Winter might appear to be something of a bleak time for the entomologist, the low temperatures putting a stop to the activities of our invertebrate fauna. However, there are insects and other small creatures to be found if you know where to look.
Perhaps one of the most profitable places to look for overwintering invertebrates is under the bark of fallen trees. Here you are likely to encounter woodlice, slugs and beetles of various kinds. Woodlice in particular make an interesting group, with 40 or so species recorded outdoors; other ‘indoor’ species are typically those that have been accidentally introduced to the UK in plants and which have established populations living within commercial and botanical glasshouses. Woodlice are an important part of the process of decomposition and nutrient recycling so finding them overwintering within dead wood or under bark is unsurprising. It is the larger species that are most commonly encountered, the smaller soil-dwelling species hidden below ground. Thanks to an excellent new atlas showing their distribution and a key to their identification produced by the Field Studies Council, this is a group that the amateur entomologist can readily tackle and to which you can make an important contribution. You’ll need a hand lens and, ideally, a microscope – some of the cheap ‘USB’ microscopes that plug into a computer and project the image on screen are ideal.
Slugs, on the other hand, are more difficult. The identification of many species requires examination of the genitalia and, since these are held internally, the slug needs to be killed and dissected, not something most of us would choose to do. There are even species that can only be reliably separated through examination of their DNA! Consequently, our understanding of which species occur in the UK, where and in what numbers, is rather limited. Of course, there are other slug species that can be readily identified in the field.
Many of our beetle species can also be identified in the field, or back at home with a hand lens and a key or guide. One species commonly found overwintering under bark is the carrion beetle Silpha atrata. While many of the carrion beetles feed on carrion, others, including Silpha atrata feed on molluscs; the elongated head that sticks out from the front of their otherwise shield-like body is probably an adaptation to this habit. Silpha atrata is usually black in colour, with pronounced ridges (or keels) on its hard wing cases (the elytra), and is often to be found in woodland. Other black beetles may be found sheltering under bark, many of them belonging to the genus Pterostichus (pronounced ter-ros-tic-us). All are equally interesting and well worth the effort at this time of year.