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INSECT PHYSIOLOGY 



membranes become separated from the general surface of the gut, but 

 tend to remain attached at the anterior end, and are therefore apt 

 to give a false impression of taking origin solely at this point. They 

 remain attached in this region largely because here the mid-gut is 

 overlaid by the oesophageal valve. That is probably the raison d'etre 

 of this invagination: it enables the peritrophic membrane to extend 

 forward beyond the point at which the food enters the mid-gut. 



Now as the food passes through the oesophageal valve it will tend 

 to press the walls outwards; and in so doing it will tend to press the 

 secretion, from the cells of this cardiac region, into a membrane. That 

 is the crudest form of mechanism by which the peritrophic mem- 

 brane is pressed out; it is met with in the larvae of Lepidoptera. But 



cm 



fig. 8. — Annular moulds producing peritrophic membrane: A, larva of 

 mosquito, Anopheles; B, tsetse-fly, Glossina; C, earwig, Forficula. 

 (Modified after Wigglesworth.) The cross-section of the membrane is 

 equal to that of the mould in each case; hence the complicated folding 

 of the membrane in the narrow part of the mid-gut in B 



cr, cuticular ring forming inner wall of press; cm, circular muscle compressing the outer 

 wall against this ring; dc, duct of crop; m, sphincter muscle; mg, mid-gut; o, oesophagus; 

 pm, peritrophic membrane; sc, cells secreting the substance of the membrane 



in many insects the conditions have become far more elaborate. The 

 cells in the cardiac region have become increasingly specialized for 

 the production of the membrane until, in the Diptera and in the 

 Dermaptera, they alone are concerned in its formation. At the same 

 time the mechanisms by which the fluid secretion from these cells 



