SMITHSONIAN MISCELLANEOUS COLLECTIONS 



VOL. IIO 



region of the back in the adult united with the tergal plates of follow- 

 ing segments in the dorsum of the prosoma. The chelicerae become 

 secondarily preoral, and in most cases the pedipalps take positions 

 at the sides of the mouth. 



HL 



4-1. 



PrC IIS Pdp IL 



Fig. I. — The prosomatic segmentation and appendages of Arachnida. 



A, young embryo of Euscorpiiis italiciis (Hbst.), extended in a plane, showing 

 cephalic lobe {HL) and postoral somites with appendage rudiments (from 

 Laurie, 1890). B, embryo of Agclena labyrinthica L. (from Balfour, 1880). 

 C, young embryo of Pcdiculopsis graminum (Reuter) in the egg just before 

 reversion, lateral (from Reuter, 1909). D, embryo of Euscorpiiis italicus 

 (Hbst.), longitudinal section through germ band to one side of median plane, 

 showing cephalic lobe {HL) extended posteriorly on dorsal surface (from 

 Laurie, 1890). E, diagram of the approximate prosomatic segmentation of an 

 adult arachnid; the primitive cephalic lobe {HL, stippled) forms the eye-bearing 

 part of the back, the epistome {Epst), and the labrum {Lm), and is invaded on 

 the sides by the primarily postoral chelicerae {Chi) ; the pedipalp coxae {Pdp) 

 turned forward and united mesally with the epistome. 



Chi, chelicera; Epst, epistome; HL, cephalic lobe of embryo; I-VI, postoral 

 somites of prosoma ; HS, sternum of pedipalp somite ; 1L-4L, legs ; Lm, labrum ; 

 Mth, mouth ; Pdp, pedipalp ; PrC, preoral food cavity. 



The prosomatic segmentation of an adult arachnid, visualized from 

 the known facts of anatomy and embryogeny, must be approximately 

 as shown diagrammatically at E of figure i. The part of the prosoma 

 derived from the cephalic lobe of the embryo (HL, stippled) certainly 

 includes the labrum (Lm), an epistomal region (Epst) differentiated 

 at the base of the labrum, and the eye-bearing region of the back ; it 



