RELATION TO OTHER ANIMALS 173 



Carbolated fish or a similar oil is used as a repellant, 

 where flies of this character are abundant; but proper 

 attention to the manure so as to prevent breeding is 

 a much more effective and satisfactory measure. 



Leading to those species that have been referred 

 to as parasitic because, in the larval stage, they are 

 confined to and dependent on the host, are a number of 

 species that in a sense are intermediate in habit. There 



FIG. 76. Screw-worm, Lucilia macellaria: a, b, c, larva and details; d, 

 pupa, e, fly; /, its head. 



are many species that are true scavengers in the larval 

 stage; maggots which are found on exposed meats, 

 fish or vegetable matter, and which in an almost in- 

 credibly short time dispose of most of the organic matter 

 of an animal of even considerable size. And in the 

 determination of what is suitable organic matter, the 

 adult flies of some species seem to follow the rule 

 that it is best to "blow" anything that might by any 

 chance be suitable. Hence while the parent of the 

 " screw- worm" normally lays her eggs in or on dead 

 animals or on exposed meats, yet sometimes, when 

 suitable food is scarce, she will select any raw or sore 



