8 ILLINOIS BIOLOGICAL MONOGRAPHS [178 



of structure and other interesting modifications, shown in the figures 

 but without direct bearing on the subject of homology, are necessarily 

 disregarded. The fixed and various movable parts of the head are dis- 

 cussed separately, as developed from the hypothetical types, the discus- 

 sions in every case proceeding from the generalized to the specialized. 



All the general conclusions pertaining to the head and mouth-parts 

 presented in the following pages are based entirely on a study of the 

 species listed under "materials", unless otherwise stated. General 

 statements in respect to the mouth-parts are true only for species having 

 them well developed. 



The names here adopted for the sclerites of the head and mouth- 

 parts have been made to agree, so far as possible, with the terms now 

 in common use for the same parts in generalized insects. The terms 

 most commonly used thruout the literature for structures peculiar to 

 this order have been adopted unless clearly unsuitable ; and new terms 

 have been applied only to structures described here for the first time 

 and to parts to which the current names are inappropriate. 



METHODS 



The greater part of this study was made from dried specimens that 

 had been soaked from two to twenty-four hours in a 10% solution of 

 potassium hydroxide. The sclerites of weakly chitinized forms show 

 more clearly when they have been soaked for only a short time. After 

 soaking, the heads were washed in distilled water to remove the potas- 

 sium hydroxide and then preserved in 70% alcohol. 



All dissections were made under a binocular microscope in 70% 

 alcohol in deep watch-glasses or in carbol-aniline oil. Studies and 

 figures were largely made from dissected parts in alcohol. Cleared 

 preparations mounted in balsam were also found useful. In making 

 such preparations the parts were dissected, stained, and cleared in 

 carbol-aniline oil. This oil evaporates slowly, will mix readily with 

 safranin or orange G dissolved in 95% alcohol, and will clear from 

 any grade of alcohol above 50%. The staining of material with safranin 

 before mounting proved to be very useful in diiferentiating the almost 

 colorless parts of some species. When using aniline oil it is necessary 

 to remove as much as possible of the oil before mounting, otherwise the 

 balsam will eventually darken. 



The material for sections was fixed with hot (80° C.) corrosive 

 sublimate (saturated corrosive sublimate in 35% alcohol plus 2% of 

 glacial acetic acid) for fifteen minutes to two hours. This was replaced 

 by 70 %> alcohol containing a few drops of iodine, and the material was 

 allowed to remain in this for twenty-four or more hours. Paraffin hav- 



