116 LEAF-MINING INSECTS 



tissues. They take just enough to sustain and mature their 

 own lives, and they injure little tissue save that which they 

 ingest. Even where they are abundant they seem not to 

 injure their hosts very seriously. 



Through a greater or less portion of the larval life they are 

 highly specialized sap-feeders, equipped by nature for obtain- 

 ing the maximum benefit from the leaves with a minimum 

 of injury to them. We have already discussed (p. 56) 

 the two forms of larvae of the different instars of this 

 group and the modifications of mouthparts for cell-shearing 

 and sap-feeding. The first form larva is always the flat, 

 sap-feeding form and the transition to the ordinary, cylin- 

 dric, tissue-feeding form comes at different instars in the 

 several genera and with varying degrees of completeness. 



In the earlier sap-feeding instars the body, head and 

 mouthparts are always very flat. The body is widest in the 

 thoracic region and tapers thence rather rapidly towards the 

 rear end. Each segment, though very thin, pouches out 

 much at either side, leaving very deep lateral incisions be- 

 tween the segments. This distribution of bulk apparently 

 facilitates bending, from side to side without causing a verti- 

 cal extension of the body. 



The legs and prolegs are so much reduced in these instars 

 that at most only the faintest rudiments are discernible. 

 When discoverable prolegs are present they occur only 

 on segments 3, 4, 5 and 10 of the abdomen. 



In all the genera of the Gracillariidae there are, according 

 to Chapman (Chambers to the contrary) at least two flat, 

 sapfeeding instars. In the most primitive genera there 

 seem to be five instars in most cases, of which the first two 

 are sap-feeders, while the last three differ very little from 

 unspecialized externally feeding caterpillars. During the 

 latter instar most of them become external though concealed 

 feeders. But in the more specialized genera, Lithocolletis, 

 and its immediate allies, there may be five or seven larval 



