Today, while out enjoying the weather in the wooded portion of the Notre-Dame-des-Neiges cemetery, I encountered a couple of common butterflies that, unusually, held still long enough for me to photograph them, so I'm going to take my opportunity to talk a little bit about this group of pollinators.
First up, we have the very common Glaucopsyche lygdamus couperi ('Silvery Blue' butterfly), a native of North America with a fairly broad range  which you have probably seen before:
|Glaucopsyche lygdamus couperi - this individual very obligingly held still for me so I could get a few photos|
|Glaucopsyche lygdamus couperi on Vicia cracca (cow vetch) - this is what the underside of the wings looks like|
So if you're a plant that would benefit from attracting butterflies and excluding other pollinators (perhaps in order to devote your resources to a single, more loyal species rather than take your chances with more fickle visitors), what kind of structure is most suitable? A deep nectar spur (pocket where nectar is produced and stored) that can only be accessed with an insecty straw, of course! So the more exclusively Lepidopteran-pollinated a species, the more likely it is to have a long nectar spur which can only be accessed via proboscis.
I got a fantastic shot of the very common Poanes hobomok ('hobomok skipper' butterfly) using its proboscis to suck up some nectar from Vicia cracca (cow vetch):
|Poanes hobomok collecting nectar from Vicia cracca|
Of course, Lepidopterans can and do get nectar from a variety of flowers, many of which aren't specialized exclusively to them, such as Vicia cracca (cow vetch). This introduced species is originally native to Europe which is now broadly distributed in North America (range map here). It is listed as invasive but with low current threat status in Minnesota . Apparently the spread of this species is also leading to a range expansion for the native Lepidopteran G. lygdamus couperi .
|Poanes hobomok on Vicia cracca|