THE ANATOMY OF THE LARVA 63 



shifts into the sixth segment. This last alteration is per- 

 formed with remarkable rapidity during the few minutes 

 the imago takes in emerging from the pupa case. 



Organs of Sense. — The eye in the encephalous larva is 

 usually stated to be of the " simple type consisting of a 

 group of ocelli with lens and retinal expansion." 



This sort of statement has been copied from one text-book 

 to another till it has become stereotyped, but as a matter of 

 fact, there is neither lens nor retinal expansion, properly 

 so-called, and the eye appears to be rather a transition stage 

 in the development of the compound eye of the imago, than 

 a structure in any way like the ocelli of adult insects, or the 

 eyes of spiders or molluscs. 



In full-grown larvae of Ciilex pipiens, the eyes, although 

 distinctly separated by an unpigmented gap, form for all 

 practical purposes a single visual organ, the thick but 

 perfectly transparent cuticle forming an unbroken spherical 

 curve over both eyes, and although this is somewhat thicker 

 in the middle line opposite the separation between the 

 deeper parts of the eyes, there is nothing whatever in the 

 form of a lens, at any rate in the optical sense of the word, 

 as not only is its internal limit formed by the irregular sur- 

 face of the pigment, but even taking this as a regular surface 

 there is so little difference in the depths of the anterior 

 and posterior curves that any image formed by it would fall 

 nowhere near the visual nerve endings, but somewhere in the 

 animal's thorax. If we examine a not too thin radial section 

 of the organ it will be seen that it consists of a number 

 of conical masses of pigment, the combined bases of which 

 form the inner boundary of the transparent cuticular layer 

 of the eye, and into the apex of each ma}^ be traced a fibre of 

 the optic nerve springing from a bilobed mass of ganglion 

 cells, almost in contact with, but yet distinct from, the large 

 lobes of the cerebral mass. 



Selecting a thinner section we find that each nerve fibril, 

 shortly after starting from the optic ganglion, begins to 

 acquire a rapidly thickening covering of pigment granules, 

 and that as soon as this has become sufficiently bulky to 

 merge with the neighbouring sheaths of pigment to form 



