165 



impairs the value of the specimens. In order to procure the best re- 

 sults in the case of soft-bodied specimens it is especially necessary that 

 the examples be placed at first in water, which should gradually be 

 brought to the boiling point, or near it, and then set to cool. If the 

 water is allowed to boil violently it often results in a distortion or 

 over-expansion of the specimens. Upon removal from the cool water 

 place the specimens in 25% alcohol to remain six or eight hours; next 

 transfer them to 50% strength, in which they should remain twenty- 

 four hours, — after which treatment the larvae may safely be trans- 

 ferred to 85% alcohol, in which they should be kept. 



Pupal exuvia that have the integument chitinized may be pre- 

 served dry, but even such forms must be placed in alcohol if the imago 

 has not emerged, as they shrink very much when preserved dry. 



I have succeeded in obtaining presentable specimens from com- 

 pletely dried-out larval and pupal exuvia by boiling them in water. 

 A larva or a pupa after drying out is rarely restored to its original 

 form except, by much patient work; but exuvia, even months after 

 they have dried out, invariably recover their form when boiled. 



The head parts are generally easily traceable in larval exuvia, 

 but in preserved larvae, especially of Brachycera and Cyclorrhapha, 

 in which the head is retracted, dissection must be resorted to in order 

 to get at the internal structures. I have had some success in ascertain- 

 ing these details when I did not wish to cut up the specimen, by boiling 

 it in 10% potash; but this is a tedious operation, especially if the 

 specimen is a large one, and I prefer to expedite matters by cutting 

 off the anterior two thoracic segments — caudad of which the cephalic 

 skeleton does not extend — which may readily be cleared so that dissec- 

 tion is possible in a few minutes'. 



The larger species have such heavily chitinized opaque heads that 

 they are not good objects for slide mounts, but the smaller ones should 

 invariably be mounted in Canada balsam. It is always necessary that 

 some system of cross-reference be used for slide, vial, and imago in 

 order to facilitate reference. 



More detailed information upon methods of mounting specimens 

 is given in my article on the Chironomidae of Illinois.* 



ECONOMIC IMPORTANCE OF THE ORDER 



No other order of insects equals the Diptera in diversity of habits 

 in larval and imaginal stages. Many of the families are largely bene- 

 ficial, but unfortunately the good done by them is counterbalanced by 



*Bull. 111. State Lab. Nat. Hist., Vol. 10, Art. VI. (1915) 



