New AIDGAP guide: Adult Caddis (Trichoptera) of Britain and Ireland

Ian Wallace has co-ordinated the UK National Caddisfly Recording Group for over 40 years. He is the co-author of a new AIDGAP guide to the adult caddis of Britain and Ireland.

The caddisflies (Trichoptera) are a cosmopolitan group of insects whose aquatic larvae are perhaps better known than the moth-like adults. Currently 202 species are known from Britain and Ireland, out of around 1400 species in Europe and over 12,000 worldwide.

Why this guide has been produced

Unfortunately adult caddis have a reputation for being difficult to identify. Although excellent reliable identification guides do exist, they tend to focus on the wing venation and genitalia of preserved specimens. Unlike moths and butterflies, there are no identification guides where comparison with pictures of whole insects is the norm. It frustrates many recorders that they cannot easily name many conspicuous caddis.

This new guide aims to solve this problem. It uses just the visual appearance of the whole living insect, assisted sometimes by magnification no higher than a x 10 hand lens. This enables the identification of adult caddis in Britain and Ireland to a group or often a species.

Development of this guide was inspired by the many naturalists who submit photographs of caddis to websites such as iRecord, iSpot, iNaturalist, the Moth Trap Intruders Facebook group and NatureSpot. All these sites provided a rich bank of material and volunteers for testing. In addition to routine AIDGAP testing, draft versions of the keys were tested at entomological workshops at World Museum, Liverpool in 2020 and 2021.

Getting started with identification

The guide recommends a structured approach to identification.

• An initial key enables a caddis to be placed in one of four major groups. This uses simple and reliable body features which can be seen with the naked eye.
• Within each major group, we have arranged similar-looking caddis into sub-groups and sections.
• The intention is that any live caddis, or photograph of it, can be placed into one of three categories: (a) identified to species, (b) identified to genus, or (c) placed in a small, artificial group.
• Unlike moths, the genitalia of caddis are not obscured amongst scales and quite easy to observe using a basic stereo microscope. The specimen will usually have to be killed but a suite of robust species can also be examined under anaesthesia and released, and the book describes the procedure. The genitalia of a large number or caddis are illustrated in the book.

Many recorders support their records with photographs. So we have included things to look out for when taking or viewing photographs. Much of this section has been developed in response to our testing with users. Side-on views tend to be much more useful than views from above. Beware of swamping the insect with uneven flash lighting.

Field survey techniques

Here are four techniques you can use, but there are others.

• Light trapping: caddis may be among the first insects to arrive at a light trap (or sheet) as darkness sets in. Many species are crepuscular rather than nocturnal. A trap set next to a river or lake is likely to catch huge numbers.
• Beating and sweeping: both of these are traditional insect collecting techniques. Unfortunately beating by itself is largely useless, as caddis readily jump or fly away. But beating into a sweep net can be highly effective. Clumps of ferns by rivers and green ash keys are good places to search.
• Searching by eye: caddis can be seen resting on vegetation or other surfaces, though they are often well camouflaged. Bankside vegetation, particularly where is overhangs water, can be a good place to look.
• Netting from a swarm: one group of caddis are well known for their gyrating day-time swarms. Vigorously waving a net while rushing into the water may catch some specimens of these insects, and at the very least it will entertain bystanders on the river bank.

The Caddis guide is available to order from the FSC online shop with an Earlybird offer of £16 if ordered by 31 August 2022.