INTERRELATIONS OF INSECTS 275 



and unaided, almost annihilated the fifth brood of the cotton worm in 

 Florida, fully ninety per cent, of the eggs of this prolific crop enemy 

 being infested by the parasite. In 1895, in the city of Washington, 

 more than ninety-seven per cent, of the caterpillars of one of our 

 most important shade-tree pests [Hemerocampa, as just mentioned] 

 were destroyed by parasitic insects, to the complete relief of the city the 

 following year. The Hessian fly, that destructive enemy to wheat 

 crops in the United States, is practically unconsidered by the wheat 

 growers of certain states, for the reason that whenever its numbers 

 begin to be injuriously great its parasites increase to such a degree as to 

 prevent appreciable damage. 



"The control of a plant-feeding insect by its insect enemies is an ex- 

 tremely complicated matter, since, as we have already hinted, the 

 parasites of the parasites play an important part. The undue multipli- 

 cation of a vegetable feeder is followed by the undue multiplication of 

 parasites, and their increase is followed by the increase of 

 hyperparasites. Following the very instance of the multiplication of 

 the shade tree caterpillar just mentioned, the writer [Howard] was able 

 to determine this parasitic chain during the next season down to quater- 

 nary parasitism. Beyond this point, true internal parasitism probably 

 did not exist, but even these quaternary parasites were subject to 

 bacterial or fungus disease and to the attacks of predatory insects. 



"The prime cause of the abundance or scarcity of a leaf-feeding 

 species is, therefore, obscure, since it is hindered by an abundance of 

 primary parasites, favored by an abundance of secondary parasites 

 (since these will destroy the primary parasites), hindered again by an 

 abundance of tertiary parasites, and favored again by an abundance of 

 quaternary parasites." - 



Entomologists have made many attempts to import and propagate 

 insect enemies of various introduced insect pests, and some of their 

 efforts have been crowned with success, as was notably the case when 

 Novius cardinalis, a lady-bird beetle, was taken from Australia to Cali- 

 fornia to destroy the fluted scale. 



Form of Parasitic Larvae. The peculiar environment of parasitic 

 larvae is responsible for profound changes in their organization. These 

 larvae, in general, are apodous, the body is compact and the head is more 

 or less reduced, sometimes to the merest rudiment. These characters, 

 occurring also in such dipterous larvae as live in a mass of decaying or- 

 ganic matter, and again in those hymenopterous larvae whose food is pro- 

 vided by the mother or by nurses, are to be attributed to the presence 



