54 LEAF-MINING INSECTS 



suture. The front is usually a triangular sclerite above the 

 mouth parts (/ of fig. 22 C) . Between it and the large side 

 sclerites are two small sclerites, called ad-frontal pieces (a). 

 In most free-living caterpillars the sutures bounding the sides 

 of the front meet halfway the epicranial suture which sepa- 

 rates the two sides. In some leaf-miners the ad-frontal 

 pieces disappear, the front extends far back and the arms 

 of the Y do not unite to form a stem. In such a case the 

 front is said to be "open." 4 



The researches of Ivar Traegardh (1913) have especially 

 pointed out the effect of the leaf-mining habit on these 

 sutures and their correlation with the flattening of the head- 

 capsule. From narrow deep sutures they become broad 

 shallow ones. From sutures meeting one-third or one-half 

 the way forward, they here meet first at the hind margin, 

 the vertical angle of the head; or indeed they may not meet 

 at all but may be joined by a bridge which increases in 

 length as the sutures become more nearly parallel to one 

 another. In the sap-feeders these sutures actually diverge 

 posteriorly. 



Besides the change in the relations of the sutures, there is 

 the difference in the angle which the head makes with the 

 axis of the body. In external feeders the dorsal side of the 

 head is longer than the ventral but the hind margins are in 

 nearly the same vertical plane since the head is bent ob- 

 liquely downwards. In the leaf -miners, on the contrary, 

 the head-capsule becomes horizontal. 



In the tissue-feeders this change is brought about by 

 having the hind parts of the dorsal side of the capsule 

 withdrawn into a fold of the prothorax, and at the sides the 

 hind margins have developed into thin blades, enormously 

 expanded to rearward for muscle attachment. These forms 

 feed by actually tearing out and swallowing mouthful 



4 The sutures must not be confused with the tentatorial arms which show 

 more plainly than the sutures in microscopic preparations. 



