THE GALL FLIES. 
297 
is absent, and places an egg near that of the bee. The egg 
of the parasite is sometimes hatched at the same time with 
that of the bee, but generally later. In the first instance, the 
larva feeds on the provisions which were supplied for the bee, 
and so starves the poor creature to death ; and in the latter 
case, it is not hatched until the young bee is large and fat, and 
capable of affording ample subsistence to the parasite, which 
fastens upon it and devours all the softer portions. 
Then there are the Cuckoo Flies ( Tachince ), which bear 
some resemblance to the common house-fly, but which are pa- 
rasitic, feeding on the larvae of other insects, and selecting the 
same species which are persecuted by the firetails. When the 
Tachina larva has eaten that of the mason bee, it forms an oval 
cocoon, and there remains until the time for becoming a perfect 
insect. A single larva of the mason bee seems to be sufficient 
for the Tachina grub, as Mr. Rennie has recorded an instance 
where two larvae of the mason bee were in a nest into which a 
single egg of a Tachina had been introduced. The parasitic 
larva devoured one of the rightful inhabitants, but did not 
touch the other, and the cocoons of the bee and the Tachina 
were formed side by side. 
We now pass to a remarkable series of insects belonging 
to the same order as the ichneumons, but parasitic upon vege- 
tables and not on animals. Their scientific name is Cynipidce , 
and they are popularly known as Gall Flies, because they 
cause those singular excrescences which are so familiar to us 
under the name of Galls. This group comprises a vast number 
of species, all of which have a strong family resemblance, 
though they greatly differ from each other in size, form, and 
colour. 
In the accompanying illustration are given several examples 
of British Galls,- most of which are tolerably common in this 
country, and some of which can be found in plenty. 
In the left hand upper corner of the illustration is a figure of 
oak-leaf, upon which are two globular projections. These 
