A. D. Imms 
75 
former, the families of the Chrysididae, Braconidae, Chalcididae and 
Ichneumonidae, with their allies, comprise an enormous assemblage of 
species. In the Diptera, the chief parasitic families are the Bombyliidae, 
Tachinidae, Pipunculidae, and Conopidae. The Oestridae and Hip23o- 
boscidae, on the other hand, are directly injurious, as they include 
parasites of domestic animals. 
Frequent instances are known where an insect has been highly 
injurious for several years, and increased to the verge of becoming a 
veritable plague. It has afterwards disappeared, as if by magic, and 
may not again attract notice as a pest for many years. Experience has 
shown it is almost always to the wmrk of parasites that the rapid dis¬ 
appearance of injurious species must be attributed. Prof Paul Marchal 
(1907, p. 8) states that the damage caused by the larvae of moths of 
the genus Hyponomeuta to fruit trees, is almost invariably stopped at 
the end of 2 or 3 years by Tachinids and other parasites. The same 
thing occurs with the larvae of Bombycid moths which defoliate coni¬ 
ferous forests. The outbreaks of the Hessian tly and oat midge in 
France in 1895 were brought to a termination through the destruction 
wrought by their parasites. Such outbreaks, how'ever, are not brought 
by Nature to a termination until the species concerned have passed 
through a number of generations, and committed widespread damage. 
Bellevoye and Laurent (1897, p. 71) have demonstrated numerically 
that it is unnecessary for a parasite to have a fecundity greater than 
that of its host. It is possible, in the course of three generations, for 
a parasite of equal fecundity to its host to desti’oy the latter completely, 
even though the parasites were outnumbered in the proportion of 
3 :1 at the commencement. Many experiments have been conducted 
with reference to the natural control of injurious insects by means 
of other species which prey upon them. In certain instances remark¬ 
able success has been achieved—in other cases failure or only 
inditferent success has resulted. The importance of this line of work 
can scarcely be overestimated. It cannot, however, be relied upon as 
a substitute to replace all the other methods commonly used in applied 
entomology, as has been advocated in some quarters. A large number 
of experiments have been made in the United States and on a smaller 
scale in Italy, Portugal, Canada, France, Peru, and other countries, but 
in England this line of work is practically a virgin field. The principle 
involved is that the injurious insects of a particular country may be 
present also in other countries, or parts of the same country, and yet 
not jjroduce any serious damage therein. In such instances it has been 
