Common  charaters of the genus Yllenus 
 Y. arabicus 
1. High cephalothorax sloping behind eye-field, 2. Legs IV the longest
3. usually cryptic coloration adapted for sandy environment.  4. Brush of setae - a scopula - on tarsus I-II.
5. Characteristic genital organs in males and females - diversified into 3 groups (see below) 
 but with intermediate forms

 ][ 
univittatus              arenarius  

About 80 species of Yllenus differ by epigynum: shape and internal  structure,
palpal organ size and shape of conductor are of limited diagnostic use 

Proposed division of the genus Yllenus into following groups of species