386 



In the four and a half-day pupa these changes are com- 

 plete. The lens is strongly developed, biconvex, with the 

 greater convexity turned inwards; pigmentation has in- 

 creased, and the iris is so disposed as to leave a rather large 

 "pupil" space, towards which the visual rods all point (fig. 

 71). An isolated adult visual cell is seen in fig. 72. It 

 measures about 24|ut, in length, of which 10*4/x is occupied 

 by the visual rod. 



Tliis description holds for the median as well as the lateral 

 ocelli; a transverse section of the former, however, shows that 

 it is strongly indented in its anterior wall. It is essentially 

 a double ocellus ; and its double nature can be recognized 

 throughout development. As is to be expected, its nerve 

 communicates with two ocellar ganglia. 



During this process the mesodermic somatopleure lining 

 the base of the ocellus has grown considerably, and occa- 

 sionally is very prominent. It is retained throughout pupal 

 life and is seen in the newly hatched wasp as a distinct mem- 

 brane, with fairly prominent nuclei, close to the ocellar wall. 



The innervation of the ocellus is quite different from that 

 of the compound eye. In the late larval stages, after defae- 

 cation has taken place, nerve fibres grow out from the brain, 

 towards the ocelli, guided apparently by the deflected somato- 

 pleure. In the newly formed pupa the nerve has already 

 come into contact with the developing visual cells of the 

 ocellus, and the only visible change undergone by the nerve 

 as development ^dvances is an increase in size; in the early 

 pupa it is long and slender, but as the ocellus is forced below 

 the surface of the head, and as the brain increases in size, the 

 nerve becomes shorter and thicker. It is not necessary to 

 describe the formation of the ocellar nerves more fully at 

 this stage beyond mentioning that the ocellar nerve is a true 

 nerve, i.e., quite devoid of cortical brain cells, and therefore i 

 quite different in nature from the optic "nerve," which is 

 essentially an outgrowth of the brain cortex. 



C. — The Respiratory System. 



The Larval Organs. 



In the newly hatched larva (fig. 1) there is a pair of 

 great longitudinal tracheal trunks passing from the second 

 segment backwards on either side of the body to the twelfth 

 segment; these are connected with one another, in front and 

 behind, by two transverse tracheal vessels, of which the 

 anterior passes over the oesophagus, the posterior under the 

 rectum. The anterior transverse vessel shows a small, 

 forwardly projecting median part, evidently indicating the 



