iv PREFACE. 
families treated of in this volume are the most difficult and least 
understood of any, and are in a most unsatisfactory state with regard 
to their systematic arrangement and identification. It is hoped that 
this account may spur younger workers on to establish new and 
important facts, so that inaccuracies may be corrected which naturally 
will be found in it, and new discoveries made. 
I have to express my thanks to numerous entomologists for kindly 
help, and for lending specimens to be figured, especially to Miss 
Ormerod for the loan of blocks from her ‘‘ Manual of Injurious 
Insects.” 
FRED. V. THEOBALD. 
CAMBRIDGE, 1892. 
