Fixation affects morphometric characters of Psoroptes cuniculi mites (Acari:Psoroptidae).

Thirteen morphologic characters were evaluated on Psoroptes cuniculi (Delafond) mites to determine if measurements for these variables were influenced by the use of different fixatives and varying periods of fixation. Morphologic structures composed primarily of chitin (outer opisthosomal setal lengths and right 3rd tarsal pedicle length) were not affected by either fixative type or duration of fixation. In contrast, alcohol and formalin fixation both resulted in significant reductions (P < 0.05) in mean body size measurements when compared with mites mounted in Hoyer's medium without prior fixation. The largest reduction in size was seen in mites fixed for the longest period, but there was no apparent association between size reduction and increasing concentration of alcohol or formalin. Surprisingly, when mites were remeasured 4 yr later, Hoyer's mounting medium was found to have had the opposite effect of fixation and caused an increase in body size. Mites mounted in Hoyer's medium without prior fixation had significantly increased body lengths, whereas mites mounted in Hoyer's following fixation in alcohol and formalin had significant increases in both body length and posterior body width. Our results indicate that fixation and mounting can significantly influence the morphometric analysis of mites and other arthropods. We recommend that morphometric studies be conducted using consistent methodology to reduce experimental bias, and these methodologies be reported in publications dealing with morphometric analyses.