12 Annals Entomological Society of America [Vol. 11, 



parts in canacla balsam, glycerin or some other medium; and 

 finally sectioning with the microtome after applying the usual 

 reagents. Used alone none of those methods is quite reliable. 

 But when one is controlled by the others and when the number 

 of specimens and forms studied is sufTficiently great, one gets a 

 clear idea of relations which would otherwise entirely escape obser- 

 vation. 



To one point I desire to call particular attention. The ter- 

 minology in this group is in as bad a shape as in any other group 

 of animals. Even the names for the three planes of the body 

 have not found universal acceptance in their application to the 

 different groups of the animal kindgom and what is still worse the 

 same term has often more than one entirely distinct application. 

 Thus the term transverse plane is used by different authors to 

 designate each of two separate intersecting planes, which results 

 in terrible confusion. In the case of appendages the terms upper 

 and lower, anterior and posterior, internal and external have 

 also been applied to the same structures, making it at times 

 extremely difficult to understand the author's exact meaning. 

 To avoid all misunderstanding I shall therefore give in this intro- 

 duction the terms as I will use them throughout the entire series 

 of articles as well as in my papers on systematics, the first of 

 which W'ill be ready for print in the near future. Whenever pos- 

 sible I preferred new terms to old ones, not out of desire to aug- 

 ment the number of terms of which there are already more than 

 needed, but to enable the reader to know at once and exactly 

 their meaning. Some of these terms are cumbersome and pos- 

 sibly could be replaced by better ones. However, clear and 

 exact terms are in nn' opinion to be preferred to those which 

 although easily remembered are at the same time easily confused. 



The three determining planes of the body, applying to all 

 animals I will call as follows: 



1. The Plane of Symmetry (called usually the sagittal or 

 chief plane). It divides the body into two symmetrical parts, 

 and not only in arthropods but even in echinoderms and coelen- 

 terates there is only one plane of symmetry. All planes that 

 run j3arallel to it I will call par asymmetrical. 



2. The Synaxonial Plane (called often the frontal, and by 

 Claus and others wrongly the transverse plane) is a plane inter- 

 secting the plane of symmetry at right angles in such a manner 

 that their line of intersection forms the chief axis of the bod\' 



