ANATOMY AND PHYSIOLOGY 



95 



Mesenteron. The ventriculus, otherwise known as the mid intes- 

 tine, or stomach, is usually a simple tube of large caliber, as compared with 



mt 



cl 



* cm 



FIG. 148. Alimentary tract of a moth, Sphinx, c, food reservoir; cl, colon; cm, caecum; 

 i, ileum; m, mid intestine; mt, Malpighian tubes; o, oesophagus; r, rectum; s, salivary 

 gland. After WAGNER. 



the oesophagus or intestine, and into the ventriculus may open glandular 

 blind tubes, or gastric cceca (Figs. 145, 146) ; these, though numerous in 

 some insects, are commonly few in number and restricted to the anterior 

 region of the stomach. The gastric ca3ca of Orthop- 

 tera secrete a weak acid which emulsifies fats, or 

 one which passes forward into the crop, there to 

 act upon albuminoid substances. In the stomach 

 the food may be acted upon by a fluid secreted by 

 specialized cells of the epithelial wall. In various 

 insects, certain cells project periodically into the 

 lumen of the stomach as papillae, which by a process 

 of constriction become separated from the parent 

 cells and mix bodily with the food. This phenom- 

 enon takes place in the larva of Ptychoptera (van 

 Gehuchten), also in nymphs of Odonata (Needham), 

 and is probably of widespread occurrence among 

 insects. The chief function of the stomach, how- 

 ever, is absorption, which is effected by the general 

 epithelium. Physiologically, the so-called stomach 

 of an insect is quite unlike the stomach of a verte- 

 brate, being more like an intestine. 



Proctodaeum. At the anterior end of the hind 

 intestine there is usually a pyloric valve, which pre- 

 vents the contents of the intestine from returning into the stomach. 

 This valve may operate by means of a sphincter, or constricting, 

 muscle, or may, as in Collembola (Fig. 144), consist of a back- 



FIG. 149. Cardiac 

 valve of young muscid 

 larva, o, oesophagus; 

 />, proven triculus; v, 

 valve. In an older 

 larva the valve pro- 

 jects into the mid in- 

 testine. After KOWA- 



LEVSKY. 



