o 



6 The Form of Insects 



fibrous columns, the larger of which {caidiculus) is 

 situated in front of the smaller [peduncle). On this 

 latter rest two elongate, trough-shaped, cellular 

 masses, the brain-cups [calyces or gyri cerebrales) j the 

 cellular envelope over these bodies is made up of 

 smaller cells than those which form the envelope 

 around the other parts of the brain ; each stalk with 

 its cup and cellular cover forms a " mushroom body " 

 (fig. 27). From the upper central region arises, on 

 either side, the large optic trunk which swells into 

 an inner and an outer lobe of nerve-fibres connected 

 with the eye. The outer lobe (fig. 30 e. op) contains 

 a series of rod-like fibres arranged radially. All these 

 parts — central masses, mushroom bodies, and optic 

 trunks and lobes make up the fore-brain {protocerebron). 

 The optic lobes are most highly developed in such in- 

 sects as dragonflies, whose eyes are greatly enlarged. 

 Development of the mushroom bodies is thought to 

 correspond with increase of intelligence (19). 



The mid-brain {deutocerehroii) consists of paired- 

 swellings, the olfactory or smelling nerve-centres, 

 whence proceed the nerves to the feelers. The hind- 

 brain (tritocerebron) comprises only that region of 

 the nerve-ring around the gullet whence arise the 

 nerves to the upper lip and food-canal described 

 above (2, 17, 18). 



Eyes. — We have next to consider the organs of 

 special sense — sight, hearing, smell, taste, touch — by 

 means of which an insect gets its knowledge of the 

 outside world. The eyes are among the most beauti- 

 ful and wonderful of all insect-structures. In the 

 Cockroach the eyes occupy externally an elongate 

 space with rounded ends, broader above, narrower 

 below, on either side of the head (fig. 4 oc). The 

 transparent skin of this region is called the cornea. 

 Examination of the surface of the cornea with a 



