228 NATURAL HISTORY OF AQUATIC INSECTS CH. 



these the surface-film is unable to penetrate. The 

 case is not completely closed. An opening is ne- 

 cessary in order that the larva may push out its head 

 and the fore-part of its body for the purpose of 

 feeding or locomotion. We should be inclined to sup- 

 pose that, on these occasions, the sheath would fill with 

 water, but this does not happen. The edges of the 

 opening are elastic, so that they open only under 

 pressure, and then to just such an extent as allows 

 the body of the Insect to protrude. The unoccupied 

 cleft is so minute that the surface-film of water is 

 unable to penetrate. Reaumur describes the opera- 

 tion of making a sheath. The larva lays hold of the 

 under side of the leaf, and gnaws a piece of it into 

 the desired shape. Then it carries the piece in its 

 mouth, and brings it against another leaf in such 

 a position that the two under-surfaces are in contact. 

 The leaves are a little concave beneath, and thus, 

 when brought together, they do not lie flat, but enclose 

 a lens-shaped cavity. The bits of leaf are then fixed 

 by threads of silk, and the second leaf is cut to the 

 same size and shape as the first. The largest sheath 

 which I have seen was if in. by f in. The larva 

 is commonly found on Potamogeton early in the 

 summer, but later on, when the leaves of the 

 Water-lily appear, it seems to prefer these both 

 for food and for the manufacture of its sheaths. It 

 is often found upon another aquatic plant, viz. the 

 Water Starwort (Callitriche verna). Before pupa- 

 tion, the Insect creeps up the stem of a tall water- 

 plant, and attaches its sheath to this well out of the 

 water. The sheath is now lined with a cocoon of 



