Prolocerebrum of Micropteryx. 135 



physiological significance. Against this view is the ad- 

 mitted fact that the bridge exists as a distinct structure in 

 all insect brains which have been investigated. 



I can find no visible connection between the bridge and 

 the nerves supplying the compound eyes, though a few 

 fibres of the ocellary nerve enter the ends of the bridge. 

 This perhaps supports Kiihnle and tends to contradict the 

 contention of Bretschn eider and others who regard the 

 bridge as a centre for the co-ordination of visual impulses, 



VI. The Visual Centres. 



A. The Ocellary Apparatus. 



In Micropteryx paired ocelli are present, but the median 

 ocellus is not developed here, or in any other Lepidopteron 

 or Trichopteron. A stream of fibres, the ocellary nerve 

 {oc. n.), leaves the back of the spherical chitinous capsule 

 in which the ocellus is contained. At the point where the 

 fibres leave the capsule there is some tendency for the nerve 

 to break, as it is very much narrowed. The sensory cells 

 are contained partly in the capsule of the organ, and some 

 of them lie along the course of the nerve away from the 

 actual ocellus, and as the nerve proceeds inwards they 

 become less and less numerous. The nerve runs straight to- 

 wards the middle line in a plane slightly anterior to the head 

 of the mushroom body (PI. VIII, figs. 10 and 11). When 

 it is over the external edge of the central body it bends 

 backwards, and at this point a few fibres leave it to pass 

 into the protocerebral lobes. From here it passes backwards 

 and inwards and continually gives off more and more of its 

 fibres, so that though there is no point at which the ocellary 

 nerve as a whole passes into the substance of the proto- 

 cerebral lobes yet the whole nerve ultimately does so. A 

 few fibres may also be seen to pass into the swollen head 

 of the bridge. 



Two small spherical bodies with rather indefinite margins 

 are found in the space beneath the outer capsule of the 

 mushroom body, posterior to the inner capsule and to 

 the middle lobe (PI. VIII, fig. 10); these are the ocellary 

 glomeruli {oc. gl.) or " tubercules du corps central " (Vial- 

 lanes). In Micropteryx I have been unable to demonstrate 

 the connection between these structures and the ocellary 

 nerve, owing to the diffuse way in which the fibres of the 

 nerve pass through the dorsal part of the protocerebrum. 



