The two Northern Lestids are very similar in appearance  and can easily be confused. Males are best separated  but viewing their anal appendages (see above).

• Lower appendages (shown black above) clubbed and inward turned - L. dryas
  
• .​Lower appendages straight - L. sponsa

Less reliable features include:

• bright blue eyes - 2nd abdominal segment (above) green in lower half - L. dryas
• dark blue eyes - 2nd abdominal segment predominantly powder blue - L. sponsa

But beware these latter features can be unreliable - appendages are vital for confirmation. They can be seen with close focusing binoculars 

The two species look similar but have quite different life styles and rarely occur together.