FIELD WORK ON THE DIPTERA. 179 



These syrphids are beneficial both as larvae and as 

 adults, many of them being predaceous in the larval 

 stage; hence they must be set free after you have looked 

 at them long enough to be able to recognize them, if you 

 were to see them again. These flower flies have some 

 representatives that like the shady woods a thing 

 rather unusual for flies, as flies notably like sunshine and 

 warmth; so an expedition with the insect net and the 

 hand lens may find some of the other syrphids, the object 

 being to discover what these beneficial flies look like and 

 where they are to be found, all with the view to letting 

 them alone after you have the knowledge sought. 



The harmful flies are often given names which tell 

 what they do, as well as often giving a clue as to where 

 to find them ; as house flies, stable flies, bot flies, horse flies. 

 On the window pane of some open room in the course 

 of an hour, several sorts may be captured; and after 

 determining with the aid of the instructor whether there 

 are any valuable flies among them, they may be put into 

 the killing bottle, and should be set up as soon as they are 

 dead. The pins should be slender and are to be thrust 

 through the thorax, the fly being set well up toward the 

 head of the pin. When a harmful fly is captured in the 

 field, it is advisable to. have the killing bottle along, 

 in order that the delicate insect may not be put into the 

 collecting box along with other larger and more powerful 

 insects; after the fly is overcome by the fumes of the 

 killing agent, it should be pinned immediately. If it is 

 allowed to shake around in the collecting box with all the 

 other finds, it will be unrecognizable when it is most 

 wanted. Few life histories of dipters are known, and 

 knowledge of this sort is needed ; here is a field for investi- 

 gation. When one goes for the study of flies in this way, 



