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ANNUAL REPOET SMITHSONIAN INSTITUTION, 1933 



tinct in their structure and function (fig. 13). The stomach of the 

 adult insect is commonly called the ventrioulus. 



Before going on, it should be mentioned that the stomach in some 

 insects does not open permanently into the proctodaeum until the 

 end of the larval stage. The larvae of such insects are mostly 

 parasites living in the bodies of other insects, or are the young of 

 wasps and bees that live in cells containing their food supply. Here 

 again is a case of adaptation to local conditions, for, just as the 

 embryo has a closed stomach to retain its food, so the larvae of these 

 insects, in order to avoid contamination of their food, have a closed 

 intestine. 



The ways in which young animals in their development depart 

 from the ancestral structure, as represented in that of their parents, 

 and then revert to the parental form is a most interesting field of 



Stom 



Vent 



DV 



Mai 



Proc 



Rect 



Mth / (^ Cr 

 vSoeGno 



FiGUKE 13. — Lengthwise section of a caterpillar showing the alimentary canal and 



other internal organs. 



AInt, anterior intestine ; An, anus ; Br, brain ; Gr, crop ; DV, dosal blood vessel : 

 Mai, Malpighian tubules ; Mth, mouth ; Oe, oesophagus ; Phv, pharynx ; Proc, proc- 

 todaeum ; Py, pylorus ; Rect, rectum ; 8kOl, silk gland ; SoeOny, suboesophageal ganglion ; 

 Stom, stomodaeum ; Vent, stomach, or ventriculus ; VNG, ventral nerve cord. 



study. In general, this temporary assertion of anatomical inde- 

 pendence on the part of young animals, and the breaking away from 

 ancestral conventions only to go back to them again, is termed 

 metamorphosis. The phenomenon is particularly prevalent among 

 the invertebrates, and, from what we have seen of the development 

 of the insect stomach, it is clear that there may be embryonic 

 metamorphosis as well as postembryonic metamorphosis. When 

 metamorphosis is very pronounced it is usually found, as in the case 

 of the insect stomach, that the primitive cells of an organ divide 

 into an active erratic group that form the temporary structure, and 

 an inactive conservative group that will later restore the organ in its 

 adult form. Without this provision, an overambitious larva might 

 get so far from the beaten path that it could not get back again. The 

 reason for metamorphosis is usually to be found in a different way 

 of living, a different habitat, or a division of labor between the 

 young and the adult stages of the same animal. 



