10 



LEAF-MINING INSECTS 



The number of larval instars, and of corresponding moults 

 varies from three to seven (possibly more in the sawfly 

 larvae), three being the usual number in leaf -mining maggots 

 and beetle grubs, and five in moth larvae. There are some- 

 times changes of habits with each succeeding moult and 

 these may in some cases be read in the appearance of the 

 completed mine. The Strigifin miner on chestnut (see fig. 40 

 on p. 123) changes its habit with four out of five of its 

 successive moults. 



1PH5 



-■ j j j 1 



* <^; 



Fig. 6. Two moth larvae. A, an ordinary caterpillar that feeds down- 

 ward; B, a leaf -mining caterpillar that feeds forward. 



Besides the sudden increase in size that follows upon 

 moulting, there are changes of form that are sometimes very 

 considerable, even before the final change to the pupa. 

 The head capsule is widened with such regular steps that it 

 is possible to read the story of the moultings from a good 

 series of the cast capsules by comparing them as to size. 

 The widths of head capsule will fall into as many groups 

 as there are instars. 



The earlier instars are the more plastic, and are more 

 specialized than the later ones. They are more precisely 

 adapted to the operations of leaf -mining. The form in the 

 later instars tends to revert to that of ordinary non-mining 



