Yes, even after seven years of "dragonhunting" there are some species that prove to be my nemesis (or should that be nemeses?) when it comes to separating them in the field, namely, the females of Black-tipped and Lance-tipped Darners.
Male dragonflies are never a problem, they are easily distinguished by their terminalia, such as, for example, this male Black-tipped Darner (Aeshna tuberculifera). The inset shows the claspers and the "black-tip" – the all black S10, although in some individuals this segment may bear tiny blue spots.
With a length of 71 mm to 78 mm the Black-tipped Darner is theoretically larger than the Lance-tipped Darner, which weighs in at 65 mm to 72 mm. But length is also tough to guesstimate in the great outdoors, and who bothers to cart a ruler around with them on a nature hike?
Lance-tipped Darner (Aeshna constricta), female:
– The lateral thoracic stripes are relatively narrow and notched.
– The dark lines separating the lateral blue abdominal spots are very straight and quite narrow.
– The blue line on S2 is continuous, and there is generally more blue than dark in this area.
The last two factors conspire to give the Lance-tipped Darner's abdomen a bluer overall aspect than the abdomen of the Black-tipped Darner.
– The lateral thoracic stripes are broad and pale, sometimes almost white, and when the dragonfly is perched in a shady place they almost seem to glow.
– The dark lines separating the lateral blue abdominal spots are somewhat irregular, and larger than in the Lance-tipped Darner.
– The blue line on S2 is broken, there is more dark than blue on this part of the dragonfly.
– The face usually bears a thin brown cross-stripe.
Note that although the colors are different in the next two examples, the patterning is still consistent with the blue form Lance-tipped Darner females studied above.