22 



physiological importance while still remaining anatomically perfect as far 

 as externals go, though probably the nerve centres or fibres have 

 degenerated. 



Many insects have been described which possess organs fully 

 developed, and yet without any function to perform ; thus PACKARD 

 writes : " In Platysamia (Lepidoptera), the cocoon-cutters of the pupa 

 though well developed, do not appear to be used at all, and the pupa 

 like that of the silk-worm and other moths protected by a cocoon, 

 moistens the silk threads by a fluid issuing from the mouth."* And 

 SHARP mentions that according to certain observers the eggs of some 

 kinds of Saw-flies, are laid not in but on the leaves, so that we may 

 conclude that in these cases the saws are not used by their possessors.f 

 Perhaps the most remarkable example of a fully developed organ 

 capable of performing its function, and yet never doing so is that of 

 Didelphys azarae, one of the opossums. Concerning this marsupial 

 MR. W. P. HUDSON writes : " In every way it is adapted to an 

 arboreal life, yet it is everywhere found on the level country far 

 removed from the conditions which one would imagine to be necessary 

 to its existence. For how many thousand of years has this marsupial 

 been a dweller on the plain, all its best faculties unexercised, its 

 beautiful grasping hands pressed to the ground, and its prehensile tail 

 dragged like an idle rope behind it! Yet if one is brought to a tree, 

 it will take to it as readily as a duck to the water, or an armadillo to the 

 earth, climbing up the trunk and about the branches with a monkey- 

 like agility." If this is true of Vertebrates why not of Invertebrates 

 also ? In the case of D. azarae it was easy enough to put the animal 

 back into its original environment, but we have no means of ascertain- 

 ing what were the original conditions under which the vibrissae were 

 used by larva No. 7. 



THE ALIMENTARY CANAL. 



(Esophagus. The mouth leads into a narrow longitudinally folded oesophagus 



with thin and transparent walls. On each side lie the convoluted lobes 

 of the salivary glands in which the enormous nuclei of the cells 



* PACKARD'S Text Book of Entomology, p. 635. 



t Cambridge Xat. Hist., Vol. V., p. 513. 



The Naturalist in La Plata, p. 18. 



