54 CHANGE OP COLOUR IN LARVJ3. 



the larva has become full fed, it has to go in search of 

 a suitable place wherein to pass the period of rest un- 

 disturbed ; and this cannot be had on the food plant. 

 Many of them pupate in pithy stems, without spinning 

 any cocoon, while others seek such situations because 

 their cocoons are thin. Hence they may have to travel 

 some little distance before finding a proper place a 

 fact shown by finding their cocoons in stems, or under 

 bark, many yards distant from the food plants. Now, 

 when a larva descends from the food plant, it enters 

 on a new mode of life, comes in contact with dangers 

 to which it had not been accustomed to, and meets with 

 new enemies. Thus a more obscure coloration would 

 be of advantage, and that it is of use, I have observed 

 with Nematus viminalis, which becomes slate-coloured 

 before leaving the galls to pupate in the ground the 

 slate-colour harmonising admirably with the sand on 

 the river-banks where it lives as it does with the 

 dried grass, &c., found in the meadows where other 

 gall-making species of similar habits live. With 

 Cladius viminalis, again, the colour at the last moult 

 becomes more brilliant. In this case several larvae 

 live on a leaf side by side, and thus they are made 

 visible ; but when they become full fed they separate 

 to seek a hiding place, which is generally under the bark 

 of a growing tree, up the trunk of which they march. 

 The increase of brightness in the colour thus is of 

 advantage, as it makes the larvae more readily seen, 

 and seen, avoided, in the case of inedible larvae. 



A few larvae would appear to be dimorphic. The 

 larva of Nematus caprece is mostly green, with white 

 longitudinal lines, but there is a rare form of it with 

 the body reddish. One or two species of Cimbex appear 

 to have dimorphic larvae also, but the subject requires 

 further investigation. 



When the larva has become full fed, it proceeds 

 to pupate. Some larvae spin no cocoon, but bore into 

 the pithy stems, or into holes made by beetles in 

 wood. Others form in the ground neatly rolled cells 



