STRUCTURES IMPLICATED IN METAMORPHOSIS. 
25 
situated beneath the membrane, was continuous with a nerve 
which is supplied to the antennae. This may be the organ of 
hearing or that of smelling ; and to whichever sense it may truly 
belong, it is clearly a new structure, developed after caterpillar 
life came to a close. 
Having thus briefly noticed some of the modifications of the 
outsides of insects during the metamorphosis, let us consider how 
the digestive, nervous, and respiratory organs are altered during 
the progressive evolution of the perfect insect from the cater- 
pillar condition. 
The digestive organs in insects are more or less tubular, and are 
continuous with the structures of the mouth. They extend from 
one end of the body to the other, and are either short and straight 
or long and convoluted. Certain swellings and contractions of 
the digestive tube mark the principal divisions of it, and enable 
us to distinguish an oesophagus, a stomach, a small and a large 
intestine. Some glandular appendages, tubular in shape, complete 
the digestive apparatus ; they are the salivary glands, the liver, 
and urinary tubes. The alimentary canal is formed of several 
layers : first there is on the outside a delicate, structureless mem- 
brane ; then beneath it a muscular coating, consisting of fibres 
arranged across and lengthwise, and which become very dense at 
both ends of the canal ; a mucous membrane is situated beneath 
this muscular coat, and its inside is covered with multitudes of 
epithelium cells, which have to do with the production of the 
fluids of the digestive function and to come in contact with the 
food. 
After the food has been masticated or sucked in, as the case 
may be, it passes into the mouth, and then into the gullet or 
oesophagus, being, first of all, mingled with saliva from the glands. 
The oesophagus is a passage possessing very dilatable walls, 
and passes through the thorax in a straight line. In the insects 
which live on fluid aliments it is usually narrow, but in those which 
devour more or less solid morsels it is large and has a considerable 
calibre. There is, in many perfect insects, a considerable enlarge- 
ment at the back part of the gullet, called the crop. It permits 
the insect to accumulate and keep a store of nourishing things, 
without digesting them all at once. This crop is very generally to 
