STRUCTURES IMPLICATED IN METAMORPHOSIS. 25 



situated beneath the membrane, was continuous with a nerve 

 which is supphed to the antennse. This may be the organ of 

 hearing or that of smelUng ; 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 contraptions 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 



