56 



SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. 



are the true sternal muscles of the head appendages, and this relation 

 brings the musculature of these appendages directly in line with that 

 of the thoracic legs, which are moved by sets of muscles arising on 

 the tergum and the sternum in each segment. In the Pterygota, it 

 will further be shown, the mandibles lose their primitive sternal ad- 

 ductors, and, by a change in the nature of the mandibular articulation 

 with the head, the primitive tergal promotor and remoter muscles of 

 the jaw become the functional abductors and adductors. 



III. THE HEz\D APPENDAGES 



The segmental appendages of the head in an adult insect are the 

 antennae, the mandibles, the maxillae, and the labium. The antennae 



Pi 



-" Lm 

 -- ,^ ' ^ , lAnt 



i^-^ vi^ -2Ant 



Mth 

 An 



A 



Lm Ant 



.Ch 



Fig. 22. — Arthropod embryos showing relative development of the trito- 

 cerebral appendages. 



A, embryo of a crayfish, Aslacus (Potatiwhius) asiactts (from Reichen- 

 bach, 1877). B, embryo of a spider, Trochosa singoricnsis (from Jaworowski, 

 1891). C, embryo of a spider, Angeleua labyrinthca (from Balfour, 1880). 

 D, embryo of an apterygote insect, Auurida maritiina (from Wheeler, 1893). 



An, anus; Ant, antenna; lAiit, first antenna; 2Ant, second antenna; Ch, 

 chelicera ; ///, tritocerebral segment ; /L, first leg ; Li, prothoracic leg ; Lm. 

 labrum ; Md, mandible ; Mth, mouth ; Pdp, pedipalp ; Pi. pit on head region ; 

 Pnt, postantennal appendage ; Pre. protocephalon. 



belong to the second, or deutocerebral, segment of the protocephalon. 

 the other appendages to the gnathal segments. In many insect em- 

 bryos there is present a pair of small lol^es on the third protocephalic 

 segment, which lobes are unquestionably rudiments of the tritocere- 

 bral appendages. Preantennal a)3pendages have been reported in Scolo- 

 pendra and in the phasmid insect, Caransius (fig. 14 A, B Prnt) . As 

 already pointed out, there is some reason for regarding the crustacean 

 eye stalks as being the appendages of the preantennal segment, though 

 the true status of these organs has not yet been demonstrated. 



