g50 Marvels of the Universe 
of the hawthorn, and sometimes, instead of the Saw-fly emerging, a dozen or more Ichneumon flies 
pierce the cocoon—the parent of them having laid its eggs in the body of the caterpillar of the 
Saw-fly. 
In the case of those caterpillars which are gregarious, the female lays her eggs in the most curious 
positions : sometimes twenty or thirty in the leaf-stem of a poplar; in another, in the ribs at the 
back of a leaf ; and again in the stem of a soft rush ; and very often the Saw-fly makes a tiny incision 
between the upper and under cuticles of a leaf and keratin lays an egg. 
Perhaps the most familiar objects on the willow-leaves growing in the hedgerows from 
May to September are the crimson swellings, or galls, sometimes a dozen or more on one 
leaf. These galls are caused by the Willow Saw-fly (page 947), which cuts a minute slit in 
the upper skin, or cuticle, of the leaf (page 948) with its saws and lays an egg, the two saws 
and two tenons then forming a tube, or egg-laying apparatus. Immediately the egg is laid in its 
place, the Saw-fly turns round, and with saliva from its mouth, hermetically seals up the egg. In 
from eight to ten hours, around each incision appears a small rust-coloured ring, which, as time 
goes on, increases, and in a month or so the two cuticles separate, and a slight blister appears on 
both sides, which slowly forms into a red spongy gall about a quarter of an inch in diameter. In 
the centre of this the egg hatches to a minute white caterpillar, feeding upon the inside material 
of the gall. A hole is bitten on the under-side, so that all waste material can fall out. In 
the photograph below the gall is shown in section, with the full-grown caterpillar of the Willow 
Saw-fly. In a month or more the grub becomes full-grown and escapes from the gall, and walking 
down the twigs and branches, it reaches the trunk ; and if it can find a loose piece of bark, it crawls to 
the back of it and spins a brown silken cocoon. Sometimes we have found a cluster of twenty 
or more all spun up together. In these cocoons the grub changes to a chrysalis and emerges in 
[by Fred Knock, F.L.S. 
SECTION OF A WILLOW-GALL. 
The grub is shown inside the Willow-gall. To the right is seen the exterior of part of the gall, with the tiny hole made by 
the grub from inside. The passage indicated by the hole is shown in the section as leading to the cavity in which lies the grub 
curled up in a characteristic attitude. 
