O SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Og 



somite, represented by the postpedes of certain holometabolous larvae, 

 and the iiropods, or cerci, of the eleventh somite. 



The pregenital appendages occur as distinctly appendicular organs 

 principally in the Apterygota and in pterygote larvae. They never 

 take the form of the thoracic pereiopods, but in a few cases they are 

 multiarticulate and suggest that they are derived from jointed legs. 

 Each appendage, when well developed, consists of a basis (fig. 2 C, 

 LB) and of a movable appendicular part {Sty), and the basis fre- 

 quently bears, mesad of the latter, an eversible and retractile vesicle 

 {Vs). The appendicular process, which commonly takes the form of 





Bnd / Tlpd Tlpd 



A B ^^^ 



Fig. 2. — Diagrams suggesting the homologies of the basal parts of the appen- 

 dages and the position of the dorso-pleural and pleuro-ventral lines {a-a, b-b) 

 relative to them. 



A, a head appendage, with basis subdivided into cardo and stipes {Cd, St), 

 the latter bearing a pair of endites {Bnd) and the telopodite {Tlpd). 



B, a thoracic appendage, v^^ith basis divided into subcoxa and coxa {Sex, Cx), 

 the latter bearing the telopodite {Tlpd). 



C, a thysanuran abdominal appendage, with limb basis undivided, and bearing 

 a stylus {Sty) and vesicle {Vs). 



D, a typical gonopod, with undivided basis (valvifer) bearing a stylus {Sty) 

 and gonapophysis {Gon). 



E, a generalized larval appendage of tenth segment (pygopod) or of other 

 segments, bearing a stylus and vesicle. 



F, a cercus (uropod) of eleventh segment, with small or rudimentary basis, 

 and unsegmented distal shaft. 



a stylus, a gill, or a tapering process, is usually provided with muscles, 

 and the muscles always have their origins in the basis. The vesicle 

 may be a small eversible sac, as in the Thysanura, a gill-bearing tu- 

 bercle, as in certain sialid larvae, or a lobe serving for locomotor pur- 

 poses, as in the larvae of Lepidoptera and chalastogastrous Hymen- 

 optera; it also is provided with muscles, which in Thysanura arise 

 within the limb basis, but which in the other forms may take their 

 origins on the dorsum of the body wall. The limb basis usually has 

 the form of a plate or lobe broadly implanted in the pleural area of 

 its segment ; it is generally immovable, but in the Thysanura and in 

 the genital segments of female Pterygota it is provided with muscles 

 arising on the body wall. 



