I METAMORPHOSES OF INSECTS 23 



state of the stomach, whether it is filled or not. 

 When the stomach is empty the juice is not secreted, 

 upon the introduction of food the glands secrete 

 actively, and the juice becomes acid. Obviously the 

 secretion must be almost continuous during the life- 

 time of a larva, which seldom ceases to eat. But 

 as the perfect stage is approached, the glands are 

 atrophied and dwindle away, and sometimes become 

 altogether obscure. Their development is best in 

 animal feeders ; in herbivorous insects they are much 

 less conspicuous. The modifications of the intestines 

 are of less consequence than those of the other 

 parts. Salivary glands are situated one on each 

 side of the oesophagus, and provide saliva* to mingle 

 with and lubricate food in the mouth, and to assist 

 in its digestion. They are largely developed in 

 Lepidopterous larvae, in which they form the web- 

 spinning apparatus and secrete silk. When the cater- 

 pillar has completed its cocoon, and turns to chrys- 

 alis, there is no further need of silk. Consequently 

 the glands become small, and their function is 

 altered. 



At first sight nothing could be more astonishing 

 than the difference in the nervous system, as regards 

 the size and relative position of its parts, at the various 

 stages of development. A minute examination before 

 and after metamorphoses reveals that it undergoes 

 no perfect alteration of plan. The nervous system 

 of the larva is merely adapted for the imago, and the 

 original design is constant, even in the structures of 

 the most highly elaborated insects. In its simple 

 form, in a vermiform * larva, the system exists in the 



