal 

 stalk, two terminal lobes, and a palpus. The base is divided into a 

 proximal cardo (Cd) , suspended from the head by a single point of 

 articulation (c), and a distal stipes (St). The cardo and stipes are 

 freely flexible on each other by a broad hinge line, and their planes 

 may form an abrupt angle at the union, but neither has an inner wall, 



