THE INSECT HEAD SNODGRASS 



451 



ing insects the pharynx and not the buccal cavity forms the sucking 

 pump ; in still others the pump is a bucco-pharyngeal structure. 



Beyond the pharynx the stomodeum generally narrows to a tubular 

 oesophagus {Oe). Posteriorly, however, the oesophagus often widens 

 into a crop^ and in some insects the crop occupies almost the entire 

 length of the oesophageal region. The stomodeum terminates in a 

 proventrlculiis, from which a valvular fold of its wall projects into 

 the stomach, or ventriculiis. 



II. EVOLUTION OF THE INSECT HEAD 



When we examine an insect embryo in an early stage of develop- 

 ment (fig. 5 A, B) we see that its head consists of an enlargement 

 (Pre) of the anterior end of the elongate body, formed of lateral 



Pre 



-Bdy 



A 



-Pre 



-Ant 



mM 



Pnti 



C 



FiQUBB 5. — Series of insect embryos showing development of flie procephalic lobes 

 (Pro) and head appendages 



A, young embryo of a roach (from Riley, 1904). B, older embryo of same, show- 

 ing enlarged procephalic lobes, with mouth (Stom) and antennae (Ant) 

 forming on under surface (from Riley, 1904). C, young embryo of Lcpisma, 

 showing third head segment (III) united with protocephalon (from Heymous, 

 1897). D, embryo of roach in later stage, showing rudiments of postantennal 

 appendages (Pnt), and developing gnathal appendages (Md, IMx, 2Mx) on 

 region behind procephalic lobes (from Riley, 1904). 



swellings of the germ band, which embryologists call the cephalic 

 lohes. On this embryonic head there are later formed the eyes, 

 the antennae (D, Ant), the mouth (Mth), and a median lobe before 

 the mouth, which is the rudiment of the labrum (Zm). In addition 

 to these parts, however, there has been observed in the embryo of a 

 walking-stick insect (Wiesmann, 1926), and also of a centipede 

 (Heymons, 1901), very small rudiments of a pair of preantennal 

 appendages; and there are commonly present in insect embryos rudi- 

 ments of a pair of postantennal appendages, though the latter are 

 sometimes situated not definitely on the region of the cephalic lobes, 

 but very close behind them (D, Pnt). 

 102992—32 30 



