THE METAMORPHOSES OF THE. NERVOUS. SYSTEM. 35 



imago, or perfect insect, they fill up the greater part of the 

 cavity which encloses them. In every case these cerebral gang- 

 lions are more or less united centrally, so as to form a bilobed 

 mass. 



The nerves of the eyes (optic nerves), of the antennse, of the 

 upper lip, and of the cords which encircle the gullet and unite 

 with those of the series of enlargements already noticed, spring 

 from the bilobed mass or brain, and also a series of nervous 

 fibres, which supply the digestive organs. 



In some insects which have two kinds of eyes, the simple 

 ocelli are situated in the middle of the head, immediately above 

 the brain — in the bee, for instance. These eyes are furnished 

 with a nerve from each side portion of the brain — a pair of 

 nerves. They are very short twigs, and are called the first pair. 

 The antennae are supplied by the second pair, which originate 

 from the under side of the brain, and pass forwards, having 

 oftentimes a considerable swelling upon them, into the so-called 

 " feelers." The size of these nerves depends upon the develop- 

 ment of the antennae, and therefore they are much larger in the 

 adult than in the early condition of insect life. 



The nerves which supply the side eyes are usually so large 

 that they seem to form a part of the brain from which they 

 spring; but there are many insects which, possessing small 

 optical organs, have correspondingly puny optic nerves. These 

 nerves form the third pair, and like those already mentioned, 

 have their size in direct relation with that of the organ they 

 supply ; consequently, they only begin to attain a considerable 

 growth in the middle life of the insect — in its pupa or nymph 

 condition. 



Some larvae which are blind have very delicate optic nerves. 

 In others which have several ocelli grouped together on either 

 side of the head, and which will become altered into a pair of 

 compound eyes when adult age commences, the nervous fibrils 

 are numerous. These unite near the brain in early life, and 

 form large and closely-connected masses as the development 

 of the eyes progresses. The larva of Dytisais affords a good 

 example of the separate condition of the nervous fibres sup- 

 plying the mass of simple eyes ; and the examination of the 



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