ORDER LEPIDOPTERA 55 



after mouthful of the tissue in the path forming a real 

 tunnel that allows considerable vertical space for moving. 



The sap-feeders, however, shear through a single layer of 

 parenchyma cells and suck their liquid contents, leaving the 

 solid parts in place, there is indeed a premium on vertical 

 space. The retraction of the head would give an obviously 

 undesirable increase in height. The horizontal position 

 of the capsule is in these brought about by the lengthening 

 of the ventral surface so that the two sides of the head are of 

 about equal length and then only the dorsal hind margin is 

 overlapped by the prothorax. 



In the sap-feeders not only is the capsule thin and free, but 

 the oval outline of the head is completely abandoned. The 

 lateral margins are almost straight, diverging backward. 

 There is a blunt transverse front border from which the 

 mouth parts project in such a way as to leave a right angled 

 notch at either side for the insertion of antennae. This 

 trapezoidal wedge-formed head is doubtless mechanically 

 efficient in severing tissue after the unique fashion of the 

 sap-feeders, and it is peculiar to these forms. 



The eyes, also, are affected. This pronounced dorso- 

 ventral flattening causes a sharp fold to occur in the head- 

 capsule in that region where the ocelli are found in external 

 feeding caterpillars. If no change occurred in their arrange- 

 ment this would cause some of the six ocelli to be found on 

 the dorsal side and some on the ventral side and directed 

 towards the epidermis in either case. As Traegardh (1913) 

 has remarked "it seems reasonable to suppose that the only 

 direction in which there is any need for them to discern 

 things is the only one in which they are moving, i.e., the 

 horizontal plane." The problem is then seen to be to have 

 them as nearly marginal as possible. 



In full-grown larvae of Ornix we have the normal arrange- 

 ment of ocelli in the free-feeding caterpillar, i.e., forward on 

 the side of the head closely behind the base of the mandibles. 



