This post is the latest in a series covering the ID of a selection of species which are likely to be in and around everyone's gardens, parks, etc, created for the 2013 Garden Bioblitz (details at http://www.naturewatched.org/gbb.html - everyone join in!)
The Brimstone moth, Opisthograptis luteola, appears to be on a one-moth mission to disprove the fallacy that moths are dull, brown, clothes-munching things:
A bright yellow flattened triangle with brown and white markings, the Brimstone has a wingspan of about 35mm. With multiple overlapping generations, adults are continuously on the wing from April to October in the south, gradually declining to a single mid-summer generation in Scotland. It is widespread and abundant almost everywhere, and frequently comes to light, though it does occasionally fly during the day. No other British species is this size and colour - the only similar species likely to be encountered is the Swallowtailed moth, Ourapteryx sambucaria, but this is much larger, paler, and flies for a short period in July.
The Brimstone caterpillar is a twig mimic, and like most caterpillars of the Geometridae family they're loopers, or inchworms. They have two colour forms, brown and green, and have a hump halfway along their body. Dietary generalists, they can be found feeding on a wide range of trees and shrubs, including hawthorn, blackthorn and rowan. Pupae (the 'chrysalis') are formed in a loosely-spun cocoon between leaves, and both can be found in any month of the year, including throughout the winter - the Brimstone is one of the few moth species to overwinter in two different stages, both caterpillar and pupa
For more info on British moths, check out the excellent UK Moths site: http://ukmoths.org.uk/
Additionally, there's a network of county moth groups all over the country, usually with notes on what moths are flying at the time, local distribution maps, hints on any local variations to look out for, and much more - the best way to find a group nearby is to google '(your county) moths'.
And of course, don't forget to join in with the Garden Bioblitz on the 1st/2nd June 2013! http://www.naturewatched.org/gbb.html for more details